Bulge Globular Clusters in Spiral Galaxies

a r X i v :a s t r o -p h /9710041v 1 3 O c t 1997HST Imaging of the Globular Clusters in the Fornax Cluster:NGC 1379Rebecca A.W.ElsonInstitute of Astronomy,Madingley Road,Cambridge CB30HA,UKElectronic mail:elson@ Carl J.Grillmair Jet Propulsion Laboratory,4800Oak Grove Drive,Pasadena,CA 91109USA Electronic mail:carl@ Duncan A.Forbes School of Physics and Astronomy,University of Birmingham,Edgbaston,Birmingham B152TT,UK Electronic mail:forbes@ Mike Rabban Lick Observatory,University of California,Santa Cruz,CA 95064USA Electronic mail:mrabban@ Gerard.M.Williger Goddard Space Flight Center,Greenbelt,MD 20771USA Electronic mail:williger@Jean P.BrodieLick Observatory,University of California,Santa Cruz,CA 95064USAElectronic mail:brodie@ReceivedABSTRACTWe present B and I photometry for∼300globular cluster candidates in NGC1379,anE0galaxy in the Fornax Cluster.Our data are from both Hubble Space Telescope(HST)and ground-based observations.The HST photometry(B only)is essentially complete and free of foreground/background contamination to∼2mag fainter than the peak of the globular cluster luminosity function.Fitting a Gaussian to the luminosity function wefind B =24.95±0.30 andσB=1.55±0.21.We estimate the total number of globular clusters to be436±30.To a radius of70arcsec we derive a moderate specific frequency,S N=3.5±0.4.At radii r∼3−6 kpc the surface density profile of the globular cluster system is indistinguishable from that of the underlying galaxy light.At r∼<2.5kpc the profile of the globular cluster systemflattens, and at r∼<1kpc,the number density appears to decrease.The(B−I)colour distribution of the globular clusters(from ground-based data)is similar to that for Milky Way globulars,once corrected for background contamination.It shows no evidence for bimodality or for the presence of a population with[Fe/H]∼>−0.5.Unlike in the case of larger,centrally located cluster ellipticals,neither mergers nor a multiphase collapse are required to explain the formation of the NGC1379globular cluster system.We stress the importance of corrections for background contamination in ground-based samples of this kind:the area covered by a globular cluster system(with radius∼30kpc)at the distance of the Virgo or Fornax cluster contains∼>200background galaxies unresolved from the ground,with magnitudes comparable to brighter globular clusters at that distance.The colour distribution of these galaxies is strongly peaked slightly bluer than the peak of a typical globular cluster distribution.Such contamination can thus create the impression of skewed colour distributions,or even of bimodality,where none exists.Key words:galaxies:individual:NGC1379-globular clusters:general-galaxies:star clusters1.IntroductionAn accumulating body of observations suggests that the distribution of colours of globular clusters,and by inference of their metallicities,varies significantly from one galaxy to the next.Of particular interest is the colour bimodality which has now been observed in the globular cluster systems of several large elliptical galaxies,and suggests the presence of distinct metal rich and metal poor populations.The best example is the Virgo cD galaxy M87(NGC4486)(cf.Elson&Santiago1996).It has one population of globular clusters with colours similar to those of the Milky Way globulars,and one which is significantly redder,with inferred metallicities∼>solar.Other galaxies whose globular cluster systems show clear bimodality include M49(NGC4472),an E2galaxy in the Virgo Cluster with the same luminosity as M87(Geisler,Lee&Kim 1996),and NGC5846,a slightly less luminous E0galaxy at the centre of a small compact group(Forbes, Brodie&Huchra1997a).Two ideas have been invoked to explain the colour bimodalities.One is that elliptical galaxies are formed during mergers in which populations of metal rich(red)globulars are created and added to a‘native’population of metal poor(blue)clusters(cf.Ashman&Zepf1992).The other is that globular cluster populations with different mean metallicities form during a multiphase collapse of a single system(Forbes, Brodie&Grillmair1997b):metal poor globular clusters are formed early in the collapse,while metal rich ones form later,roughly contemporaneously with the stars.To understand fully the implications of the observed colour distributions for the origin of globular cluster systems,a much larger body of accurate data for systems surrounding galaxies of a variety of types and in a variety of environments is required.A question of particular importance,for example,is whether all elliptical galaxies have bimodal globular cluster systems,or whether such systems are restricted to large galaxies in rich environments.The data best suited to address this question are those acquired with the Hubble Space Telescope(HST).The resolution of HST allows even the crowded central regions of galaxies at the distance of Fornax and Virgo to be probed and allows most background galaxies to be eliminated. At these distances samples of globular clusters are complete and uncontaminated to well past the peak of the luminosity function.This paper and the others in this series are contributions to this growing database. Forbes et al.(1997a)and Grillmair et al.(1997)discuss the globular cluster systems of the Fornax cD galaxy NGC1399,its neighbour the E1galaxy NGC1404,and the peculiar galaxy NGC1316which may have undergone a recent merger.Here we present observations of the globular cluster system of NGC1379, a normal E0galaxy in the Fornax cluster.Hanes&Harris(1986)used photographic data to study the NGC1379globular cluster system toB=23.6(about a magnitude brighter than the peak of the luminosity function).They measured the profile of the outer part of the system(5<r<35kpc),and estimated the total population to number∼800. More recently the globular cluster systems offive galaxies in the Fornax cluster,including NGC1379,have been studied by Kohle et al.(1996)and Kissler-Patig et al.(1997a)using V and I−band photometry obtained with the100-inch telescope at Las Campanas.Their data are50%complete at B∼24,and cover a radial range∼3−10kpc.Our observations,obtained both from the ground at the Cerro Tololo Interamerican Observatory (CTIO),and from space using HST,are described in Section2.Section3presents the results,including a caveat concerning the need for accurate background corrections for ground-based data.Ourfindings are summarized in Section4.2.Observations and Data ReductionIn this section we describe the HST and CTIO images upon which our results are based,and the process for detecting,selecting,and determining magnitudes for the globular cluster candidates in each case.We also discuss completeness,and contamination from foreground stars and background galaxies.At an adopted distance of18.4Mpc(m−M=31.32;Madore et al.1996),1arcsec corresponds to89pc. Reddening in the direction of the Fornax cluster is assumed to be negligible(Bender,Burstein,&Faber 1992).2.1.HST ImagingFive images of NGC1379were obtained with HST on1996March11,using the F450W(∼B)filter. (Due to technical difficulties,the complementary I-band images were not acquired,and are anticipated in 1997.)Three images were taken at one pointing,and two more were offset by0.5arcsec.The total exposure time was5000seconds.The centre of NGC1379was positioned at the centre of the Planetary Camera (PC)chip to afford the greatest resolution in the most crowded regions.Details of the reductions are given by Grillmair et al.(1997).Briefly,the images were reduced using the standard pipeline procedure.TheVISTA routine SNUC was used tofit and subtract the underlying galaxy.We ran DAOPHOT II/ALLSTAR (Stetson1987)separately on the sum of thefirst three images and the last two images,requiring that detections appear in both lists to qualify as real objects.We adopted a detection threshold of3σand measured magnitudes byfitting a point-spread function(PSF).Extended objects were eliminated by visual inspection.Count rates were converted to B magnitudes using the gain ratios and zeropoints given by Holtzman et al.(1995;1997,private communication).Photometry is available on request from CJG.Figure1shows a mosaic of the four WFPC2chips,with the galaxy subtracted.The total area of the field,excluding two60pixel wide unexposed borders on each chip,is4.8arcmin2.The scale of the Wide Field Camera(WFC)is0.0996arcsec pixel−1,and of the PC,0.0455arcsec pixel−1.At the distance of NGC1379,one WFC pixel corresponds to∼9pc and one PC pixel to∼4pc.A globular cluster with size typical of those in the Milky Way(core radius∼2pc,half-mass radius∼10pc,and tidal radius∼50pc) will thus appear essentially unresolved in our images.A total of∼300objects were detected and measured in ourfield.To determine the completeness of this sample,3000artificial PSFs(100at each of30magnitude levels)were added to the images,and the images were then processed in a manner identical to that for the original data.The completeness of the sample as a function of magnitude is shown in Figure2.The sample is∼100%complete to B=26for the WFC chips(80%of the sample),and to B=25.5for the PC chip.At B>26the completeness begins to drop rapidly.Photometric errors areδB≈0.10mag at B∼<25,rising to0.15mag at B=26.Next we consider the extent to which our sample may be contaminated by foreground stars and background galaxies.Few foreground stars are expected in an area of only4.8arcmin2at this Galactic latitude,and most background galaxies are resolved and thus easily distinguished from globular clusters. The main source of contamination is compact,spherical background galaxies.To determine the expected level of contamination in our sample,we observed a backgroundfield located∼1.4degrees south of the center of the Fornax cluster.The exposure time was5200seconds,so the limit of detection is comparable to that for the NGC1379sample.The image was processed and the sample selected in the same way as for the NGC1379field(see Grillmair et al.1997).Figure3shows a colour-magnitude diagram(CMD)for the84unresolved objects detected in the backgroundfield.The sample becomes incomplete at B>26.5,but as we shall see,this is∼1.5magnitudes fainter than the peak of the luminosity function,and so will not affect our results.At B∼<26.5the objects have a wide range of colours,with the majority concentrated around(B−I)∼1.The B luminosityfunction for the background sample is plotted in Fig.4,which also shows the luminosity function for the ∼300candidate globular clusters.The background luminosity function is tabulated in Table1.Since these background number counts are applicable to HST studies of any unresolved population at high latitude, we also include in Table1the I-band luminosity function for the backgroundfield.This is plotted as the solid histogram in Fig. 5.As a check on the consistency of the sample,and to assess the amplitude of spatialfluctuations in the background on this scale,we compared the luminosity function in Fig.5with the Medium Deep Survey(MDS)star count data from17high latitudefields obtained with HST in the V and I bands(Santiago,Gilmore&Elson1996).The dotted histogram in Fig.5shows the I-band luminosity function for the MDS data,normalized to an area of4.8arcmin2.The two distributions are in excellent agreement to I≈23.5which is the limiting magitude of the MDS star count data.Fainter than this stars can no longer be reliably distinguished from compact galaxies.The MDS stellar luminosity function in V, normalized to the same area,is also included in Table1.Finally,Fig.5also shows a luminosity function for stars and galaxies from the Canada-France Redshift Survey(CFRS)(Lilly et al.1995;S.Lilly1997,private communication).The magnitudes were measured in an aperture of diameter3arcsec.The I-band data cover an effective area of∼425arcmin2,and have again been normalized to an area of4.8arcmin2.These data illustrate the much larger degree of background contamination to be expected in the extreme case where no galaxies(with I∼>21)can be distinguished morphologically from unresolved objects.Background contamination in typical ground-based samples of globular clusters in distant galaxies will fall somewhere between the CFRS and the HST histograms.2.2.Ground–based ImagingBroadband B and I images of NGC1379were taken with the CTIO1.5m telescope,using a Tek2048 x2048array with a pixel scale of0.44arcsec pixel−1.Total exposure times were9000and3900seconds for the B and I images respectively.Although the seeing was only∼1.5arcsec,conditions remained photometric throughout the night of1995December24.Reduction was carried out in the standard way(i.e. bias and dark subtraction,flat–fielding and sky subtraction).After combining,the images were calibrated using aperture photometry from the catalogs of of Longo&de Vaucouleurs(1983)and de Vaucouleurs& Longo(1988).This procedure gave an rms precision of better than0.05mag.For the selection of globular cluster candidates in the CTIO images,we employed an iterative procedure using DAOPHOT II.We measured the background noise in both images and set the threshold for singlepixel detection at5σ,i.e.five times the noise due to the background.The other important detection parameters,SHARPness and ROUNDness(designed to weed out extended objects and cosmic rays),were initially given a large range.For each detected object we measured a3pixel radius aperture magnitude and applied an aperture correction based on a curve-of-growth analysis for a dozen isolated globular clusters. The rms error in the aperture correction is∼0.05mag.We then compared our B-band candidate list with the positions and B magnitudes of globular clusters detected with HST’s WFC.Our candidate list was matched to the HST list with the condition that the HST globular cluster lie within3CTIO pixels of our object.With this condition,28objects were matched. The average magnitude difference between the CTIO and HST B magnitude is0.03mag.Such excellent agreement is gratifying from a photometric standpoint,and reassures us that we have matched the data sets correctly.The matched clusters have SHARPness parameters0.3to0.7and ROUNDness−0.4to0.4. Assuming that these globular clusters are representative of all globular clusters in the CTIO image,we re–ran DAOPHOT II with the new restricted range in SHARPness and ROUNDness parameters.This resulted in the exclusion of about half the objects in the original sample,which are presumably background galaxies.The same parameters were used for both the B and I images.Figure6shows a CMD for365 objects selected in this way.A comparison of the CTIO object list and the HST list,within the area in common,will also indicate how complete our detection is as a function of magnitude.The completeness function estimated this way is shown in Fig.7.Our sample is∼100%complete at B∼21and∼50%complete at B∼23.5.The photometric errors are<0.1mag for all magnitudes brighter than our50%completeness limit.At this point it is necessary to investigate the level of contamination of our ground-based sample by foreground stars and background galaxies.Since thefield is much larger than that observed with HST(211 arcmin2compared to4.8arcmin2),and since the resolution is not sufficient to distinguish many background galaxies from unresolved sources,contamination from both stars and galaxies will be significant.Since no background comparisonfield was obtained,we rely instead on the CFRS data in the B and I-bands, discussed above,to estimate a correction for contamination.Figure8shows histograms of(B−I)for the full NGC1379sample in Fig.6,and for the CFRS sample of objects with19<B<23.Since selection effects in the two samples are different,we do not normalize the CFRS sample directly using the known area of the survey.Rather,we normalize it to match the tail of objects with(B−I)>2.5in Fig.8,whose colours are too red to be globular clusters.From this wecan infer the relative number of contaminants with(B−I)<2.5.The CFRS sample is strongly peaked at (B−I)∼1.0,which is just∼0.4mag bluer than the peak of the NGC1379sample.The NGC1379sample is lacking many of the bluest objects in the CFRS sample;this is probably because fainter galaxies are on average bluer,and the CFRS sample is much more complete than the NGC1379sample at the faintest magnitudes.Also,our process of excluding galaxies from the CTIO sample on the basis of ROUNDness would preferentially eliminate edge-on spirals,which are systematically bluer than ellipticals.The most important point illustrated by Fig.8is that in the ground-based data,much more so than in the HST data,failure to properly correct for foreground/background contamination may lead to a significant error in the deduced colour of the peak of the globular cluster colour distribution,and possibly to the erroneous impression of a bimodal colour distribution.With ground-based data obtained with better seeing it would of course be possible to exclude a greater proportion of background galaxies,so that any skewing of the colour distribution would be less severe.3.Properties of the NGC1379globular cluster systemThe principal properties of a globular cluster system which any theory of its origin and evolution must account for are the luminosity function(in particular,the absolute magnitude of its peak,and the width of the distribution),the colour distribution(in particular,the colour of the peak and the presence or absence of any bimodality),the radial distribution,and the total number of clusters,N tot,and therefore the specific frequency,defined as S N=N tot100.4(M V+15),where M V is the absolute magnitude of the galaxy.With B tot=12.07(Tully1988)and(B−V)=0.89(Faber et al.1989)we infer an apparent magnitude for NGC 1379of V tot=11.18.With distance modulus31.32,this implies M V=−20.16.This is the integrated magnitude within a radius of∼70arcsec.3.1.Luminosity functionThe luminosity function for the NGC1379globular cluster candidates derived from the HST datais shown in Fig. 4.This sample includes∼300unresolved objects and is essentially complete and uncontaminated well past the peak.The background-corrected luminosity function,that is,the differencebetween the solid and dashed histograms in Fig.4,is shown in Fig.9.Fitting a Gaussian function to the luminosity function in Fig.4,using a maximum-likelihood technique which is independent of binning, and takes into account completeness,background contamination,and photometric errors(Secker&Harris 1993),we derive a peak magnitude of B =24.95±0.30,and a widthσB=1.55±0.21.This Gaussian is shown as the solid curve in Fig.9.We know of no other direct measurements of B for this system to compare with ours.However,our values are the same,to within the errors,as the values measured for the globular cluster systems of two other Fornax galaxies,NGC1399and NGC1404(Grillmair et al.1997).Also,for four elliptical galaxies in the Virgo cluster Harris et al.(1991)find an average value B =24.77±bining this with the value(m−M)F ornax−(m−M)V irgo=0.08±0.09(Kohle et al.1996)implies B =24.85±0.22for NGC 1379.This is in good agreement with our value.We may either use the measured peak magnitude to infer a distance modulus for NGC1379,assuming a‘universal’value for the absolute magnitude of the peak,or we may adopt a distance modulus and infer an absolute magnitude.While the value of M V has been measured for many globular cluster systems, there are fewer B−band studies.Sandage&Tamman(1995)quote M B =−6.93±0.08for the Milky Way and M31globular clusters,while Ashman,Conti&Zepf(1995)give M B =−6.50for the Milky Way clusters.Adopting the Cepheid distance for NGC1379and our measured peak magnitude,implies M B =−6.37±0.36,which is in good agreement with the Ashman et al.value and somewhat fainter than the Sandage&Tamman value.As we shall see below,the(B−I)distribution of NGC1379globular clusters appears very similar to that for the Milky Way globular clusters.It should therefore be safe to assume that the(B−V)distribution is also similar.Adopting B−V =0.7for the Milky Way globulars(Harris1996),we may convert our value for the peak magnitude of the luminosity function, B =24.95±0.30,to V =24.25±0.30.This value is somewhat fainter than the value found by Kohle et al.(1996)of V =23.68±0.28,but their data reach just to the peak of the luminosity function,and the errors in their faintest bin are large.The sense of the difference is consistent with our result above thatfitting only the brighter part of the luminosity function results in a peak magnitude which is too bright.With the Cepheid distance modulus of31.32,our V value implies M V =−7.07±0.36,which is typical for elliptical galaxies of this absolute magnitude (Harris1991).Our value ofσB=1.55±0.21for the NGC1379globular cluster system is larger than the valuesσB=1.07and0.89quoted by Sandage&Tamman for the Milky Way and M31respectively,but is consistent with the valuesσB=1.37±0.07andσB=1.39±0.12found by Grillmair et al.(1997)for the globular cluster systems of NGC1399and NGC1404respectively.It is also consistent with the value σB=1.46±0.07for four elliptical galaxies in the Virgo cluster(Harris et al.1991).3.2.Radial profileHarris&Hanes(1987)compared the radial profile of the NGC1379globular cluster system with the surface brightness profile of the galaxy itself from Schombert(1986),over the radial range5−35kpc. They detected no difference between the two,although their uncertainties were large.Kissler-Patig et al. (1997a)found the same result from their ground-based data over the range3−10kpc.Radial surface density profiles for our HST sample of NGC1379globulars are shown in Figs.10a and b.Corrections have been made to compensate for the fraction of each annulus which falls outside thefield of view of the WFPC2.In Figure10a profiles are plotted for both the complete,uncontaminated sample with B<25.5, and for the objects with B>26.5(with no correction for completeness),which are expected to be primarily background galaxies.Indeed,the fainter sample shows almost no radial gradient.The dashed line is the background level(∼9.0objects per arcmin2)measured from the backgroundfield for B>26.5.The agreement is excellent.The background level for the brighter sample,again measured from the background field,is2.7objects per arcmin2,and is shown as the solid line.The difference between the radial profile and the background level is probably due to small numbers,but may indicate a small amount of residual contamination:thefive outermost points represent an average of<4objects each,and the total number of objects in the backgroundfield with B<25.5is just13.The radial profile of the brighter sample decreases smoothly from∼10−80arcsec(∼1−7kpc),at which point the globular cluster system is lost in the background.Figure10b shows the radial profile for the sample with B<25.5,with a background of2.7objects per arcmin2subtracted.Superposed is a curve representing the surface brightness profile of the underlying galaxy from Kissler-Patig et al.(1997a),scaled arbitrarily to match the profile of the globular cluster system.The two profiles agree well at∼35−70 arcsec(3–6kpc).There is no evidence that the surface brightness profile of the globular cluster system is shallower than that of the underlying galaxy light out at least to the limit of our data at r∼7kpc.The logarithmic slope of the profile in Fig.10b is−2.4at r∼>35arcsec.Inwards of r∼30arcsec(∼2.5kpc)the profile of the globular cluster systemflattens out.This core structure seems to be a common feature of the globular cluster systems of elliptical galaxies,and the radius at which theflattening occurs correlates with the galaxy luminosity(Forbes et al.1996).The mean surface density within∼10arcsec of the centre of NGC1379is∼200±60clusters per arcmin2.The core radius, where the surface density has fallen to half its central value,is r c∼23±6arcsec(2.0±0.5kpc).This is consistent with values for other galaxies with absolute magnitude comparable to that of NGC1379.Such a core structure is not present in the underlying galaxy light,which,while it changes slope slightly at r∼50 arcsec,rises with constant slope inwards to at least10arcsec(Schombert1986).Inwards of∼10arcsec(∼1kpc),the surface density of globular clusters appears to decrease.This radius corresponds to220pixels on the PC,and as can be seen from Fig.1,crowding even in these inner regions is not severe.Extrapolating a smoothly rising profile to the center of the galaxy would require6 clusters instead of2in the innermost bin,and14instead of12in the second bin.If the radial distribution really were steadily rising,this would imply that our data are only∼70%complete in these combined bins. Closer analysis of our completeness tests reveals that we are in fact97%complete for B<25.5in the region r<220pixels(the slight reduction over the PC-averaged completeness value being a consequence of the increased noise due to the integrated stellar light of the central regions of the galaxy).We conclude that the central dip in the cluster surface density distribution is not an artifact of our analysis,and may indeed indicate a quite substantial drop in the volume density of clusters near the nucleus of the galaxy.Radii less than1kpc from the nucleus are where we expect tidal stresses to begin to take a toll on the numbers of globular clusters(Lauer&Kormendy1986,Grillmair,Pritchet,&van den Bergh1986),either during their formation or subsequently through tidal stripping(Grillmair et al.1995),particularly if the clusters are on box orbits.We now turn to the ground-based data which cover a much greater area than the HST data.The radial distribution for the full sample from Fig.6is shown in Fig.11.The surface density for the CTIO sample has been increased by0.74in log N to match the HST sample,which is shown as the solid curve.At r∼>100arcsec,the profile of the CTIO sample is essentiallyflat,suggesting that the sample is composed overwhelmingly of foreground/background objects at these radii.Even a colour selection is unlikely to help disentangle the background contamination since,as shown in Fig.8,background galaxies have a similar colour distribution to the full NGC1379sample.We therefore conclude that,despite the larger spatial coverage of the ground-based sample,in the absence of a suitable background calibrationfield it does not contribute much to our knowledge of the radial structure of the NGC1379globular cluster system notcovered by our HST data.It is also unable to probe the innermost regions of the cluster system,due to crowding.3.3.Colour distributionIn the absence of an I-band image from HST,we attempt to extract a sample of globular clusters from the ground-based data which is as uncontaminated as possible,to investigate the(B−I)colour distribution.One approach is to select only objects with r<100arcsec,since these show a radial gradient in surface density(Fig.11).This gives a sub-sample of35objects.The background level inferred from the radial distribution of the CTIO sample at r>100arcsec is1.6arcmin−2,implying that14of the35 globular cluster candidates at r<100arcsec,or40%of this sample,are background galaxies or foreground stars.The colour distribution for the35objects at r<100arcsec,along with those for the full CTIO sample and for the CFRS sample,is shown as the dashed histogram in Fig.8.The r<100arcsec sample clearly peaks at a redder colour than the full sample,underlining the fact that the colour distribution of the uncorrected sample is misleading.Figure12shows the same histogram for the r<100arcsec sample,along with the histogram of residuals obtained by subtracting the normalized CFRS histogram from the full CTIO histogram in Fig.8.A histogram for95globular clusters in the Milky Way(Harris1996)is also shown.The colour distributions of the NGC1379sample with r<100arcsec,and with the CFRS sample subtracted,have a very similar peak colour,(B−I)≈1.6,which is indistinguishable from that for the globular cluster system of the Milky Way.Using the relation between(B−I)colour and metallicity from(Couture et al.1990)we infer from the peak colour a metallicity of[Fe/H]∼−1.5.Forbes et al.(1997b)plot a relation between mean metallicity of globular clusters and parent galaxy magnitude for11galaxies with−21<M V<23,with bimodal globular cluster colour distributions.Theyfind that,while in the metal rich populations([Fe/H]>−0.5)there is a strong correlation between mean globular cluster metallicity and galaxy magnitude,for the metal poor populations the scatter is much greater and the correlation much weaker.There is,however,a trend for less luminous galaxies to have globular clusters with a lower mean metallicity,and our results for NGC1379are consistent with this trend.There is a tail of bluer objects in the NGC1379samples which is probably comprised of residual。

剑桥商务英语听说星系The Milky Way GalaxyThe Milky Way is the galaxy that contains our Solar System, with the Earth and Sun. This galaxy is a vast, spinning collection of stars, planets, dust and gas, held together by gravity. It is just one of hundreds of billions of galaxies in the observable universe.The Milky Way galaxy is estimated to contain 100-400 billion stars and have a diameter between 100,000 and 180,000 light-years. It is the second-largest galaxy in the Local Group, with the Andromeda Galaxy being larger. As with other spiral galaxies, the Milky Way has a central bulge surrounded by a rotating disk of gas, dust and stars. This disk is approximately 13 billion years old and contains population I and population II stars.The solar system is located about 25,000 to 28,000 light-years from the galactic center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. The stars in the Milky Way appear to form several distinct components including the bulge, the disk, and the halo. These components are made of different types of stars, and differ in their ages and their chemicalabundances.The Milky Way galaxy is part of the Local Group, a group of more than 50 galaxies, including the Andromeda Galaxy and several dwarf galaxies. The Local Group in turn is part of the Virgo Supercluster, a giant structure of thousands of galaxies. The Milky Way and Andromeda Galaxy are moving towards each other and are expected to collide in about 4.5 billion years, although the likelihood of any actual collisions between the stars themselves is negligible.The Milky Way has several major arms that spiral from the galactic bulge, as well as minor spurs. The best known are the Perseus Arm and the Sagittarius Arm. The Sun and its solar system are located between two of these spiral arms, known as the Local Bubble. There are believed to be four major spiral arms, as well as several smaller segments of spiral arms.The nature of the Milky Way's bar and spiral structure is still a matter of active research, with the latest research contradicting the previous theories. The Milky Way may have a prominent central bar structure, and its shape may be best described as a barred spiral galaxy. The disk of the Milky Way has a diameter of about 100,000 light-years. The galactic halo is a spherical component of the galaxy that extends outward from the galactic disk, as far as 200,000 light-years from the galactic center.The disk of the Milky Way Galaxy is marked by the presence of a supermassive black hole known as Sagittarius A*, which is located at the very center of the Galaxy. This black hole has a mass four million times greater than the mass of the Sun. The Milky Way's bar is thought to be about 27,000 light-years long and may be made up of older red stars.The Milky Way is moving with respect to the cosmic microwave background radiation in the direction of the constellation Hydra with a speed of 552 ± 6 km/s. The Milky Way is a spiral galaxy that has undergone major mergers with several smaller galaxies in its distant past. This is evidenced by studies of the stellar halo, which contains globular clusters and streams of stars that were torn from those smaller galaxies.The Milky Way is estimated to contain 100–400 billion stars. Most stars are within the disk and bulge, while the galactic halo is sparsely populated with stars and globular clusters. A 2016 study by the Sloan Digital Sky Survey suggested that the number is likely to be close to the lower end of that estimate, at 100–140 billion stars.The Milky Way has several components: a disk, in which the Sun and its planetary system are located; a central bulge; and a halo of stars, globular clusters, and diffuse gas. The disk is the brightest part of theMilky Way, as seen from Earth. It has a spiral structure with dusty arms. The disk is about 100,000 light-years in diameter and about 13 billion years old. It contains the young and relatively bright population I stars, as well as intermediate-age and old stars of population II.The galactic bulge is a tightly packed group of mostly old stars in the center of the Milky Way. It is estimated to contain tens of billions of stars and has a diameter of about 10,000 light-years. The Milky Way's central bulge is shaped like a box or peanut. The galactic center, which lies within this bulge, is an extremely active region, with intense radio source known as Sagittarius A*, which is likely to be a supermassive black hole.The Milky Way's halo is a spherical component of the galaxy that extends outward from the galactic disk, as far as 200,000 light-years from the galactic center. It is relatively sparse, with only about one star per cubic parsec on average. The halo contains old population II stars, as well as extremely old globular clusters.The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's spiral arms. Different studies have led to different results, and it is unclear whether the Milky Way has two, four, or more spiral arms. The Milky Way's spiral structure is thought to be a major feature of its disk, and it may berelated to the generation of interstellar matter and star formation.The Milky Way's spiral arms are regions of the disk in which the density of stars, interstellar gas, and dust is slightly higher than average. The arms are thought to be density waves that spiral around the galactic center. As material enters an arm, the increased density causes the material to accumulate, thus causing star formation. As the material leaves the arm, star formation decreases.The Milky Way's spiral arms were first identified in the 1950s, when radio astronomers mapped the distribution of gas in the Milky Way and found that it was concentrated in spiral patterns. Since then, astronomers have used a variety of techniques to study the Milky Way's spiral structure, including observations of the distribution of young stars, star-forming regions, and interstellar gas and dust.One of the key challenges in studying the Milky Way's spiral structure is that we are located within the disk of the galaxy, which makes it difficult to get a clear view of the overall structure. Astronomers have had to rely on indirect methods, such as measuring the distances and motions of stars and gas clouds, to infer the shape and structure of the galaxy.Despite these challenges, our understanding of the Milky Way's spiral structure has advanced significantly in recent years, thanks tonew observations and more sophisticated modeling techniques. Ongoing research is continuing to shed light on the nature and evolution of the Milky Way's spiral arms, and the role they play in the overall structure and dynamics of the galaxy.。

The Milky WayMilky Way probably looks likeAndromeda.The band of light we see is really 100 billion starsMilky WayBefore the 1920’s, astronomers used a “__________model” for the galaxyTried to estimate our location in the galaxy by counting stars in different __________Because some stars are _______ by dust, the true shape of this group of stars was unclear.A Globular ClusterFinding the Centerthe Solar System.The Milky WayParts of OurGalaxyDisk: The ____ Resides in theNuclear Bulge: The dense_______ regionHalo: Spherical regionsurrounding the disk where the_______ ________ live.Questions:How big is the Milky Way?Where are stars forming (or not forming)?How much mass is in the Milky Way?What’s going on at the center?so stars are still forming Car Headlights are standard candles:We use them to determine the car’s distanceHenrietta Leavitt Cepheid stars change in brightness. They pulsate in a very regular way. Large, bright Cepheids pulsate_____, while small, dim Cepheids pulsate _______.Milky Way Galaxy, we map out its structureA modern map of the Milky WayMeasuring the Mass of the Milky WayWe use the Sun’s ______around the center of the MilkyWayThe greater the mass insidethe orbit, the ______ the Sunhas move around the center.This way we can measure themass of the Milky Way.Total mass: about ___ _______ MThe Center of the Milky Wayat the center of the galaxy!Chapter 13Galaxies____)M 100NGC 300Less gas and dustAre generally ______ than spirals and ellipticals_______ Galaxies (E): Classified according to shape (E0-E9)_______ GalaxiesA Barred Spiral Galaxy with only 2 arms.Candles••Supernova in galaxy NGC4526 (HST Image)Hubble’s Original DataHubble Law/ Hd = vrClassifying Galaxies Lecture Tutorial: Page 127•Work with a partner or two•Read directions and answer all questions carefully. Take time to understand it now!•Discuss each question and come to a consensus answer you all agree on before moving on to the next question.•If you get stuck, ask another group for help.•If you get really stuck, raise your hand and I will come around.。

a r X i v :a s t r o -p h /0311188v 1 7 N o v 2003Astronomy &Astrophysics manuscript no.h4350February 2,2008(DOI:will be inserted by hand later)The Globular Cluster System of NGC 4374M.G´o mez 1and T.Richtler 21Depto.Astronom´ıa y Astrof´ısica,P.Universidad Cat´o lica de Chile,Casilla 306,Santiago 22,Chilee-mail:mgomez@astro.puc.cl 2Grupo de Astronom´ıa,Departamento de F´ısica,Universidad de Concepci´o n,Casilla 160-C,Concepci´o n,Chile e-mail:tom@coma.cfm.udec.clReceived /AcceptedAbstract.We study the globular cluster system (GCS)of the giant elliptical NGC 4374(M84)in the Virgo cluster using Band R photometry.The colour distribution is bimodal with peaks at B-R =1.11and B-R =1.36,fitting well to those found in other early-type galaxies.The radial profile of the cluster number density is flatter than the galaxy ing the luminosity function we derive a distance modulus of µ=31.61±0.2,which within the uncertainty agrees with the distance from surface brightness fluctuations.Blue and red clusters show similar radial concentrations and azimuthal distributions.The total number of clusters is N =1775±150,which together with our distance modulus leads to a specific frequency of S N =1.6±0.3.This value is surprisingly low for a giant elliptical,but resembles the case of merger remnants like NGC 1316,where the low specific frequency is probably caused by the luminosity contribution of an intermediate-age population.A further common property is the high rate of type Ia supernovae which also may indicate the existence of a younger population.However,unlike in the case of NGC 1316,one cannot find any further evidence that NGC 4374indeed hosts younger populations.The low specific frequency would also fit to a S0galaxy seen face-on.Key words.galaxies:distances and redshifts –galaxies:elliptical and lenticular,cD –galaxies:individual:NGC 4374–galax-ies:interactions –galaxies:star clusters1.IntroductionThe study of globular clusters in early type galaxies has reached a state,where surprises have become rare when only individual galaxies are studied.Progress in understanding the relation be-tween the morphology (i.e.the specific frequency of clusters,their spatial as well as their colour distribution)of a GCS and the host galaxy properties normally emerges from analyzing larger galaxy samples (e.g.Kundu &Whitmore 2001;Larsen et al.2001)However,from time to time,one encounters galax-ies which exhibit some peculiarity in their GCS,which one would like to understand in a more general framework.For example,it is well known that “normal”elliptical galaxies have specific frequencies higher than about 3(see Sect.3.6for the definition and Elmegreen 1999for a review of specific frequencies).NGC 1316,the brightest galaxy in the Fornax cluster,has nevertheless a low specific frequency of only ∼0.9(Grillmair et al.1999;G´o mez et al.2001).Since it is known as a merger remnant,it is tempting to seek an ex-planation not in the low number of globular clusters,but in the high luminosity due to the presence of an intermediate-age stellar population which formed in the merger a few Gyrs ago.Indeed,Goudfrooij et al.(2001),by means of spectroscopy of the brightest clusters,identified several intermediate-ageglob-2M.G´o mez and T.Richtler:The Globular Cluster System of NGC 4374NEFig.1.The observedfields in the three nights.NGC4374is at the center.The bright elliptical at the left is NGC4406(M86). Table1.The details of the observations.See Fig.1for the ori-entation of thefields.nightfield B R2.Observations,reductions and photometry2.1.ObservationsThe observations were carried out at the 3.5m telescope at Calar Alto,Spain,run by the Max-Planck Institute for Astronomy,Heidelberg.The observation period was19to21 March,1999.The instrument was the focal reducer MOSCA (www.caha.es/CAHA/Instruments/MOSCA/index.html) equipped with a Loral2K x2K CCD.The pixel scale was0.513”/pix and the usable unvignettedfield of view∼13′x13′(1.5K x1.5K).Thefilters in use were Johnson B and R.In thefirst night several frames,centered on NGC4374,were acquired,as well as the Landoltfields SA98,SA101and SA107at different airmasses.The observation log is given in Table1.Two additionalfields were also observed during the second and third night,with an offset of∼8′to the north and south, respectively,as shown in Fig.1.Bias subtraction andflat-fielding were performed using standard IRAF procedures,resulting in aflat-field accuracy of about1%.The images were aligned using the centers of many bright stars as reference points.Bad pixels were replaced by the average of4neighboring pixels.After this,we combined typically5frames with exposure times ranging from400to 600seconds in both B and R.Cosmics were removed by aσ-clipping algorithm.The combined frames show a seeing of1.7”in B and1.5”in R.A medianfilter was then applied to the combined frames to model and subtract the galaxy light.For the photometry,we used DAOPHOT under IRAF. Daofind was run on each of these frames with a detection threshold of3times theσof the sky level.We then performed PSF photometry with allstar.The output lists were matched to leave only objects detected in bothfilters.For these objects we used the stellarity index of SExtractor (Bertin&Arnouts1996)for distinguishing star-like objects from galaxies.The stellarity index ranges between1.0(star-like)and0.0(extended).This classification,however,becomes progressively more uncertain for fainter objects.As we do not resolve globular clusters at the Virgo distance, no light loss due to a different shape of clusters and stars is ob-servable.An aperture correction was applied in the usual way to calibrate our PSF-photometry with that of the standard stars, which were measured using a much larger aperture.2.2.The calibration of the photometryOnly thefirst of3nights was photometric.About40Landolt (1992)standard stars were observed at airmasses from1.0to 1.8,giving a total of∼100datapoints.Their colour range ex-ceeds that of the globular clusters.We then performed aperture photometry with radii from4to30pixels.A curve-of-growth was constructed for each standard,and only stars whose instru-mental magnitudes converged with increasing aperture radius were selected for the calibration.The resulting transformation equations are:b=B+0.874(±0.026)+0.223(±0.021)·X−0.176(±0.021)·(B−R)(1) r=R+0.027(±0.022)+0.096(±0.016)·X+0.046(±0.020)·(B−R)(2) with b,r,B,R the instrumental and standard magnitudes in the B and Rfilter.X is the airmass.The rms of thefit was0.029 and0.027for B and R,respectively.The calibration has been compared with photoelectric aper-ture photometry of NGC4374published by Poulain(1988),us-ingfive aperture sizes.The mean difference was∼0.02mag for each band,without any systematic trend.Local standard stars were then defined in thefield to calibrate the non-photometric nights.We also compared the R magnitudes of the cluster can-didates with about100clusters in common with Ajhar et al. (1994).The mean difference is0.006,with aσof0.085.No trend with magnitude is apparent(Fig.2).Regarding colours, no straightforward comparison is possible.However,we can plot V-I colours of Ajhar et al.versus our B-R colours and ask whether this relation matches the(B-R)-(V-I)relation for galac-tic globular clusters.This is done in Fig.3.The agreement isM.G´o mez and T.Richtler:The Globular Cluster System of NGC 43743202122R−0.4−0.20.20.4∆Rparison of common objects in the photometry of Ajhar et al.(1994)and the present work in the R-band.∆R means the magnitudes quoted by Ajhar et al.minus ours.The mean di fference is 0.006mag with a σof 0.085mag.0.40.60.81 1.2 1.4(V−I)00.811.21.41.61.8(B −R )0Fig.3.Colour comparison of common objects in the VRI pho-tometry of Ajhar et al.(1994)and our BR photometry (filled symbols).We chose the V-I colour because of its widespread use in the photometry of globular cluster systems.Overplotted are galactic globular clusters,where the dereddened colours are taken from the compilation of Harris (1996).There is no obvi-ous o ffset between the clusters of NGC 4374and galactic glob-ular clusters,demonstrating the quality of our B-R colours.quite satisfactory.A few outliers may be caused by photometric errors.2.3.Selection of cluster candidates886objects have simultaneously been detected in the B and R bands.However,a fraction of them might be foreground stars and background galaxies.To statistically select globular clus-ters among them,we have used the following set of criteria:i)B >20.5ii)0.7<B −R <1.8iii)uncertainty(B),uncertainty(B-R)<0.3iv)stellarity index >0.35These criteria arise from the assumption that clus-ters around NGC 4374should resemble the Galactic ones.Regarding the color range,we have used the McMaster data (Harris 1996)and chosen galactic clusters with reddening smaller than E B −V =1.0,resulting in the above colour limits.The colours were de-reddened and the magnitudes extinction-corrected using the maps of Schlegel et al.(1998).We adopt E B −V =0.04,A B =0.173and A R =0.107.In the following,only reddening corrected values appear.pleteness correctionFor the later determination of the globular cluster luminosity function (GCLF)we need to know what fraction of globular clusters is found in each magnitude interval.A common way to evaluate the completeness is to perform artificial star ex-periments.Many artificial “star-like”objects are added to the frames and the detection,photometry and selection procedures are performed in exactly the same way as with the real cluster candidates.The number of recovered and selected objects per magnitude bin,divided by the initial number of artifical clusters gives an estimation of the completeness correction.To get su fficiently good statistics,one needs a large number of artificial objects,but care must be taken not to increase the crowding.Therefore,consecutive tests with a small number of artificial objects are preferred.Using the known PSFs,we added 200artificial clusters in steps of 0.1mag,starting from B =20.0down to B =26.0,and applied the same photometric treatment and selection criteria as we did for the cluster candidates.We expect the completeness to vary with galactocentric distance because of the radially dependent galaxy light back-ground.However,a decrease in the completeness was only noticed for clusters located between radii of 50and 150pix-els (25.′′65and 76.′′95).At larger radii,the completeness cor-rections stays constant with increasing distance.Fig.4(upper panel)gives the resulting completeness in dependence on B-magnitude for a colour of B-R =1.2.We estimated the dependence of the completeness on colour using clusters of colours B-R =0.9,B-R =1.2and B-R =1.5.In all,140000clusters were added for each colour.Fig.4(lower panel)shows that the completeness for red clus-ters is about 0.3mag fainter than for blue ones.This di fference must be taken into account for the calculation of the radial pro-file of red and blue cluster candidates (Sect.3.3).2.5.Background correctionDespite the applied selection criteria,there remain point sources,which may be foreground stars or unresolved back-ground galaxies.The surface density profile levels out at a ra-dial distance of about 500′′(see Fig.11).We therefore have used the outermost parts of the two fields observed during the second and third nights (see Fig.1).That is,two rectangular ar-eas of 12.′8x 4.′3at the top of the upper field and at the bottom4M.G´o mez and T.Richtler:The Globular Cluster System of NGC 437420222426B0.20.40.60.81c o m p l e t e n e s s20222426B0.20.40.60.81c o m p l e t e n e s sFig.4.Top :The completeness for three annuli centred on NGC 4374,as function of the B magnitude,for clusters hav-ing B-R =1.2.The annuli are described by the following inner and outer radii (in pixels):50<r <150,150<r <300and r >300.Only the inner annulus has a noticeable lower completeness.Bottom :The mean completeness (averaged over the three an-nuli),for clusters having colours of B-R =0.9,1.2and 1.5.These are represented by the three solid lines from the left to the right.The shift with colour is evident.The two dashed lines indicate the magnitude at which the mean completeness falls to 0.5for blue (left)and red (right)clusters.The di fference is ∼0.3mag.of the lower field are our background fields.The photometry for these fields was done in the same way as for the central field and it was calibrated using the local standards defined in the overlapping regions.Completeness tests were also run on these two fields.The two background populations were averaged concerning colour distributions,luminosity function and surface density.3.ResultsWe first present the colour-magnitude diagram and the colour distribution of those cluster candidates identified simultane-ously in B and R.After that we study the morphological prop-erties of blue and red candidates separately.In these subsec-0.511.5(B−R)020212223B 020212223Bbackground populationFig.5.The B-R colour-magnitude diagram of cluster candi-dates around NGC 4374(top)and for the background popu-lation (bottom).tions we consider only sources brighter than B =23.8,where the mean completeness is ∼60%(see Fig.4).For deriving the GCLF (Sect.3.5)and the specific fre-quency (S N ),we used only the R frame.The reason for doing so is that we reach more than half a magnitude deeper using the R frames alone,which might be crucial for the detection of the turn-over magnitude (hereafter TOM).We keep the crite-ria regarding magnitude range,photometric error and stellarity index.Completeness and background corrections for the R frames have been performed in the same way as for the combined sam-ple.3.1.The colour-magnitude diagramThe B-R colour-magnitude diagram is shown in Fig.5.In the upper panel,cluster candidates brighter than B =23.8and hav-ing a radial distance less than 500′′are plotted.The lower panel shows the same diagram for the background population in an area which is smaller by a factor 1.3than that for the upper panel.It is apparent that objects bluer than B-R =0.9are mainly foreground /background objects.This is in good agreement with the fact that B-R =0.9is also a limit for the galactic clusters,as can be seen from Fig.3.3.2.Colour distributionThe colour histogram is given in Fig.6.The upper panel shows the colour distribution of all sources having a radial distance less than 500′′(bold line).The dotted line is the backgroundM.G´o mez and T.Richtler:The Globular Cluster System of NGC 43745normalised to the same area.The lower panel shows the colourdistribution for the background corrected sample (solid line).For comparison the B-R distribution of the galactic clusters is shown as well (dashed line),showing those clusters,for which B-R photometry is available and which havereddenings less than 1.0(81clusters from the compilation of Harris 1996).The little peak at B-R =0.85can be a residual from an inappropri-ate background subtraction.Without having further clues,we do not regard these objects to be globular clusters.The galac-tic distribution has the peak at B-R =1.1,qualitatively agree-ing with the blue peak of the NGC 4374clusters.Although the comparison in the red regime is made di fficult by the fact that many metal-rich galactic clusters with higher reddenings are omitted,it seems that the distribution of NGC 4374clusters is more extended to the red and hence presumably to more metal-rich clusters.The appearance of the histogram does not depend much on the bin center which is demonstrated by the dotted curve.This is the sum of the contributions from all clusters,where each one is represented by a Gaussian at the correspond-ing colour.The only parameter is the σof the Gaussian which we choose to be 0.05to match the bin size.There are not many B-R photometries of GCSs available,so a comparison only with 3galaxies,NGC 1380(Kissler-Patig et al.1997),NGC 1199and NGC 6868(da Rocha et al.2002)is principally possible.Unfortunately,in all three cases the colour distribution was not background corrected,which may hamper the comparison.However,the blue peak in these colour distri-bution was found at B-R =1.1in all cases,resembling the galac-tic system and,as we will see,that of NGC 4374.Ajhar et al.(1994)and Gebhardt &Kissler-Patig (1999)re-port V-I photometry for GCs in NGC 4374.Ajhar et al.pointed out a strikingly narrow V-I colour distribution in comparison with other Virgo and Leo ellipticals,but again a more detailed comparison is made di fficult by their small number statistics (moreover,no background subtraction has been attempted).Gebhardt &Kissler-Patig analyse the first 4moments in the V-I colour distribution of a sample of 50GCSs.NGC 4374shows one of the highest skewness parameters in their sample,i.e.its colour distribution is strongly weighted to the red.The general appearance of the colour distribution looks bi-modal.A bimodal colour distribution has been found for many GCSs (e.g.Ashman &Zepf 1992;Larsen et al.2001;Kundu &Whitmore 2001),often interpreted as a signature of two episodes of cluster formation,sometimes in the context of a merger scenario.However,see Dirsch et al.(2003)for the ef-fects of a non-linear colour-metallicity relation.Following a common statistical approach,we performed a KMM-test (Ashman,Bird &Zepf 1994)on the colour distri-bution.This test is applied to the distribution uncorrected for the background population,in both homoscedastic and het-eroscedastic versions.It returns that the colour distribution is best represented by two Gaussians with positions at B-R =1.11(the blue peak)and B-R =1.36(the red peak),and with σ-values of 0.08and 0.13,respectively (heteroscedastic mode).In the homoscedastic case,the peaks are at 1.14and 1.42with σ=0.10.The P-value,which gives the probability of hav-ing a unimodal distribution,is practically zero in both ver-sions,i.e.a unimodal distribution is excluded.Fig.7shows the0.51 1.5(B−R)0204060N 0204060N Fig.6.Top :the B-R colour distribution of cluster candidates around NGC 4374.The bin size is 0.05mag.The thick solid line represents the colours before subtraction of the background population,whose distribution is shown with the dotted line.Bottom :the B-R colour histogram after subtraction of the background population,together with a so-called “generalised histogram”(solid line).This is constructed by placing a gaus-sian in the abscissa at each B-R colour,and then adding the contribution from all gaussians together.The latter is indepen-dent of the bin center,and in our case agrees with the “nor-mal”histogram.The dashed line shows the comparison with the galactic clusters.colour distribution together with the returned Gaussians in the heteroscedastic fit.In all GCSs investigated so far,the peaks exhibit approximately constant colours.Weak dependence on galaxy luminosity has been found for the blue and red peaks by Larsen et al.(2001),while Forbes &Forte (2001)see only a dependence in case of the read peak.As mentioned,also NGC 4374fits to this notion compared with GCSs,for which B-R photometry is available.However,most work has been done in V-I (rsen et al.2001;Kundu &Whitmore 2001)where the peaks are located at V-I =0.95and V-I =1.15.As can be seen from Fig.3,these values fit well to the B-R peak colours quoted above.Is there a colour gradient?Fig.8plots the B-R colour ver-sus the projected distance from the galaxy centre (in logarith-mic scale).The scatter is large,but a slight trend that the clus-ters become bluer at larger radii,may be recognizable.6M.G´o mez and T.Richtler:The Globular Cluster System of NGC 437420406080N 0.51 1.5(B−R)Fig.7.This plot shows the colour distribution of the GCS of NGC 4374(not background corrected)together with the two Gaussians,which best represent the distribution in the sense of a KMM-test.Only clusters redder than 0.925have been in-cluded in the test.110radius (arcmin)0.511.5(B −R )00.511.5(B −R )Fig.8.The colour gradient for cluster candidates brighter thanB =23.8.The upper panel shows the B-R colour of individual clusters vs.the projected distance to the optical center.The B-R colour of the galaxy is shown for comparison (triangles).In the lower panel the mean color in concentric anuli (each 51.′′3wide)is plotted.The error bars are the σin the colour for each bin.The solid lines are the least-square fits.3.3.Blue and Red clustersWe divided the clusters in two subpopulations at B-R =1.25in order to search for possible morphological di fferences between the blue and the red clusters.The resulting counts are given in Table 2,and the radial pro-files of red and blue clusters are shown in Fig.9after the sub-traction of the corresponding red and blue background levels.Red and blue clusters do not significantly di ffer in their radial distributions.They are well represented by:ρ(r )red ∝r −1.22±0.12and ρ(r )blue ∝r −1.00±0.15.100radius (arcsec)110G C /a r e a (a r c m i n−2)500Fig.9.Radial profile of red (open circles)and blue clusters (filled circles),for objects brighter than B =23.8.They are not clearly distinguishable in their concentration.The solid lines are least-square fits.Table 2.The counts for the radial profile of blue and red sub-samples,using both B and R frames.The counts have been corrected for completeness and background contamination.r [′′]area [⊓⊔′]N blue N redTo investigate possible di fferences regarding the azimuthal distribution,we counted the cluster candidates in 12equally-sized sectors,each spanning an angle of 30degrees.The re-gion inside the radius r =51.′′3(or,equivalently,∼5kpc)was left out as the completeness is significantly lower.Clusters out-side r =350′′were not considered because the sectors become geometrically incomplete.The results (Fig.10)show that,on average,red and blue clusters are spherically distributed around NGC 4374.The galaxy’s position angle of the major axis is in-dicated by the arrows at 135and 315degrees.There are some cases where blue and red clusters show sig-nificant di fferences in their azimuthal distributions,for example NGC 1316(G´o mez &Richtler 2001),NGC 3115(Kavelaars 1998),and NGC 1380(Kissler-Patig et al.1997).In these galaxies,the red clusters resemble the shape of their host galax-ies,while the blue clusters are more spherical.The interpreta-tion as di fferences between halo and bulge /disk populations is suggestive.The elongations of the above galaxies,if caused by inclined disks,are strong enough to make a disk population of clusters easily distinguishable from a spherical halo popula-tion.This is not the case for NGC 4374with its low ellipticity.M.G´o mez and T.Richtler:The Globular Cluster System of NGC 4374790180270360PA (deg)0102030405060N Fig.10.The azimuthal distribution of the clusters.The posi-tion angle is counted north over east.The histogram at the top (solid line)represents the number of cluster candidates on each sector.The binning angle is 30degrees.The dashed and dot-ted lines are the angular distribution of blue and red clusters,respectively.No systematic di fference exists and both red and blue clusters show a circular distribution.The galaxy’s posi-tion angle of the major axis is indicated by the arrows at 135and 315degrees.Clearly,we cannot draw conclusions about halo and bulge /disk cluster subpopulations from their azimuthal distribution.3.4.Radial ProfileWe calculated the cluster radial profile by counting the clus-ter candidates in several annuli centered on NGC 4374,each 25.′′65wide,starting from a radius of r =89.′′8up to r =346.′′4,and 102.′′6wide thereafter.The innermost ring (r =77.′′0)was given a width of 51.′′3because of the low statistics.The results are listed in Table 3.The bin center and size are the first two columns.The area of the annuli is given in the third column.The fourth column shows the number of candidates up to the 60%detection limit,before correcting for the completeness.The corrected number of clusters and its density are given in columns five and six.The remaining columns are used in the derivation of the specific frequency S N (see Sect.3.6).The radial profile in R is shown in Fig.11.The upper panel plots the GC number surface density and it is apparent that the cluster population extends at least out to r =350”.At larger radii the background dominates the profile.The background level is well determined and its value of 8.36±0.60arcmin −2has been subtracted from the tabulated densities,to derive the true radial profile,which is shown in the lower panel.The galaxy light in the R-band,arbitrarily shifted,is also plotted.The solid lines are:ρ(r )gcs ∝r −1.09±0.12and ρ(r )gal ∝r −1.67±0.02,with ρ(r )gcs being the cluster surface den-sity (in units of arcmin −2)and ρ(r )gal the surface brightness of the galaxy light.It is also apparent that the radial profile of the cluster den-sity around NGC 4374is flatter than the galaxy light.100radius (arcsec)110100G C /a r e a (a r c m i n −2)0510********G C /a r e a (a r c m i n −2)Fig.11.The radial profile of the clusters’surface density for candidates brighter than R =23.5.Top :the number of candi-dates per square arcmin vs.the mean projected distance to the optical center.Bottom :the radial profile,after the correction for background objects.The galaxy light,arbitrarily shifted (crosses)and the least-squares fits (solid lines)are also plot-ted.3.5.The Luminosity FunctionThe GCLF has been widely used as a distance indicator (seeHarris 2001and Richtler 2003for recent reviews).GCLFs of a broad range of galaxy types are well characterised by log-normal or t 5functions,the latter being of the form:t 5(m )=85πσt1+(m −m 0)28M.G´o mez and T.Richtler:The Globular Cluster System of NGC4374Table3.This table shows the result of the radial profile for the surface density of cluster candidates.Thefirst three columns list the center of each annulus(in arcsecs),its size and its area,in square arcmin.The observed number of cluster candidates brighter than R=23.5on the corresponding annuli is given in column4.After correcting for completeness,the actual number is shown in column5.Column6lists this number divided by the area of the annulus,or the surface density of clusters.The next three columns are used for the derivation of the specific frequency S N.They are the number of candidates up to the observed TOM before and after the completeness correction(columns7and8),and the total number of clusters in each annulus,assuming the LF to be symmetric around the TOM(column9).The background level has been subtracted for these latter counts.For radii larger than333.′′5,the annuli contain only background sources,so these four last rows are left blank.r[′′]size[′′]area[⊓⊔′]N(R)obs N(R)corr GC/area[1/⊓⊔′]N(R)TOMN(R)TOM,corrN ringTable4.The counts for the GCLF in the R band.Thefirst column is the bin center.The second column gives the observed number of candidates.The completeness factors(in column3)lead to the completeness-corrected number of candidates in column4.The observed background population and background completeness are listed in columns6and7.The completeness correction for the backgroundfield is slightly smaller than for the centralfield due to better seeing.Finally,after subtraction of the background population(scaled to the area of the clusters),the total number of clusters is given in column8.R N obs ffield N corr N bkgobsf bkg N bkg corr N terror bars account for poisson error in the binning process and for the completeness correction.According to thisfit,the TOM in the R band is23.56±0.15.Leavingσt a free parameter,we get for the TOM(R)= 23.84±0.35andσt=1.22±0.11.We take the absolute TOM(R) from Della Valle et al.(1998)who quote M R=−8.14±0.07for the galactic system.Another absolute calibration comes from the Andromeda nebula.Barmby et al.(2001)give R=16.40±0.14for the apparent TOM of M31.Together with a distance modulus ofµ=24.38±0.07(Freedman et al.2001), one gets M R=−7.98±0.15.We average these two values and adopt M R=−8.06±0.15.This leads to distance moduli of31.61±0.20(keepingσt= 1.1)and31.90±0.38(leavingσt free)for NGC4374.In the following,we consider only the TOM withσt=1.1.。

星河的英文带翻译The Milky Way: Our Home in the Universe。

The Milky Way is a barred spiral galaxy that contains our solar system and is home to billions of stars, planets, and other celestial objects. It is one of the most studied galaxies in the universe and has captivated the imaginations of astronomers, scientists, and stargazers for centuries.Structure and Composition。

The Milky Way has a diameter of about 100,000 light-years and is composed of a central bulge, a disk, and a halo. The central bulge is a dense, spherical region that contains mostly old stars and a supermassive black hole at its center. The disk is a flattened region that contains most of the galaxy's stars, gas, and dust, and is where most star formation occurs. The halo is a roughly spherical region that surrounds the disk and contains mostly oldstars and globular clusters.The Milky Way is made up of various types of celestial objects, including stars, planets, gas clouds, and dust. It is estimated to contain between 100 billion and 400 billion stars, including our own sun. The galaxy also contains a significant amount of dark matter, which is a mysterious substance that cannot be directly observed but is thought to make up about 85% of the galaxy's total mass.Observing the Milky Way。

星系介绍英文作文1. The Milky Way。

The Milky Way is a barred spiral galaxy that contains our solar system. It is estimated to have between 100 and 400 billion stars, and spans a diameter of approximately 100,000 light-years. The center of the Milky Way is home to a supermassive black hole, which has a mass of approximately 4 million times that of our sun. The Milky Way is also home to numerous other celestial objects, including gas clouds, star clusters, and nebulae.2. Andromeda。

Andromeda, also known as M31, is a spiral galaxy that is located approximately 2.5 million light-years away from the Milky Way. It is the closest galaxy to our own and is visible to the naked eye in the night sky. Andromeda is estimated to have between 1 and 2 trillion stars, making it the most massive galaxy in our local group. It also has asupermassive black hole at its center, which has a mass of approximately 140 million times that of our sun.3. The Large Magellanic Cloud。

有关银河的英文文章The Milky Way, often referred to as the Galaxy, is a vast and magnificent spiral of stars, dust, gas, and other celestial bodies that we call home. It is named for its appearance in the night sky as a hazy, milky band of light that stretches across the heavens. This ethereal glow is actually the combined light of billions of stars that are too far away to be seen individually. The Milky Way is not just a beautiful sight to behold; it is also a complex and fascinating system that has captivated the minds of astronomers and scientists for centuries.The Milky Way is a barred spiral galaxy, meaning it has a central bar-shaped region with spiral arms extending outward from it. It is enormous, containing an estimated 200 billion stars and spanning a diameter of approximately 100,000 light-years. Our own Sun is just one of these stars, located on the inner edge of one of the spiral arms, about 26,000 light-years from the Galactic Center.One of the most intriguing aspects of the Milky Way is its structure. The galaxy is composed of three main components: the disk, which contains the stars, gas, and dust; the halo, a spherical region that extends beyond the disk and is populated by older stars and globular clusters; and the central bulge, a dense region at the heart of the galaxy that contains mostly older stars.The disk of the Milky Way is where most of the action takes place. It is made up of stars, gas, and dust that are organized into spiral arms. These arms are not solid structures, but rather regions of higher density that are separated by gaps. The arms are home to star-forming regions, where clouds of gas and dust collapse under their own gravity to form new stars. The M ilky Way’s spiral structure is thought to be caused by gravitational interactions between the stars and gas in the disk, as wellas the influence of the central black hole.The halo of the Milky Way is a spherical region that surrounds the disk and extends outward for hundreds of thousands of light-years. It is populated by older stars that are metal-poor and have orbits that take them far away from the plane of the disk. The halo also contains globular clusters, which are tightly packed groups of thousands to millions of stars that orbit the center of the galaxy.At the heart of the Milky Way lies the central bulge, a dense region that is packed with stars. This region is thought to be the site of intense star formation in the early history of the galaxy. It is also home to a supermassive black hole known as Sagittarius A*, which has a mass equivalent to millions of Suns. This black hole exerts a powerful gravitational influence on the surrounding stars and gas, shaping the structure of the galaxy.Studying the Milky Way has been a challenging task for astronomers due to our position within it. We cannot see the galaxy as a whole, as we are embedded within its disk. However, advances in technology and observation techniques have allowed us to piece together a comprehensive picture of our galactic home. We have mapped its structure using radio waves, X-rays, and visible light, revealing the locations of stars, gas, dust, and other components.The Milky Way is not static; it is constantly evolving. New stars are being born in star-forming regions, while older stars are dying and expelling their outer layers into space. The galaxy is also growing through the accretion of smaller galaxies and star clusters. In fact, our own Milky Way is destined to merge with our nearest neighbor, the Andromeda Galaxy, in several billion years.Despite our advances in understanding the Milky Way, there are still many mysteries surrounding it. We do not fully understand how spiral galaxies like our own form and evolve. We also know little about the nature of dark matter, which is thought to make up a significant portion of the mass of the galaxy but has never been directly detected.In conclusion, the Milky Way is more than just a pretty sight in the night sky; it is our home, a vast and complex system that contains billions of stars and countless other celestial bodies. It has captivated the imaginations of people throughout history and continues to inspire awe and wonder in those who gaze upon it. As we continue to explore and study our galactic home, we will undoubtedly uncover more secrets and mysteries that lie hidden within its depths.。

a r X i v :a s t r o -p h /0703385v 1 15 M a r 2007Astronomy &Astrophysics manuscript no.rejkuba cESO 2008February 5,2008Bright globular clusters in NGC 5128:the missing link betweenyoung massive clusters and evolved massive objects ⋆M.Rejkuba 1,P.Dubath 2,3,D.Minniti 4,and G.Meylan 51ESO,Karl-Schwarzschild-Strasse 2,D-85748Garching,Germany e-mail:mrejkuba@2Observatoire de Geneve,ch.des Maillettes 51,CH-1290Sauverny,Switzerland 3INTEGRAL Science Data Centre,ch.d’Ecogia 16,CH-1290Versoix,Switzerland e-mail:pierre.dubath@obs.unige.ch4Department of Astronomy and Astrophysics,Pontificia Universidad Cat´o lica de Chile,Vicu˜n a Mackenna 4860,Santiago 22,Chile e-mail:dante@astro.puc.cl 5Ecole Polytechnique F´e d´e rale de Lausanne (EPFL),Observatoire,CH-1290Sauverny,Switzerland e-mail:georges.meylan@epfl.chReceived 01October 2006;accepted 13March 2007ABSTRACTContext.Globular clusters are the simplest stellar systems in which structural parameters are found to correlate with their masses and lumi-nosities.Aims.In order to investigate whether the brightest globular clusters in the giant elliptical galaxies are similar to the less luminous globular clusters like those found in Local Group galaxies,we study the velocity dispersion and structural parameter correlations of a sample of bright globular clusters in the nearest giant elliptical galaxy NGC 5128(Centaurus A).Methods.UVES echelle spectrograph on the ESO Very Large Telescope (VLT)was used to obtain high resolution spectra of 23bright globular clusters in NGC 5128,and 10clusters were observed with EMMI in echelle mode with the ESO New Technology Telescope.The two datasets have 5clusters in common,while one cluster observed with UVES had too low signal-to-noise (S /N).Hence the total number of clusters anal-ysed in this work is 27,more than doubling the previously known sample.Their spectra were cross-correlated with template spectra to measure the central velocity dispersion for each target.The structural parameters were either taken from the existing literature,or in cases where this was not available,we have derived them from our VLT FORS1images taken under excellent seeing conditions,using the ISHAPE software.The velocity dispersion and structural parameter measurements were used to obtain masses and mass-to-luminosity ratios (M /L V )of 22clusters.Results.The masses of the clusters in our sample range from M vir =105−107M ⊙and the average M /L V is 3±1.The three globular clusters harbouring X-ray point sources are the second,third and sixth most massive in our sample.The most massive cluster,HCH99-18,is also the brightest and the largest in size.It has the mass (M vir =1.4×107M ⊙)an order of magnitude larger than the most massive clusters in the Local Group,and a high M /L V ratio (4.7±1.2).We discuss briefly possible formation scenarios for this object.Conclusions.The correlations of structural parameters,velocity dispersion,masses and M /L V for the bright globular clusters in NGC 5128extend the properties established for the most massive Local Group clusters towards those characteristic of dwarf elliptical galaxy nuclei and Ultra Compact Dwarfs (UCDs).The detection of the mass–radius and the mass–M /L V relations for the globular clusters with masses greater than ∼2×106M ⊙provides the missing link between “normal”old globular clusters,young massive clusters,and evolved objects like UCDs.Key words.Galaxies:elliptical and lenticular,cD –Galaxies:Individual:NGC 5128–Galaxies:star clusters1.IntroductionThe properties of globular clusters and the observed corre-lations between their various internal structural and dynami-cal parameters o ffer empirical constraints not only on the for-mation of globular clusters themselves,but also on the his-2M.Rejkuba et al.:Bright globular clusters in NGC5128cal relations:M/L=const.and E b∼L2.05,where M is the mass, L the luminosity,and E b the binding energy of the cluster.The brightest and the most massive globular cluster of our Galaxy,ωCen(Meylan et al.1995),is peculiar in many of its characteristics:e.g.it is the mostflattened Galactic globular cluster(White&Shawl1987)and it shows strong variations in nearly all element abundances(e.g.Norris&Da Costa1995; Pancino et al.2002).A scenario that may explain some of its characteristics is thatωCen is the nucleus of a former dwarf elliptical galaxy(Zinnecker et al.1988;Hughes&Wallerstein 2000;Hilker&Richtler2000).The same scenario was pro-posed for M54,another very massive globular cluster.It is a candidate for the nucleus of the Sagittarius dwarf(e.g. Bassino&Muzzio1995;Layden&Sarajedini2000),a galaxy that is currently being accreted by the Milky Way.M31has 4globular clusters for which Djorgovski et al.(1997)mea-sured velocity dispersionsσ>20km s−1implying masses at least as large as the one ofωCen.The most massive of them,G1,shares also other particular properties ofωCen,like theflattening and metallicity dispersion(Meylan et al.2001). From the recent work of Ma et al.(2006),the most luminous M31globular cluster,037-B327,has been suggested to be the most massive Local Group cluster,with a total mass of (3.0±0.5)×107M⊙,determined photometrically.These au-thors estimate the(one-dimensional)velocity dispersion for 037-B327of(72±13km s−1).However,a later paper by Cohen (2006)challenged this result,based on the measured velocity dispersion ofσ=21.3±0.4km s−1,which is comparable to that of G1(σ=25.1±0.3km s−1;Djorgovski et al.1997). She concluded that037-B327is not the most massive cluster in the Local Group,and that probably M31clusters G1,G78 and G280are more massive than037-B327.Going to galax-ies beyond the Local Group,very similar to G1in M31is the cluster n1023-13in NGC1023(Larsen2001).With luminosities and masses larger than globular clus-ters are the so-called ultra compact dwarfs(UCDs)or dwarf-globular transition objects(DGTOs)discovered in Fornax and Virgo galaxy clusters(Hilker et al.1999;Drinkwater et al. 2000;Has¸egan et al.2005;Hilker et al.2007).While their ori-gin and relation to globular clusters is still debated in the literature,it has been established that very massive young star clusters can form in major star forming events.Such clusters,with masses of the order of>106,or even> 107M⊙(Maraston et al.2004;Bastian et al.2006),show sim-ilar scaling relations as UCDs/DGTOs,but might be differ-ent from the less massive globular clusters based on examina-tion of their mass-velocity dispersion and mass-radius relations (Kissler-Patig et al.2006).However,we point out that the ages of UCDs/DGTOs are similar to those of globular clusters,in contrast to the massive young star clusters forming in mergers.To populate the transition region between“normal”globu-lar clusters and more massive DGTOs,it is of interest to look at the massive elliptical galaxies which harbour globular cluster systems which are an order of magnitude more populous than those of the Local Group spiral galaxies.The nearest easily ob-servable elliptical galaxy is NGC5128.It has a large number of bright globular clusters with luminosities exceeding the bright-est Local Group globulars.This makes it an ideal target.The most recent distance determination to this galaxy is 3.42±0.18±0.25Mpc(thefirst is random and the sec-ond systematic error),obtained using Cepheid PL relation (Ferrarese et al.2006).Here we use the distance of3.84±0.35Mpc(Rejkuba2004),which is the same value used in a previous work by Martini&Ho(2004)who presented veloc-ity dispersions and mass-to-light ratios for14bright globular clusters in NGC5128.In this work we present new high reso-lution spectra and derive M/L ratios,thus more than doubling the sample of bright globular clusters with similar data in the literature.A decade ago Dubath(1994)presented at a conference the first measurements of velocity dispersions of10bright globular clusters in NGC5128.Since these results have not been pub-lished in a refereed journal yet,they are included here along with the more recent observations of23clusters from UVES high-resolution echelle spectrograph of ESO Kueyen(UT2) telescope of Very Large Telescope(VLT).This paper is organized as follows:Sect.2describes the observations and data reduction,Sect.3.1shows the results of the cross-correlation technique for radial velocity and metallic-ity standard stars,while Sect.3.2presents the results from the radial velocity and core velocity dispersion measurements of globular clusters in NGC5128.The comparison with previous measurements of clusters’radial velocity and velocity disper-sion is in Sect.3.2.1.In Sect.4the structural parameters for22 clusters are presented.For those clusters which had no previ-ous determinations of structural parameters in the literature we derive them from our high resolution ground-based imagesfit-ting the King profile(King1962)using ISHAPE(Larsen1999, 2001)programme.In Sect.5we derive mass-to-luminosity ra-tios for the clusters and in Sect.7discuss the correlations and fundamental plane.Finally,in Sect.8we summarize our re-sults.2.Sample selection and observationsThe observations of10bright clusters(selected from the lists of van den Bergh et al.1981;Hesser et al.1986;Harris et al. 1992)were taken in March-April1993with the echelle mode of EMMI(Dekker et al.1986),the multi-mode instrument of the ESO New Technology Telescope(NTT).These data have previously been presented at a conference(Dubath1994),and are published here together with the observations of23clusters obtained with UVES echelle spectrograph(Dekker et al.2000) of the ESO VLT in April2002.There are5clusters observed with both instruments and these have been used to check for the systematics in the data and errors.The sample of globular clusters selected for observations with UVES contains the brightest NGC5128clusters with ei-ther membership confirmed through published radial veloci-ties(van den Bergh et al.1981;Hesser et al.1986;Harris et al. 1992,cluster names starting with VHH81,HHH86,and HGHH92)or the structural parameters and colours typical for globular clusters in the Milky Way(Holland et al.1999; Rejkuba2001,cluster names starting with HCH99and with R,respectively).M.Rejkuba et al.:Bright globular clusters in NGC512832.1.EMMI spectroscopyThefirst high-resolution integrated-light spectra of bright glob-ular clusters in NGC5128were obtained with EMMI at ESO NTT telescope during three nights,March31to April21993. The red arm of EMMI was used in Echelle mode(REMD)with grating#10and grism#3(CD2),yielding the resolving power of30,000,corresponding to10km s−1,and the wavelength cov-erage was from4500to9000Å,divided among65useful or-ders.In total14spectra of10of the brightest globular clusters, selected from the catalogues of van den Bergh et al.(1981), Hesser et al.(1984),and Harris et al.(1992),were secured. The ThAr calibration lamp spectra were taken before and af-ter each cluster spectrum.In addition,the following four K gi-ant radial velocity standard stars were observed on each of the three nights:NGC2447-s28,NGC2447-s4,HD171391and HD176047.All the spectra were reduced with the INTER-TACOS software developed by Queloz&Weber in Geneva Observatory(see e.g.Queloz et al.1995).2.2.UVES spectroscopyThe UVES observations were carried out on the nights of19 and20April2002in visitor mode.The red arm of UVES spec-trograph was used with the standard CD#3setting centered on 580nm.It is equipped with two CCDs,covering the total wave-length range from4760Åto6840Å,with a gap of50Åcen-tered on5800Å.The slit was1”wide,giving the resolution ofλ/∆λ∼42,000.The sky conditions were clear and the see-ing varied between0.′′6to1.′′3,but it stayed most of the time around0.′′8.Globular clusters observed with UVES have V-band mag-nitudes ranging from17.1to18.8for22clusters.The faintest observed cluster had V=19.44.The typical exposure times were1200sec for the brighter or1800sec for the fainter clus-ters,except for the faintest19.4mag cluster which was exposed for2700sec.Four clusters have been observed twice and one cluster three times during the two night run.The multiple expo-sures have been averaged to increase the signal-to-noise(S/N), but were also reduced independently in order to provide esti-mates of measurement errors.The observation log for all the clusters is in Table1,where we list(1)the name,(2)the obser-vation date,(3)the exposure times in seconds,(4)the typical S/N measured on the blue side of the Hαline at∼6550Åus-ing the splot IRAF task.The last column lists the V magnitude of the clusters taken from the literature.Apart from the globular cluster targets,we have observed 17different G and K-type giant stars with a range of metallic-ities(−2.6<[Fe/H]<+0.3dex)to be used as templates for cross-correlation.Some stars were observed several times,thus yielding a total of28high S/N stellar spectra.The observation log of the template stars is in Table2.After each target spectrum,globular cluster or star,we have obtained the ThAr lamp spectrum at the same telescope posi-tion.The bias andflat-field calibration data were taken at the end of each night.Table1.Observations log for globular clusters:observations in 1993were done with EMMI at NTT and in2002with UVES at Kueyen VLT.The nomenclature of the clusters is following that of the Peng et al.(2004a)catalogue,and the magnitudes given in the last column are from the same catalogue where available. For the clusters for which there are no measurements in that catalogue,we take the magnitudes from the original discovery publications.(1)(2)(3)(4)(5)ID Date Exp.S/N@Vyyyy-mm-dd sec6550Åmag The data reduction was done both using the echelle pack-age in IRAF(Willmarth&Barnes1994)and the MIDAS based ESO-UVES pipeline(Ballester et al.2000),where we have taken care to assign the wavelength calibration spectrum taken after each target spectrum in order to have the highest preci-4M.Rejkuba et al.:Bright globular clusters in NGC5128Table2.Template stars observed with UVES during the2002run.The columns list:(1)identifier,(2)number of observed spectra, (3)spectral type,(4)apparent V band magnitude from the literature,(5)metallicity from the literature,(6)radial velocity from the literature,(7)measured radial velocity,(8)reference for catalogue value of radial velocity,and averageσCCF measured from cross-correlation with all the other stars for(9)lower CCD,and(10)upper CCD(see Eq.1for the definition ofσCCF).(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)ID N Sp.Typ V(mag)[Fe/H]V R(cat)V R(UVES)Ref.σlCCF σu CCFReferences:All the radial velocities except those from D97(Dubath et al.1997)are compiled from .ar/catalogue/catalogue.html,which lists references to the sources:N04(Nordstr¨o m et al.2004);F05(Famaey et al.2005);COR(Udry et al.1999,see also:http://obswww.unige.ch/udry/std/stdcor.dat).sion in the wavelength calibration.Due to low S/N ratio of the spectra,the MIDAS pipeline was not used in optimal extraction mode.Thefinal spectra were normalized using the continuum task in IRAF and the cosmic rays were excised by hand.After some tests to ensure that the pipeline results were giving the same results as manual reductions done within IRAF,we have decided to later use spectra reduced within the MIDAS pipeline because the different echelle orders were combined in one long 1D spectrum per CCD,offering thus the maximum number of lines for cross-correlation.3.Cross-correlationIn order to measure radial velocities and velocity dispersions of all our targets we have used cross-correlation technique (Tonry&Davis1979).Slightly different implementation of the cross-correlation technique has been adopted for the EMMI and UVES spectra.The comparison of the resulting velocity dispersion measurements for the5targets in common between the two datasets is providing a useful check on the results ob-tained with these two slightly different methods.EMMI spectra were cross-correlated with a numerical mask specially designed for globular clusters.This has been described in greater detail by Dubath et al.(1990,1992)and thus we do not repeat it here.The globular cluster spectra observed with UVES have been cross-correlated with the high signal to noise spectra of template radial velocity stars observed during the same run,us-ing the IRAF task FXCOR in the RV package.All the spectra were Fourier-filtered prior to cross-correlation,to remove the residual low frequency features arising from imperfect con-tinuumfitting to the spectra with combined echelle orders. The features at frequencies higher than the intrinsic resolu-tion of the spectrograph were also cut.The peak of the cross-correlation function(CCF)traces the radial velocity,and the width is a function of the velocity dispersion and of the instru-mental width.The latter is measured by cross-correlating the template stars spectra with each other.Since we have observed a large number of radial velocity standard stars as well as gi-ant star templates with a range of metallicities,we could check that the template miss-match does not produce spurious results. This is described in detail in the next section.3.1.Template starsThe measured radial velocities for all the stars observed dur-ing the2002run with UVES are given in Table2in column7. They were measured by cross-correlating each Fourier-filtered stellar spectrum against each other.The resulting radial veloci-ties for each individual spectrum were averaged and we report here the average radial velocity and standard deviation for each star.In column6we list the radial velocity from the literature. These were compiled from Dubath et al.(1997)and the web database of stellar radial velocities(see table footnote).In all but one case the difference between our measured radial veloci-ties from UVES spectra and those from the literature is smaller than1km s−1and the measurements are consistent with the cat-alogue values within the errors.Since the stars were observed with a1.0arcsec slit and seeing was sometimes as good as 0.6-0.7arcsec,part of the error in radial velocities may comeM.Rejkuba et al.:Bright globular clusters in NGC51285 Fig.1.The points with error bars are the average raw veloc-ity dispersion measurements(σCCF)obtained from7differenttemplate stars whose spectra were broadened to simulate thedifferent input velocity dispersion(σin values).The solid line isfrom Equ.1which is also displayed in each panel.The two di-agrams are for the lower and upper CCDs of UVES which havedifferentσre f values.The inserts show the difference betweenthe obtained and input true velocity dispersion as a function ofinput velocity dispersion.from slit centering errors.The star that shows the largest differ-ence,HD103295,has less certain value of the radial velocityand the quoted error from the literature is evidently underes-timated.Leaving HD103295out,the average difference be-tween our radial velocity measurements and catalogued valuesis V R(UVES)−V R(cat)=0.07±0.27km s−1indicating thatthe systematic errors due to slit centering are not significant.For the cross-correlation of stars with cluster spectra we adoptour measured radial velocity for HD103295,and literature val-ues for all the other stars.The projected velocity dispersions(σ)for globular clustersare derived from the broadening of the cluster cross-correlationfunction(CCF)produced by the Doppler line broadeningpresent in the integrated-light spectra due to the random spa-tial motion of stars.The raw measurement(σCCF)is howevera quadratic sum of theσand the intrinsic instrumental width(σre f)(Dubath et al.1992):σ2CCF=σ2+σ2re f(1)Theσre f value is determined for both UVES CCDs by takingthe average value ofσCCF measurements obtained by cross-correlating18selected best template stellar spectra,belongingto13different stars,with all the other stars.This same set ofstars is used in cross-correlation of globular cluster spectra aswell as in all the simulations(see below).Since all these starsare late-type giants they are not expected to exhibit line broad-ening due to rotation.For3stars,HD66141,HD107328,andHD161096,de Medeiros&Mayor(1999)provide measure-ments of rotational velocities which are,1.1,1.3,and<1.0km s−1,respectively,with uncertainties which are of the sameorder of magnitude.The fact that for all the stars CCF has sim-ilar width(see Table2)implies that rotation is not a concern.The weighted average of the stellar CCFs are12.6±0.2km s−1for the lower and11.8±0.2km s−1for the upper CCD.Thedifference in the instrumental width reflects the different reso-lution of the two spectral ranges.Excluding from the averageHD150798,the star that shows the widest cross-correlationpeak for both spectral ranges,does not change the averagevalue ofσre f.Figure1shows the validity of the Equ.1for the lower andupper CCDs of UVES.The solid line is from Equ.1and thepoints represent the average projected velocity dispersion mea-surements obtained from7different template stars whose spec-tra were broadened by convolving each of them with Gaussianswith known sigma(σin)of5,10,15,20,25and30km s−1.The velocity dispersion was then measured on these broadenedspectra by convolving them with the18selected best templatestellar spectra and averaging the resulting velocity dispersions.The smaller inserts in each of the panels in Fig.1show the dif-ference between the average measured and input velocity dis-persion as a function of the input velocity dispersion,whilethe main panel displays the averages of the raw measurements(σCCF)at eachσin value.We have carefully selected the regions for cross-correlation,avoiding strong lines such as Hβ,Hα,sodium re-gion,as well as Mgb region.When we included for exampleHαline,we noticed a strong trend of cross-correlation width asa function of metallicity.In thefinal selection,this dependenceis not present as can be seen from Fig.2.In Fig.3we tested the dependence of the measured ve-locity dispersion on S/N of the input spectra.The measuredvelocity dispersions from the most broadened spectra,withσin=30km s−1and with the lowest S/N,appear to be slightlyunderestimated.However,these have,as expected higher un-certainty and are consistent with the input values within theerrors.The observations of radial velocity standards were also se-cured during the1993EMMI run and were used to check thatthe CCF has Gaussian shape and that its width does not dependon the stellar metallicity(see also Dubath et al.1990,1992).The average sigma of the stellar CCFs,derived from10mea-surements of K-giant radial velocity standard stars observed6M.Rejkuba et al.:Bright globular clusters in NGC5128Fig.2.Width of the cross-correlation function for stars is plot-ted as a function of metallicity.The measurements on the lower CCD are plotted with filled and on the upper CCD with opencircles.Fig.3.Di fference between the measured and simulated velocity dispersion as a function of S /N of the spectra.These simula-tions are based on randomly selected 7template stars whose spectra were broadened by convolving them with Gaussians with widths of 10,20,and 30km s −1and had S /N degraded to simulate noisy spectra,more similar to globular cluster tar-gets.(See the electronic edition of the journal for the colour version of this figure.)during that same run,is 6.2±0.3km s −1,where 0.3km s −1is the standard deviation around the mean.3.2.Cluster radial velocities and velocity dispersionsThe initial estimate of the radial velocity for all the clusters was obtained by fitting the H αline.In all but one cluster spectrum the line was well defined and could be fitted with a Gaussian profile using splot task in IRAF.Then the precise radial veloc-ities and velocity dispersions were measured using fxcor IRAF task.Table 3.Radial velocity measurements for globular clusters observed with EMMI and UVES are listed in column (4).For clusters with multiple observations the reported value is a weighted average of velocities measured from individual spec-tra and the combined spectrum.The last column is the ra-dial velocity from Peng et al.(2004a)catalogue.B −V and V −I colours were taken from Peng et al.(2004a)when avail-able,otherwise from original discovery papers.They have been dereddened assuming only foreground reddening of E(B–V)=0.11(Schlegel et al.1998),except for clusters observed by Holland et al.(1999,HCH99)which have individual redden-ings from that work.Reddening assumed for HGHH92-C23is 0.31,derived from strong interstellar NaD absorption lines (see text for more details).(1)(2)(3)(4)(5)ID(B −V )0(V −I )0V R V R (P04)(mag)(mag)(km s −1)(km s −1)Radial velocity measurements for all the clusters are listed in Table 3.After the identifier,de-reddened (B −V )0and (V −I )0colours of the targets are given.The radial velocities measured from our spectra are in column 4,and the velocities from the catalogue of Peng et al.(2004a)are shown for comparison in the last column.For clusters with both EMMI and UVES spec-tra the radial velocities reported are weighted averages of both measurements.The individual radial velocity and velocity dis-persion measurements for clusters with multiple observations are reported in Table 4.We note that the values listed as “com-bined”are not the averages of individual measurements,but rather the measurements of the radial velocity and velocity dis-persion from the combined UVES spectra,constructed by aver-M.Rejkuba et al.:Bright globular clusters in NGC51287Table4.Radial velocities and velocity dispersions measured on individual spectra for cluster targets with multiple observa-tions.The values listed as“combined”are not the averages of individual measurements,but rather the measurements from the combined UVES spectra,constructed by averaging individual exposures.(1)(2)(3)(4)ID V RσInst.km s−1km s−1HGHH92-c61857.3±2.119.5±1.7EMMI2847.9±3.224.5±3.0EMMIHGHH92-c11combined753.1±0.518.4±1.0UVESA752.4±1.218.2±2.0UVESB753.1±0.819.4±0.4UVESHGHH92-c171777.8±2.816.5±2.5EMMI2783.7±2.323.7±2.0EMMIHGHH92-c23combined671.4±1.330.9±1.5UVESA674.5±1.730.5±0.2UVESB675.8±1.528.6±2.1UVES1673.4±2.126.3±1.8EMMI2675.8±2.925.8±2.7EMMIaging individual exposures.These spectra have slightly higher S/N and the agreement between the values obtained from the individual and these combined spectra indicates the absence of significant systematic errors(Fig.3).Dubath et al.(1997)have made detailed numerical simula-tions in order to understand and estimate the statistical errors on their radial velocity and projected velocity dispersionσp measurements obtained by applying a cross-correlation tech-nique to integrated-light spectra.They show that statistical er-rors,which can be very important for integrated-light measure-ments of Galactic(nearby)globular clusters,because of the dominance of a few bright stars,are negligible in the present case where sampling problems are not present,thanks to the larger distances of our targets.An integrated light spectrum of an NGC5128globular cluster is well approximated by the con-volution of the spectrum of a typical globular cluster star with the projected velocity distribution.The influence of binary stars is negligible(e.g.Olszewski et al.1996,for dSph galaxies).The measurements of velocity dispersions for all the clus-ters are given in Table5.In thefirst column is the identifier, then we list velocity dispersion measured on UVES spectra.In the third column the velocity dispersions measured on EMMI spectra are given,while in the last column the results from Martini&Ho(2004)are shown for comparison.In Fig.4we plot the spectrum of the highest S/N cluster,the combined three20min exposures of HGHH92-C7,centered on some of the characteristic absorption lines.Overplotted are broadened spectra of HD103295made by convolving with Gaussians of5(red),15(blue)and25km s−1(green).The bestfitting template is the one broadened to25km s−1in agreement with24.1±1.6km s−1obtained by cross-correlation (Table4).The differences between the template broadened with 25km s−1and the cluster spectra are shown below the spectra in each panel.The narrow lines at5890.0and5895.9Å,cannot befitted by the broadened stellar templates.This is expected, because they are resonance lines(Na D1and Na D2)due to in-terstellar ions of NaI.The equivalent width of these lines can be used to constrain the interstellar extinction towards each glob-ular cluster(Munari&Zwitter1997).The equivalent width of Na D1line in this spectrum implies E(B−V)≃0.07mag, somewhat lower than the average reddening of0.11mag in the line of sight of NGC5128from Schlegel et al.(1998)maps. However,we note that both our measurement and the calibra-tion have considerable uncertainty and that the error on the red-dening is probably of the same size as the derived value.In Fig.5we plot cross-correlation functions for all the clus-ters.In each panel,next to the cluster name,the velocity disper-sion is given in parenthesis.The x-axis scale has been corrected for the relative velocity shift between the template star and the cluster so that the peak of the CCFs correspond to the heliocen-tric velocity of each cluster.All the CCFs show a single well defined Gaussian peak,except for R122,which has additional peaks at velocities59and at114km s−1.We have inspected the through-slit image taken at the start of the exposure as well as deep V-band images taken with FORS1under excellent see-ing conditions,but there is no sign of a spatially resolved blend at this position.The two additional peaks at the above given velocities are present when cross-correlating the cluster spec-trum with all the template stars and the height of these peaks is comparable,and sometimes larger than that corresponding to a velocity plausible for a cluster in NGC5128.We note that the spectrum of this object also displays additional absorption features.We do not have any explanation for this,other than an unfortunate spatially unresolved blend with foreground MW stars,which is not uncommon given the low Galactic latitude (b=+19.4◦)of NGC5128.To calculate the mean and sigma of the velocity dispersion for each spectrum,wefirst calculate the straight mean and stan-dard deviation of the18independent measurements,each ob-。

a rXiv:as tr o-ph/19298v118Se p21Extragalactic Star Clusters IAU Symposium Series,Vol.207,2001Eva K.Grebel,Doug Geisler,and Dante Minniti,eds.Formation and Disruption of Globular Star Clusters S.Michael Fall Space Telescope Science Institute,3700San Martin Drive,Baltimore,MD 21218,USA Qing Zhang Space Telescope Science Institute,3700San Martin Drive,Baltimore,MD 21218,USA Abstract.In the first part of this article,we review observations of the mass and luminosity functions of young and old star cluster systems.We also review some of the physical processes that may determine the characteristic mass of globular clusters and the form of their mass function.In the second part of this article,we summarize our models for the disruption of clusters and the corresponding evolution of the mass function.Much of our focus here is on understanding why the mass function of globular clusters has no more than a weak dependence on radius within their host galaxies.1.Background The most basic attribute of any population of astronomical objects is its mass function.In our notation,ψ(M )is the number of objects per unit mass M ,and Ψ(log M )is the number of objects per unit log M .These two forms of the mass function are related by Ψ(log M )=(log e )−1Mψ(M ).They contains infor-mation about the physical processes involved in the formation and subsequent evolution of the objects.There are well-known reasons,for example,why starsand galaxies have roughly the masses they do and not some others,even if we lack definitive theories for the detailed forms of the stellar and galactic mass functions.This article addresses the question:What physical processes deter-mine the mass function of star clusters,especially that of globular clusters?The upper panel of Figure 1shows the empirical mass function of young star clusters in the interacting and merging Antennae galaxies (from Zhang &Fall 1999).This function declines monotonically,approximately as ψ(M )∝M −2,over the entire observed range,104∼<M ∼<106M ⊙.In a young cluster system,such as the one in the Antennae galaxies,where the spread in the ages of the clusters is comparable with their median age,the luminosity function need not reflect the underlying mass function,since the clusters have a wide range of mass-to-light ratios.However,luminosity functions are easier to determine than mass functions and are known for more cluster systems.For all the young cluster systems studied so far,the luminosity functions are well-approximated by power laws (Milky Way,van den Bergh &Lafontaine 1984;LMC,Elson &Fall 1985;12Fall&ZhangFigure1.Empirical mass functions of young star clusters in the An-tennae galaxies and old globular clusters in the Milky Way.The formeris from Zhang&Fall(1999);the latter is based on data compiled byHarris(1996,1999).The dashed curve is the usual lognormal represen-tation of the mass function,corresponding to a Gaussian distributionof magnitudes.M33,Christian&Schommer1988;Antennae,Whitmore et al.1999).In fact, the mass and luminosity functions of young star clusters are remarkably similar to the mass function of interstellar clouds in the Milky Way(Dickey&Garwood 1989;Solomon&Rivolo1989),as emphasized by several authors(Harris& Pudritz1994;Elmegreen&Efremov1997).The lower panel of Figure1shows the empirical mass function old globular clusters in the Milky Way.This was derived from the luminosities of all the clusters in the Harris(1996,1999)compilation,with afixed mass-to-light ratio (M/L V=3),since the spread in the ages of the clusters is smaller than their median age.The mass function of the globular clusters in the Milky Way,like those in the spheroids of other well-studied galaxies,rises to a peak or turnover at M p≈2×105M⊙and then declines.The corresponding feature in the luminosity function,at¯M V≈−7.3,is sometimes used as a standard candle in distance determinations.The empirical mass function is often represented by a lognormal function,although,as Figure1indicates,the former is actually shallower than the latter for small masses.The crucial point here is that the mass and luminosity functions of young cluster systems are scale-free,whereas those of old cluster systems have a preferred scale.Why is this?Formation and Disruption of Globular Star Clusters3Figure2.Survival triangle in the mass-radius plane from Fall&Rees(1977).The solid lines show where the timescales for disruptionby disk shocks and two-body relaxation are equal to the Hubble time.The dashed lines indicate some of the uncertainties in these timescales.The dots represent globular clusters in the Milky Way;filled and opensymbols indicate clusters closer to and farther from the Galactic centerthan the Sun(about8kpc).Clusters outside the triangle will be de-stroyed within the next Hubble time,whereas those inside will survivefor longer.Two explanations have been proposed for the age-dependence of the mass functions.Thefirst is that the conditions in ancient galaxies and protogalaxies favored the formation of clusters with masses∼105–106M⊙but that these conditions no longer prevail in modern galaxies.For example,the Jeans mass could have been much higher in the past,as a result of less efficient cooling by heavy elements and/or more efficient dissociation of molecular hydrogen(Fall &Rees1985;Kang et al.1990).It is sometimes stated in observational papers that this theory is ruled out by the lack of correlation between the luminosities (masses)of globular clusters and their metallicities.However,this argument is not correct,because the dependence of the Jeans mass on the abundances of heavy elements and molecules is essentially bimodal.The relevant Jeans mass is determined by whether or not gas in the protoclusters cools rapidly at temperatures below104K.If it does,the Jeans mass is∼<102M⊙;if it does not, the Jeans mass is∼106M⊙.In thefirst case,one is making stellar associations or small open clusters;in the second,one is making globular clusters.The other explanation for the differences between the mass functions of young and old cluster systems is that they represent the erosion of the initial mass function by the gradual disruption of low-mass clusters.Star clusters are relatively weakly bound objects and are vulnerable to disruption by a variety of processes,including mass loss by stellar evolution(supernovae,stellar winds,4Fall&Zhangetc.),evaporation by two-body relaxation and gravitational shocks(against the galactic disk and bulge),and dynamical friction(see Spitzer1987for a review). Figure2shows the survival region for globular clusters in the mass-radius plane defined by some of these processes(from Fall&Rees1977).Recent work shows that disruptive processes,especially two-body relaxation,operating for a Hubble time,would cause the mass function to evolve from a variety of initial forms into one resembling that of old globular clusters(Vesperini1997,1998;Baumgardt 1998;Fall&Zhang2001).There is,however,a potentially serious objection to the idea that disruption is responsible for the low-mass form of the mass function of old globular clusters: the chief disruptive processes operate at different rates in different parts and different types of galaxies(Caputo&Castellani1984;Gnedin&Ostriker1997). For example,the rate at which stars escape by two-body relaxation depends on the density of a cluster,which is determined by the tidalfield,and hence is higher in the inner parts of galaxies than in the outer parts.The rate at which stars escape by gravitational shocks is also higher in the inner parts of galaxies because the orbital periods are shorter and the surface density of the disk is higher there.Moreover,disks are absent in elliptical galaxies.Thus,if the mass function were strongly affected by disruptive processes,one might expect its form to depend on radius within a galaxy and to vary from one galaxy to another.This,however,is contradicted by many observations showing that the mass function of old globular clusters varies little,if at all,within and among galaxies(Harris1991).2.New ModelsWith these issues in mind,we have developed some new models to compute the evolution of the mass function of star cluster systems(Fall&Zhang2001). Our models include disruption by two-body relaxation,gravitational shocks,and stellar evolution.We describe these processes by approximate formulae that can be solved largely analytically.The clusters are assumed to orbit in a galactic potential that is static,spherical,and has a logarithmic dependence on the distance R from the galactic center.Each cluster is assume to be tidally limited at the pericenter of its orbit and to lose mass at a constant mean internal density. The population of orbits is specified by the distribution function f(E,J),defined as the number of clusters per unit volume of position-velocity space with energy and angular momentum per unit mass near E and J.The distribution function determines how much the clusters are mixed in radius and hence how much the mass function varies with radius.We consider two simple models for the initial distribution function.The first is the Eddington modelf0(E,J)∝exp(−E/σ2)exp[−1/2(J/R Aσ)2].(1) This has velocity dispersionsσR=σandσT=σ[1+(R/R A)2]−1/2in the radial and transverse directions,where the anisotropy radius R A marks the transition from a nearly isotropic to a predominantly radial velocity distribution.The second initial distribution function we consider has the formf0(E,J)∝exp(−E/σ2)J−2β.(2)Formation and Disruption of Globular Star Clusters5Figure3.Initial densities of clusters positions(solid lines)andpericenters(dashed lines)for the Eddington and scale-free distributionfunctions.Note that the distribution of pericenters is narrower thanthe distribution of cluster positions for the Eddington model but notfor the scale-free model.In this case,the radial and transverse velocity dispersions areσR=σand σT=σ(1−β)1/2.We refer to this as the scale-free model.In most cases,we adopt R A=5kpc andβ=0.5,so that both models have the same velocity anisotropy at the median radius of the globular cluster system(R h=5kpc).For our purposes,the most important difference between the Eddington and scale-free models is that,in the former,the velocity anisotropy increases outward, whereas in the latter,it is the same at all radii.Thus,the distribution of pericenters is narrower in the Eddington model than it is in the scale-free model, as shown in Figure3.We consider four models for the initial mass function of the clusters:(1)a pure power law,ψ0(M)∝M−2,(2)the same power law truncated at M l=3×105M⊙,(3)a Schechter function,and(4)a lognormal function.Figure4shows the evolution of the mass function,averaged over all radii,for the Eddington initial distribution function.In all four cases,the mass function develops a peak,which,after12Gyr is remarkably close to the observed peak,despite the very different initial conditions.Below the peak,the evolution is dominated by two-body relaxation,and the mass function always develops a low-mass tail of the formψ(M)=const.This can be traced to the fact that,in the late stages of disruption,the masses of tidally limited clusters decrease linearly with time.The predicted low-mass form of the mass function agrees nicely with the6Fall&ZhangFigure4.Evolution of the mass function,averaged over all radii, for the Eddington initial distribution function and four different initial mass functions.These are(clockwise from upper left):a pure power law,a truncated power law,a Schechter function,and a lognormal function.Each mass function is plotted at t=0,1.5,3,6,and12Gyr;the arrows indicate the peak at t=12Gyr.The histograms depict the empirical mass function of globular clusters in the Milky Way(the same as in Fig.1).Note that the peak mass in the models is similar to that in the observations for the four different initial conditions.Formation and Disruption of Globular Star Clusters7Figure5.Evolution of the number density profile of the clustersystem for the Eddington and scale-free initial distribution functions.The profiles are plotted at t=0,1.5,3,6,and12Gyr.The data pointsdepict the empirical profile for globular clusters in the Milky Way.Notethat thefinal profiles in the models are in reasonable agreement withthe empirical profile.observed form.Above the peak,the evolution of the mass function is dominated by stellar evolution at early times and by gravitational shocks at late times. These processes shift the mass function to lower masses but leave its shape nearly invariant.Thus,the present shape of the mass function at high masses is largely determined by its initial shape.The radially averaged mass function for the scale-free model(not shown)is similar to that for the Eddington model. The evolution of the density profiles of the cluster systems for both models are shown in Figure5.Where the Eddington and scale-free models differ most is in the radial variation of the mass function of the clusters.This is shown in Figures6and7, which display the evolution separately for clusters inside and outside R=5kpc. In both models,the peak mass is larger at small radii.This is caused mainly by the higher rate of evaporation by two-body relaxation,resulting from the larger mean densities of clusters with small pericenter distances.In the Eddington model,the radial variation of the peak mass is weak enough to be consistent with observations,whereas in the scale-free model,the variation is too strong. The reason for this is that the distribution of pericenters is narrower in the8Fall&ZhangFigure6.Evolution of the mass function,averaged over inner radii (R<5kpc)and outer radii(R>5kpc),for the Eddington initial dis-tribution function and the Schechter initial mass function.Each mass function is plotted at t=0,1.5,3,6,and12Gyr;the arrows indicate the peak at t=12Gyr.The histograms depict the empirical mass functions of globular clusters in the Milky Way in the corresponding ranges of radii.Note that the shift in the peak mass in the models between inner and outer radii is relatively small.Figure7.Evolution of the mass function,averaged over inner radii (R<5kpc)and outer radii(R>5kpc),for the scale-free initial dis-tribution and the Schechter initial mass function.Each mass function is plotted at t=0,1.5,3,6,and12Gyr;the arrows indicate the peak at t=12Gyr.The histograms depict the empirical mass functions of globular clusters in the Milky Way in the corresponding ranges of radii.Note that the shift in the peak mass in the models between inner and outer radii is relatively large.Formation and Disruption of Globular Star Clusters9 Eddington model,leading to a smaller range of disruption rates,than in the scale-free model.The lesson here is that radial mixing is a necessary but not sufficient con-dition for weak radial variations in the mass function of globular clusters.The other requirement is that the radial anisotropy in the initial velocity distribution of the clusters increase outward,as in the Eddington model.The present veloc-ity distribution of globular clusters in the Milky Way appears to have little or no radial anisotropy(Frenk&White1980;Dinescu,Girard,&van Altena1999). This is qualitatively what we would expect,since most clusters on elongated orbits would already have been destroyed,leaving behind a nearly isotropic or tangentially biased velocity distribution.These conclusions are based on models with static,spherical galactic potentials,in which each cluster returns to the same pericenter on each of its revolutions around a galaxy.In galaxies with time-dependent and/or non-spherical potentials,however,the pericenters of the clusters may change from one revolution to the next.This will tend to homoge-nize the disruption rates of the clusters and hence to weaken the radial variation in their mass function.Whether these effects are significant in galaxies like the Milky Way is not yet known.It is important to test the models of disruption against observations.A clear prediction of the models is that the peak mass M p should increase with the ages of clusters.This might be observable in galaxies in which clusters formed continuously over long periods of time.Alternatively,the evolution of the peak mass might be observable in galaxies with bursts of cluster formation at different times,such as in a sequence of merger remnants.This test may be difficult,however,because the luminosity corresponding to the peak mass is relatively small for young clusters(since M p varies more rapidly with t than M/L V does).Another prediction of the models is that the peak mass should decrease with increasing distance from the centers of galaxies,unless this has been completely diluted by the mixing of pericenters mentioned above.Searches for radial variations in the peak mass have so far been inconclusive.This test is difficult because the diffuse light of the host galaxies also varies with radius, making it harder tofind faint clusters in the inner regions.Finally,the strong dependence of the peak mass on the ages of clusters and the weak dependence on their positions within and among galaxies cast some doubt on the use of the peak luminosity as a standard candle for distance estimates.This method may be viable,however,if the samples of clusters are carefully chosen from similar locations in similar galaxies.The models of disruption help to dissolve some of the boundaries in the classification of star clusters.The shape of the mass function above the peak is largely preserved as clusters are disrupted and hence should reflect processes at the time they formed.Below the peak,however,the shape of the mass function is determined entirely by disruption,mainly driven by two-body relaxation,and hence contains no information about how the clusters formed.If there were any feature in the initial mass function,such as a Jeans-type lower cutoff,it would have been erased.In our models,the only feature in the present mass function,the peak at2×105M⊙,is largely determined by the condition that clusters of this mass have a timescale for disruption comparable to the Hubble time.Thus,it is conceivable that star clusters of different types(open,populous,10Fall&Zhangglobular,etc.)formed by the same physical processes with the same initial mass function and that the differences in their present mass functions reflect only their different ages and local environments,primarily the strength of the galactic tidal field.Our results therefore support the suggestion that at least some of the star clusters formed in merging and other starburst galaxies may be regarded as young globular clusters.Further investigations of these objects may shed light on the processes by which old globular clusters formed.ReferencesBaumgardt,H.1998,A&A,330,480Caputo,F.,&Castellani,V.1984,MNRAS,207,185Christian,C.A.,&Schommer,R.A.1988,AJ,95,704Dickey,J.M.,&Garwood,R.W.1989,ApJ,341,201Dinescu,D.I.,Girard,T.M.,&van Altena,W.F.1999,AJ,117,1792 Elmegreen,B.G.,&Efremov,Y.N.1997,ApJ,480,235Elson,R.A.W.,&Fall,S.M.1985,PASP,97,692Fall,S.M.,&Rees,M.J.1977,MNRAS,181,37PFall,S.M.,&Rees,M.J.1985,ApJ,298,18Fall,S.M.,&Zhang,Q.2001,ApJ,in press(astro-ph/0107298)Frenk,C.S.,&White,S.D.M.1980,MNRAS,193,295Gnedin,O.Y.,&Ostriker,J.P.1997,ApJ,474,223Harris,W.E.1991,ARA&A,29,543Harris,W.E.1996,AJ,112,1487Harris,W.E.1999,http://www.physics.mcmaster.ca/Globular.htmlHarris,W.E.,&Pudritz,R.E.1994,ApJ,429,177Kang,H.,Shapiro,P.R.,Fall,S.M.,&Rees,M.J.1990,ApJ,363,488 Solomon,P.M.,&Rivolo,A.R.1989,ApJ,339,919Spitzer,L.1987,Dynamical Evolution of Globular Clusters(Princeton:Prince-ton Univ.Press)van den Bergh,S.,&Lafontaine,A.1984,AJ,89,1822Vesperini,E.1997,MNRAS,287,915Vesperini,E.1998,MNRAS,299,1019Whitmore,B.C.,Zhang,Q.,Leitherer,C.,Fall,S.M.,Schweizer,F.,&Miller,B.W.1999,AJ,118,1551Zhang,Q.,&Fall,S.M.1999,ApJ,527,L81。

a r X i v :0711.0997v 1 [a s t r o -p h ] 7 N o v 2007Mon.Not.R.Astron.Soc.000,000–000(0000)Printed 2February 2008(MN L A T E X style file v2.2)Keck spectroscopy of globular clusters in the spiral galaxyNGC 2683Robert N.Proctor 1⋆,Duncan A.Forbes 1,Jean P.Brodie 2,Jay Strader 21Centre for Astrophysics &Supercomputing,Swinburne University,Hawthorn,VIC 3122,Australia 2Lick Observatory,University of California,Santa Cruz,CA 95064,USA2February 2008ABSTRACTWe analyse Keck spectra of 24candidate globular clusters (GCs)associated with the spiral galaxy NGC 2683.We identify 19bona fide GCs based on their recession velocities,of which 15were suitable for stellar population analysis.Age and metallicity determinations reveal old ages in 14out of 15GCs.These old GCs exhibit age and metallicity distributions similar to that of the Milky Way GC system.One GC in NGC 2683was found to exhibit an age of ∼3Gyr.The age,metallicity and α-element abundance of this centrally located GC are remarkably similar to the values found for the galactic centre itself,providing further evidence for a recent star formation event in NGC 2683.Key words:globular clusters:general –galaxies:individual:NGC 2683–galaxies:star clusters.1INTRODUCTIONDespite their historical importance in understanding the for-mation processes of our own Galaxy (Eggen,Lynden-Bell &Sandage 1962;Searle &Zinn 1978;Mackey &Gilmore 2004;Forbes,Strader &Brodie 2004),detailed studies of the stellar populations of globular cluster (GC)systems in spiral galaxies beyond the Local Group are somewhat lim-ited.It is important that this be rectified,not only to inform formation models of spiral galaxies,but also to constrain formation models of other morphological types.For exam-ple,Ashman &Zepf (1992)proposed that the GC systems of elliptical galaxies represent the merged systems of spiral galaxies plus the addition of newly formed red (metal-rich)GCs.Bedregal et al.(2006)have argued that the GC sys-tems of S0s are consistent with faded spirals.A better un-derstanding of GC systems in spirals,with a range of types and luminosities,is needed to test these ideas.Imaging studies of GC systems exist for about a dozen spirals (e.g.Kissler-Patig et al.1999;Larsen,Forbes &Brodie 2001;Goudfrooij et al.2003).When sufficient num-bers of GCs are present,they reveal a bimodal colour distri-bution (similar to those seen in elliptical galaxies)with the red (metal-rich)subpopulation associated with the galaxy bulge component (see Forbes,Brodie &Larsen 2001).Spec-troscopic studies of GCs in spirals beyond the Milky Way and M31(Burstein et al.1984;Beasley et al.2004)are even⋆rproctor@.aumore limited.Schroder et al.(2002)investigate the stellar population properties of 16individual GCs in M81.Similar,or smaller,numbers have been investigated in M104(Larsen et al.2002),NGC 253and NGC 300(Olsen et al.2004),and M33(Chandar et al.2006).These studies generally find old ages with a wide range of metallicities for the GCs.Some GC systems reveal bulk rotation,while others do not,but small numbers and the lack of edge-on systems make such analyses uncertain.Based on the HST Advanced Camera for Surveys (ACS)imaging study of Forde et al.(2007),we have obtained Keck telescope spectra of GC candidates in the nearby,edge-on Sb spiral NGC 2683.A variety of distance estimates exist in the literature for NGC 2683.Here we adopt the surface bright-ness fluctuation distance modulus of Tonry et al.(2001)modified by the correction found by Jensen et al.(2003).This is gives m–M =29.28±0.36or 7.2±1.3Mpc which lies near the midpoint of the literature estimates.With a lu-minosity of M V =–20.31mag it has a lower luminosity (by a factor of 2)than the Milky Way or M31.We note that,although possessing a Hubble type and bulge size similar to the Milky Way and M31,NGC 2683exhibits a disc size and HI gas mass that are significantly smaller.Rhode et al.(2007)show that the extent of the GC system of NGC 2683is also rather small,with the projected density of the sys-tem falling to background levels within ∼8kpc.This can be compared to the Milky Way GC system in which a fraction of the GC system lies outside ∼30kpc.Some properties of2Proctor,Forbes,Brodie&StraderMilky Way S(B)bc1–-20.91 2.53 5.05 4.05160±2010.70 M31Sb10.78-21.21 2.43 6.45 3.05400±551 1.32 NGC2683Sb27.2-20.3 2.54 1.720.62120±4060.90Globular Clusters in NGC26833 gc018:52:27.833:24:40.4S/N<1(Sky)–––––––––gc028:52:27.833:24:40.4Star–––147.496.320.9719.10 1.87 1.24 gc038:52:37.933:23:24.4–368(3)559(7)525(20)111.7-46.921.1219.52 1.60 1.92 gc048:52:35.333:24:31.4–602(16)552(1)590(20)87.323.522.8720.86 2.02 2.27 gco58:52:35.533:24:39.7Contam283(14)546(2)593(20)79.727.623.2821.39 1.89 2.66 gc068:52:38.233:24:07.5–491(3)563(2)530(20)78.6-19.022.1420.44 1.70 2.31 gc078:52:35.333:25:22.5–557(14)532(4)497(20)59.651.222.8321.33 1.51 2.86 gc088:52:35.633:25:35.5–560(4)396(5)437(20)66.239.321.2419.72 1.52 4.17 gc098:52:41.033:24:51.7–388(4)481(2)471(20)22.5-12.620.9518.98 1.97 2.96 gc108:52:43.533:24:16.8–317(15)447(31)477(20)25.0-59.422.8420.86 1.98 2.67 gc118:52:42.033:25:02.9–430(3)461(8)460(20) 5.7-13.521.3319.82 1.51 5.55 gc128:52:42.333:25:11.3Contam432(3)432(3)424(20)-2.9-10.221.8820.00 1.89 3.09 gc138:52:43.633:24:59.4S/N<10–500(27)–-6.0-30.222.7720.89 1.88 2.04 gc148:52:44.433:25:18.6–304(5)426(2)424(20)-26.6-23.721.2719.43 1.84 2.22 gc158:52:42.133:26:30.8Star–351(5)386(20)-57.347.820.8618.47 2.39 1.01 gc168:52:42.833:26:45.3–217(5)344(5)–-73.851.822.0519.97 2.08 2.96 gc178:52:43.133:26:54.1–435(5)385(7)–-82.755.421.5719.84 1.739.66 gc188:52:46.733:25:52.9–244(6)365(2)–-71.2-19.822.4820.99 1.49 2.18 gc198:52:47.733:25:54.1–352(4)371(4)304(20)-80.9-27.820.8519.20 1.65 1.89 gc208:52:48.033:26:26.1S/N<10247(21)295(2)237(20)-106.2-7.823.6121.62 1.99 1.77 gc218:52:50.433:25:46.9–711(5)259(5)329(20)-99.8-56.821.8920.50 1.40 2.52 gc228:52:49.033:26:30.9–307(49)307(2)329(20)-118.5-13.323.0121.54 1.47 2.67 gc238:52:50.033:26:45.9S/N<10456(43)285(4)–-137.9-11.523.3521.38 1.97 3.07 gc248:52:53.033:27:15.3Star–166(85)252(20)-185.2-17.322.0920.58 1.52 1.31Index Offset ErrorTable3.Index offsets required to match the Lick system(Section√3.2).Errors are taken as the error on the mean(i.e.rms/4Proctor,Forbes,Brodie &StraderFigure 1.A sample of GC spectra after broadening to the Lick resolution.The age sensitive Balmer lines are highlighted in grey.Metallicity sensitive features are marked by lines.The young GC (gc11;bottom)is compared to old GCs with similar colours (gc03and gc08).Candidate gc11has similar Balmer line-strengths,but significantly stronger metallicity-sensitive features than gc03and gc08,indicating a younger,more metal-rich stellarpopulation.Figure 2.The distribution of GC candidates with respect to the galaxy NGC 2683is shown.The red diamond marks the centre of the galaxy,while the line identifies the major axis.Red stars are candidates identified as being stars.Blue stars represent candi-dates contaminated by OB associations.Filled symbols are GCs for which stellar population analysis was performed.Open cir-cles are candidates whose signal-to-noise was too low for stellar population analysis.The open circle onthe extreme right is the candidate used for sky estimation (Section 2).Figure 3.ACS imaging showing three GC candidates (from top to bottom;gc05,gc11and gc12).Each image (B band;left and I band;right)is 10arcsec (∼360pc at 7.2Mpc)on a side.Candi-dates gc05and gc12are clearly projected against OB associations in NGC 2683itself,and contamination appears highly likely.On the other hand,there is no evidence for an OB association affect-ing the young GC candidate;gc11.The comparisons to SSP models were carried out us-ing the χ2-fitting procedure of Proctor &Sansom (2002)(see also Proctor et al.2004a,b and Proctor et al.2005)to measure the derived parameters;log(age),[Fe/H],[Z/H]and [E/Fe](a proxy for the ‘α’–abundance ratio;see Thomas et al.2003for details).Briefly,the technique for deriving these parameters involves the simultaneous comparison of as many observed indices as possible to models of single stellar pop-ulations (SSPs).The best fit is found by minimising the deviations between observations and models in terms of the observational errors,i.e.χ.We have shown this approach to be relatively robust with respect to many problems which are commonly experienced in the measurement of spectral indices and their errors.These include poor or no flux cal-ibration,poor sky subtraction and poor calibration to the Lick system.The method is similarly robust with respect toGlobular Clusters in NGC 26835many of the uncertainties in the SSP models used in inter-pretation of the measured indices;e.g.the second parameter effect in horizontal branch morphologies and the uncertain-ties associated with the Asymptotic-Giant Branch.It was shown in Proctor et al.(2004a)and Proctor et al.(2005)that the results derived using the χ2technique are,indeed,significantly more reliable than those based on only a few indices.The process by which the candidate spectra were com-pared to the models was iterative.First,fits were obtained for all the candidates using all the available indices.The pat-terns of deviations from the fits obtained was then used to identifyindividual indices that matched the models poorly (see Fig.4).These included the H δ,CN indices for which flux levels were generally too low for accurate determination and Mg 1and Mg 2indices which suffer from flux calibration sensitivity.These indices were excluded from the analysis and the fits performed again.These fits were carried out using a clipping procedure in which indices deviating from the model fit by more than 3σwere excluded,and the fit performed again.Many of these poorly fitting indices could be associated with known problems,e.g.the contamination of the Mgb index by the 5202˚A sky-line in low signal-to-noise candidates.Indices that are excluded on this basis are in parentheses in Table A1.On average,after all exclusions,10indices were used in each of the final fits.For each GC in the sample,errors in the derived param-eters (log(age),[Fe/H],[E/Fe]and [Z/H])were estimated us-ing 50Monte-Carlo realisations.Best-fit model indices were perturbed by Gaussians,the width of which were set equal to the observational errors added in quadrature to the er-rors in offset to the Lick system (Table 3).Error estimates in the derived parameters are therefore highly sensitive to the estimates of index errors.The process also makes no al-lowance for the correlated components of the observational index errors,such as velocity dispersion,flux calibration and background subtraction errors.The errors are modelled in-stead as purely random Gaussian distributions.As a conse-quence,our error estimates must be considered to include both random and systematic errors.4RESULTS4.1Results of recession velocity analysisThe results of our analysis of recession velocities are given in Table 2and are presented in Fig.5.The value assumed for the galactic centre (442.8km s −1)was estimated such that the value of the least-squares fit to the stellar RVs with radial distance (solid line in Fig.5)passes through 0.0km s −1at a radial distance of 0.0arcsec.Note that candidates identified as stars and the single object with signal-to-noise <1are omitted from these and all subsequent plots of GC data.We therefore present recession velocities for 19GCs.The stellar and gas emission-line data (Fig.5)clearly show an increasing rotation speed with increasing radial dis-tance from the galactic centre,and can be seen to be essen-tially cylindrical (i.e.there is little scatter and no particular trend in RV with distance above or below the least-square fit).The figure also shows the rotation of gas and stars to be in very good agreement.Figure 4.shown in terms of observational errors (i.e.χ).The open symbols identify the indices excluded from the fitting procedure.Error bars show rms scatter.Figure 5.Recession velocities are shown against radial distance from the galactic centre perpendicular to the minor axis.Solid black points are stellar data,while open symbols for the gas.The stellar and gas data clearly show rotation.The least-squares fit to the stellar data is shown as a line.The small scatter about the fit shows the rotation to be roughly cylindrical.Coloured points are the GC data with colour representing actual GC colour from Forde et al.(2007)for red (B-I >1.8)and blue (B-I <1.8)candidates.6Proctor,Forbes,Brodie&StraderIt is evident that we do not reach the radii at which rota-tion is observed toflatten.However,our results are neverthe-less consistent with the rotation of Casertano&van Gorkom (1991)and Broeils&van Woerden(1994),whofind similar rotation curves with aflattening/peak lieing just beyond the range probed by our data.The apparent dip in stellar and gaseous RVs at radial distance65arcsec in the rotation pro-file of Barbon&Capaccioli(1975)is also present at similar radii in our data(Fig.5;lower left),although we note that theyfind a significantly steeper rotation curve than Caser-tano&van Gorkom(1991),Broeils&van Woerden(1994) or ourselves.Our‘dip’is also significantly deeper than that observed by Casertano&van Gorkom,with both stars and gas rotating in the opposite sense to the rest of the galaxy at similar radii.The RVs measured in the GC spectra are also shown in Fig.5.GC recession velocities are generally consistent with those of the stars and gas.However,we lack sufficient numbers to unambiguously identify rotation in the GC system.4.2Results of stellar population analysisThe results of our age and metallicity determinations are given in Table4and plotted in Fig.6.Candidates with signal-to-noise<10are excluded from our analysis,leaving 15GCs suitable for stellar population analysis.In Fig.6our results are compared to the values for Galactic GCs from de Angeli et al.(2005)and Pritzl, Venn&Irwin(2005).It is shown in Mendel et al.(2007) that ages and metallicities derived from Lee&Worthey (2005)SSP models agree extremely well with Galactic GC measurements from colour-magnitude diagrams and high resolution spectral studies.For[Fe/H],Mendel et al.find only a0.028±0.024dex average offset between the value derived from Lee&Worthey SSPs models and the Harris(1996)values for42Galactic GCs.A similar offset (∼–0.024±0.021dex or–0.28±0.24Gyr)was found in the comparison of the derived ages with the data from de Angeli et al.(2005).Finally,the average values of[E/Fe] derived by Mendel et al.(2007)for Galactic GCs are offset from the Pritzl et al.(2005)values by–0.024±0.02dex(T. Mendel2007;private communication).The Mendel et al. (2007)results are consequently fully consistent with the literature data.We therefore have reasonable confidence in our comparison of the ages and metallicities of GCs of NGC2683with those of the Milky Way.4.3Consistency checkingHowever,before interpreting the ages and metallicity es-timates,we sought to gain further confidence in our re-sults by using them to predict the B-I colours of our GC sample(using the SSP models of Bruzual&Charlot2003) for comparison to the observed HST colours(Forde et al. 2007).The comparison is shown in Fig.7.The predictions compare quite favourably with the observed values,particu-larly given the∼0.15mag overestimation of predicted(B–I) colour found by Pierce et al.(2005,2006)in similar studies.This is believed to be primarily the effect of the poor mod-elling of the horizontal branch(see also Strader&Smith 2007).Scatter should also be expected to be relatively high in our study due to the highly variable internal extinction in NGC2683.There is,however,one clearly aberrant GC–gc11;an apparently young GC(Table4).We note that the spectrum of this GC is clearly different from other GCs of the same colour in a sense consistent with the derived younger age and higher metallicity,i.e.similar Balmer line strengths and stronger metal lines(see Fig.1).It is clear from Fig.7 that this effect is not the result of extinction.However,the proximity of this GC to the galactic centre makes contam-ination by the background galaxy a concern.We therefore experimented with adding galaxy light back into the GC spectrum and then subjecting the resultant spectrum to our age/metallicity analysis.We found that when50%of the galaxy light was recombined with the GC spectrum the derived age increased by0.2dex(i.e to5Gyr),while the metallicity fell by a similar amount,resulting in a similar predicted colour.This is both a relatively small change(for a relatively large amount of galaxy contamination)and is in the opposite sense to that required to explain the young age by galaxy contamination.We therefore conclude that background contamination in the spectroscopic analysis is unlikely to be the cause of the observed young age of this GC,or the discrepancy with its predicted colour.The cause of the discrepancy between observed and predicted colours therefore remains unknown.4.4Stellar population parametersHaving gained some confidence in our measured stellar pa-rameters we now return our attention to the age and metal-licity estimates.Our stellar population analysis identifies a single young GC,with a derived age of3.3Gyr.This is similar to the luminosity-weighted age of4.7Gyr found for the galactic centre by Proctor&Sansom(2002).The central[Fe/H]=–0.03±0.09and[E/Fe]=0.20±0.04found in Proctor&San-som(2002)are also similar to the values found for this young GC(–0.17±0.04and0.16±0.03respectively;Fig.6).This suggests the possibility that this GC formed in the same event that fuelled the central star-burst.Wefind the remaining14of15GCs to possess ages older than10Gyr(Fig.6).Infive cases wefind an age equal to the oldest age modelled by Lee&Worthey(2005). It is apparent that the scatter in GC age estimates is smaller than the error given by our Monte-Carlo analysis(Section 3.2).We take this to be a combination of three effects;i)the error includes both random and systematic errors,ii)the scatter is slightly suppressed by the GCs hitting the oldest age,iii)a slight over-estimation of observational errors is also a possibility(see Section3.2).The14GCs found to be old span a broad range of metallicities(Fig.6),similar to that observed in other spiral galaxy GC systems(Burstein et al.1984;Beasley et al.2004; Schroder et al.2002;Larsen et al.2002;Olsen et al.2004). They also span a similar range to the Milky Way GC system (de Angeli et al.2005;Fig.6).Fig.6also shows a comparison of[E/Fe]values fromGlobular Clusters in NGC26837 gc0311.9 1.08(0.15)-1.40(0.13)0.21(0.08)-1.20(0.16)gc0411.2 1.05(0.13)-0.99(0.17)0.09(0.08)-0.90(0.15)gc0611.9 1.08(0.20)-1.57(0.33)0.21(0.14)-1.38(0.24)gc0711.9 1.08(0.18)-1.63(0.26)0.24(0.11)-1.40(0.22)gc0810.0 1.00(0.08)-1.95(0.09)0.24(0.15)-1.73(0.13)gc0910.0 1.00(0.18)-0.72(0.17)0.05(0.07)-0.68(0.21)gc1010.0 1.00(0.18)0.16(0.18)-0.25(0.24)-0.08(0.24)gc11 3.30.53(0.02)-0.17(0.03)0.16(0.02)-0.03(0.03)gc1410.0 1.00(0.07)-0.53(0.12)0.01(0.11)-0.53(0.08)gc1611.2 1.05(0.07)-0.71(0.10)0.14(0.10)-0.58(0.12)gc1711.2 1.05(0.14)-1.47(0.12)0.15(0.05)-1.33(0.09)gc1810.0 1.00(0.10)-1.59(0.24)0.36(0.25)-1.25(0.16)gc1911.9 1.08(0.15)-1.26(0.11)0.12(0.05)-1.15(0.12)gc2111.2 1.05(0.04)-2.55(0.19)0.24(0.11)-2.33(0.18)gc2211.9 1.08(0.26)-2.05(0.56)0.27(0.27)-1.80(0.52)8Proctor,Forbes,Brodie&StraderFigure 6.Metallicity–age(top)andα-element abundance–metallicity(bottom)relations for NGC2683GCs.Values for theGalactic GC system are shown as small solid symbols.These arefrom de Angeli et al.(2005)(top)and Pritzl et al.(2005)(bot-tom).The relation for local Galactic stars is shown as a dashedline in the bottom plot for reference only.GCs in NGC2683with(B-I)0>1.8are shown as red squares,those with(B-I)0≤1.8asblue circles.Arrows indicate GCs whose derived age equals themaximum modelled by Lee&Worthey(2005).One GC(gc11;solid blue star)exhibits age,[Fe/H]and[E/Fe]similar to thethose measured in the galactic centre by Proctor&Sansom(2002)(open black stars).Combinedsystematic and random errors fromour Monte Carlo analysis(Section3.2)are indicated in each plot.6ACKNOWLEDGEMENTSWe thank Soeren Larsen for help preparing the slit maskand Kieran Forde for providing information prior to pub-lication.We also thank Lee Spitler for assistance with thephotometric analysis.Part of this research was funded byNSF grant AST-02-06139The data presented herein wereobtained at the W.M.Keck Observatory,which is operatedFigure7.A comparison between(B-I)values predicted fromBC03models using our derived ages and metallicities and the ob-served Galactic extinction corrected colours of Forde et al.(2007).Symbols as Fig.6.The solid line represents the one-to-one rela-tion.The extinction correction corresponding to A V=0.2is shownin the bottom right.Figure8.[Fe/H]is plotted against projected distance along themajor axis.Symbols as Fig.6.Lines represent the Harris(2000)trends for the average[Fe/H]with radius in Galactic GCs whensubdivided into red and blue subpopulations.as a scientific partnership among the California Institute ofTechnology,the University of California and the NationalAeronautics and Space Administration.The Observatorywas made possible by the generousfinancial support ofthe W.M.Keck Foundation.This research has made useof the NASA/IPAC Extragalactic Database(NED),whichis operated by the Jet Propulsion 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A., Fletcher A.B.,Luppino G.A.,Metzger M.R.,Moore C.B., 2001,ApJ,546,681Trager S.C.,Worthey G.,Faber S.M.,Burstein D.,Gon-zarlez J.J.,1998,ApJS,116,1van den Bergh S.,1999,A&ARv,9,273Worthey G.,Ottaviani D.L.,1997,ApJS,111,377 APPENDIX A:LICK INDICES10Proctor,Forbes,Brodie&Stradergc030.613 2.496(-0.773) 1.357 2.3560.878 1.6670.856 2.407(3.319)(0.789) 1.459 1.3540.4730.1580.308(0.284)0.1500.3820.1950.2850.4130.202(0.396)(0.195)0.2300.2780.203gc040.826(2.314)-1.802(0.650) 4.043(2.170) 3.129 3.0860.849 3.717 1.704 1.537 1.434 1.0240.252(0.497)0.484(0.283)0.644(0.335)0.5130.7760.3470.7060.3270.3880.4570.341gc050.491 4.129-1.598 1.097(0.016) 1.410 2.178 3.120 2.710 5.260(3.361) 2.823 2.081(2.848)0.2970.5150.5430.316(0.800)0.4000.5960.8980.3750.847(0.406)0.4810.558(0.388)gc060.558 1.760 1.102 1.957(-0.892)0.026(4.640)-0.698(2.973)–(2.502)(2.594)(3.365)0.9750.2720.4980.4570.268(0.756)0.368(0.505)0.823(0.334)–(0.349)(0.401)(0.459)0.360gc070.674 1.421(2.184) 2.137 1.481(1.445) 1.969 1.189(1.944)(-0.967) 1.3590.712(2.443)(1.234)0.2590.493(0.455)0.2730.706(0.354)0.5670.853(0.357)(0.811)0.3670.445(0.508)(0.392)gc080.270(2.277) 1.095(2.398) 1.5250.543 1.005(-0.971) 2.817 1.963(0.889)0.527(-0.296)0.4780.139(0.277)0.245(0.121)0.3350.1700.249(0.354)0.1790.341(0.159)0.196(0.239)0.168gc090.987 4.497(-1.567)(1.870) 4.216 1.480 2.695(3.496)(3.054) 4.700 2.082 1.809 2.001 1.1390.1460.288(0.274)(0.138)0.3470.1760.261(0.375)(0.192)0.3740.1770.2130.2500.183gc10(2.085)(4.412)-6.486(-0.820) 5.984 1.888 3.711(-0.545)(3.132) 4.335(5.628) 3.803 3.382(-1.105)(0.266)(0.571)0.672(0.397)0.8060.4170.621(1.018)(0.378)0.908(0.402)0.4960.589(0.508)gc11(1.282) 4.757-3.7180.044 4.424 1.341 2.790 4.384 2.302(4.093) 3.153 2.577– 1.314(0.101)0.2260.1930.0810.2210.1100.1430.1600.136(0.192)0.0800.109–0.092gc14(2.066)(8.325)(-6.007)-0.872 3.526 1.497 2.655 2.910(2.760)(7.069)(6.047)(3.092) 2.487(2.594)(0.152)(0.310)(0.349)0.1980.4270.2190.3140.454(0.214)(0.407)(0.181)(0.232)0.278(0.193)gc16(1.335) 4.365-4.644-0.772 4.343 1.316 1.977(0.293) 1.886(5.444)(1.715) 2.616 1.721 1.147(0.177)0.3500.3640.2110.4540.2360.354(0.530)0.237(0.463)(0.227)0.2550.3110.226gc170.812 2.337-0.019(1.640) 1.692(0.951) 1.631(0.399)(2.379) 2.857 1.007(1.963)(1.882)0.6600.1340.2770.247(0.124)0.331(0.167)0.241(0.340)(0.177)0.3240.151(0.178)(0.220)0.159gc180.602(4.593)-1.0770.433 1.818 1.729(-0.295)(-3.244)(3.752)(6.285)(6.797) 1.817(2.002)0.3620.191(0.358)0.3790.2230.5530.259(0.452)(0.660)(0.273)(0.597)(0.260)0.364(0.429)0.340gc190.347 2.742-0.704 1.413 2.5080.893 2.543(1.323) 1.885 3.133 1.415(2.099) 1.6940.6570.1680.3190.2990.1600.4040.2080.307(0.454)0.2140.4260.205(0.241)0.2920.217gc210.4140.368 2.799 3.054 1.5600.6140.488(-3.468) 3.386 1.508(1.695)-1.085 1.1140.0110.2500.4700.4060.2280.6290.3260.533(0.810)0.3160.732(0.347)0.4510.5100.392gc220.554 1.470 2.364(3.414) 1.936 1.382-1.338––(-0.760)–(-2.791)(-3.424)(3.388)0.3500.7950.672(0.393) 1.0150.5700.967––(1.420)–(0.840)(1.076)(0.642)。

播放银河英语作文As I gaze into the night sky, a canvas of infinite black, adorned with a myriad of stars, I am captivated by the celestial spectacle that unfolds before me. The Milky Way, our home galaxy, stretches across the horizon, a luminous river of ancient light, each star a silent testament to the universe's grandeur.The composition of the Milky Way is a cosmic symphony of elements, with a central bulge of stars at its heart, spiraling outwards into a vast disc. This disc is punctuated by spiral arms, where new stars are born amidst the nebulae of gas and dust. At the edges of the galaxy, the halo of the Milky Way is home to globular clusters, ancient spheres of stars that have witnessed the eons pass in their silent orbits.The Milky Way is not just a collection of stars; it is a dynamic ecosystem of celestial bodies. Planets, both within and beyond our solar system, orbit their respective stars, some with the potential to harbor life. The gravity of black holes, those enigmatic entities where space and time are warped, influences the dance of the stars around them.Our solar system, a tiny part of the Milky Way, is a microcosm of the larger galaxy. Earth, our blue marble, is a unique haven for life, orbiting the Sun, which is but one of the hundred billion stars that make up our galaxy. The Moon,our faithful companion, orbits Earth, their relationship a delicate balance of forces that has shaped our world.The exploration of the Milky Way is not just a journey into space; it is a voyage into the unknown, a quest for knowledge that pushes the boundaries of our understanding. Telescopes, both ground-based and space-borne, allow us to peer into the depths of the galaxy, revealing the secrets of its structure, the life cycles of its stars, and the potential for extraterrestrial life.As I continue to observe the Milky Way, I am filled with a sense of awe and humility. The stars, distant suns, burn with a light that has traveled for years to reach my eyes. Each one a potential source of heat and light for other worlds, just as our Sun does for Earth. The galaxy is a reminder of our place in the universe, a speck of dust in the cosmic expanse, yet it is also a testament to the potential for discovery and the pursuit of knowledge.In conclusion, the Milky Way is a celestial masterpiece, a vast and complex system that invites us to explore its mysteries. It is a reminder of the beauty and the vastness of the universe, and it inspires us to continue reaching for the stars, both literally and metaphorically. As we continue to study and understand our home galaxy, we also gain insights into our own existence and the possibility of life beyond our planet. The Milky Way is not just a subject for an English composition; it is a story that is still unfolding, a narrative written in the stars.。

PEOPLE25war and tensions, while others wereslave labors that signed contracts withlabor smugglers.All these people migrated in orderto survive and to survive, it is importantto unite as one. Those migrants broughtChinese clan culture to the local area.In today’s Kuala Lumpur, you will seealmost everywhere the Chinese char-acters like “Sun”, “Zhou”, “Shen” and“Chen”. At the streets of Labuan, youwill also see the labels of large court-yards including Chen Family Hall andZhou Family Hall. This strong clanculture based on blood ties created ahuge cohesive force and enabled theseChinese migrants to survive in foreignlands. The affairs of many Chinesefamilies are still being discussed anddecided by the head of the clan.As China is becoming more andmore powerful in the world, the statusand influence of Chinese people hasbeen increasing in Southeast Asia.The little-known Labuan is becominga financial center that matches HongKong, Singapore and Dubai, thanks tothe contributions and constructions bycoconut juice.The beautiful scenery is really at-tractive for tourists, and the local socialenvironment makes it easy for Chinesepeople around the world to intermingle.Malaysia is a Chinese society.Drive along the streets of Kuala Lum-pur and Labuan, you will see variousChinese banks like CCB, ICBC andBOC, and also Chinese characters onthe shop labels. You will also be able toeat traditional Fujian cuisine, Guang-dong cuisine and Sichuan cuisine.Chinese people account for 25%of Malaysia’s total population. Mostof them are descendants of Chinesemigrants from Fujian, Guangdong,Guangxi and Hainan during the Qingand Minguo era. In recent years, localChinese and Malaysian people workedtogether during the fight for national in-dependence. There is a popular saying inlocal society, which is that the Belt andRoad symbolizes a development historyof overseas Chinese. The Chinese repre-sent the second-largest ethnicity of Ma-laysian society and Malaysia ranks firstin terms of the proportion of Chinesepeople in its national population.Actually people are very familiarwith “Chinese Malaysians”. The moviestar Michelle Yeoh and badminton starLee Chong Wei are well-known exam-ples of Chinese Malaysians. Guo Henianhas been the richest person in Malaysiafor 10 years. The Chinese Malaysianshave controlled Malaysia’s key sectors likefinance, real estate, lottery, constructionand tele-communication. Many of themhave obtained national honors for theirgreat contributions to Malaysia.Most of their ancestors were Chi-nese people migrating to SoutheastAsia during the 19th and 20th centu-ries. Some of them sought to escape— A Rising Starof the Belt and Road Regionhere is a small island in thenorth of the Brunei Bay andsouth of South China Sea,with an area of only 92 sq.m.It is described as one of the most mysticalplaces of Asia, surrounded by coral reefwith beautiful sands beaches and elegantmaritime scenery. This island is called asthe “garden island” of Borneo. There arealso four sunken ships at the bottom ofthe sea area, bringing challenges and ex-citement to the adventurous divers.The small island is called Labuan,and local Chinese migrants named itas “Endless Happiness”. Located in thenortheast of Sabah State, Labuan is be-coming a rising star of the Belt and Roadregion. As one of the federal territories ofMalaysia, it legally became an internation-al offshore financial center on October. 1,1990. On June 12, 2018, Labuan was de-clared as a new platform for internationalcooperation of the Belt and Road regions.Labuan is a source of pride forChinese Malaysians. Datuk Liu Fuwen,chairman of the BVA and SNE Group,a Chinese entrepreneur born in the1980s, had planned its construction andfactored more cultural contents into itsdevelopment.A Chinese society easy tointegrateMalaysia has many beautiful touristdestinations, including the paradise-likeLabuan, Langkawi with clear sea water,the “heart of the sea” Redang island andPangkor island with its primitive beauty.When summer comes, the soft sand onthe sea beach is really enticing and var-ious aquatic creatures, like fire octopus,ringed octopus, color-changing octopusand living corals, are mesmerizing to theeye. Tourists can enjoy musang kingsunder the palm trees and take a sip of26the new generation of Chinese people. These constructors have received good education and inherited the spirit of hard work from their ancestors, to dis-play new styles to the world. Great contributions by Chinese Datuk born in the 1980sTake a flight from Kuala Lumpur and fly for about 2.5 hours, you will arrive Labuan. Our journalist met with Datuk Liu Fuwen who was born in 1980s. This native Malaysian Chinese said to our journalist with a sense of humor, “Now I was born in 1980s and one year earlier I would have been born in the 1970s.” Liu Fuwen received a Chinese and Malaysian education since he was born, and he later studied in Australia. He excelled in Chinese, Malaysian and English, and he is able to communicate in the multi-lingual environment. Lubuan was originally named Namin and thanks to Liu Fuwen’s work, the local government renamed the place as Labuan. It means limitless happiness, and was renamed to attract Chinese tourists. Luban is the only deep-water port of Malaysia. It has clean island, beautiful sunset, delicious seafood and tax-free products. It also features a sea-side holiday hotel. As chairman of the BVA and SNE Group, Liu Fuwen received the title of Datuk when he was 28. Such an honor was not hereditary, but the result of his special contributions to the local social economy. According to the explanation by the Malaysian government, Datuk is not a government position, but a national honor conferred to people with high social status and influence. It represents a per-sonal honor and contributions to society. Liu Fuwen deserved such an honor, as he developed a project worthy of 1.6 billion Malaysian Ringgit. He said that it may be a small project in Kuala Lumpur, but in Labuan it is one of the hugest in-vestment projects. Take Labuan Outlets for example, they sell Malaysia’s cheapest brand products and needs to cooperate with local tourism companies. Besides, customer data collection and analysis is important for the Outlets and sometimes they need to invite professional experts to make the data analysis. It will create big commercial value if the combined analy-sis of data is well implemented. Liu Fuwen is also seeking to de-velop e-finance in Labuan, a strategy that perfectly fits the development goal of Malaysia’s national economy. Although Alipay has not entered Ma-laysia, e-money is already used in the country. To create an e-money sales network in Labuan will help release the island from currency-based consump-tion and also collect consumer data to establish a database platform. Its com-mercial value will be limitless. “We plan to establish a database that initially covers the data of 100 thousand people. Then our service will extend to other areas of the Sabah State. If people in those areas have accepted the database service, we will replicate such a model to big cities like Kuala Lumpur and foreign countries including Indonesia, Brunei and Singapore. It would be difficult to establish a database in big cities as the population is large and competition is fierce,” said Liu Fuwen. It is Liu Fuwen’s idea to establish a big data center in Labuan. As a fi-nancial center, Labuan can easily win local government support and obtain financial certifications. Also, its beau-tiful environment has attracted a lot of Malaysian big data talents, which is an important factor for this project. Liu Fuwen said that it needs an investment of 50 million Ringgit into the big data center project. He provided half of the finance, and the other half is financed from bank loan and social in-vestment. Such an investment amounts to USD 10 million, which will create huge commercial value for the local economy. “Because of the big data center, the significance of Labuan as an offshore financial center will become more pro-nounced.” Liu Fuwen said proudly that with the basis of big data center, it will be able to establish offshore finance com-panies in Labuan and attract inflow of foreign capital. Besides, offshore finance companies will enjoy favorable tax policies and traders will not like to bring money back home. BVA has set up a finance consulting department to help foreign investors design investment portfolio and provide investment consulting services.Seeing the unique advantages of Labuan, Liu Fuwen contacted with local government officials and presented his plan: “T o transform Labuan into oriental Monte Carlo to attract rich people.” La-buan sells cheap car, wine and cigarette and also has a lot of other advantages. BVA is able to introduce clubs to attract wealthy people of surrounding coun-tries and promote mutual development. Labuan has rich natural resources, such as oil and methanol. The local gov-ernment had been running well with oil tax income when the oil price was high. But when Liu Fuwen came to Labuan in 2014, the world oil price slumped to less than USD 50 per barrel. The bad situation was worsened with a downturn in the tourism market and local economy shrunk by 70%. Liu Fuwen thought that to re-vitalize local economy, it would develop the real estate and tourism industries and attraction of more people. The rise of the population will provide basis for the com-27mercial operation of the big data platform, and the big data platform will provide basis for the establishment of the offshore financial center, which will bring in a lot of capital, improve consumption environ-ment and create more jobs. This smart idea and logical thinking helped Liu Fuwen win local government trust and support.To increase popularity, the BVA group is working with the local govern-ment to organize sailing activities. It will also invite pop stars from the Chinese mainland, Hong Kong and Taiwan to make romance films in Labuan. For ex-ample, it has invited a film star born in the 1990s to produce the film Summer Holiday 2. The recreational industry is also indispensable to attract a large population. “Labuan is a safe place. If you park your car at night and leave the car key, you will find your car still there the next morning.” Liu Fuwen said that Malaysia’s housing prices are still low and have huge potential for inflation. “We are thinking to attract Chinese investors to buy real estate here.” Create a slow-paced Shenzhen As a Chinese Malaysian born in 1980s, Liu Fuwen always admired his ancestors that travelled across the ocean to settle down in Malaysia 80 years ago. Malaysia was once controlled by British colonists, and then fell in the hands of the brutal Japanese invaders. T o seek free in-dependence, Chinese Malaysians worked with local people and developed a close relationship. They constructed rubber plantations and created job opportunities. There were few conflicts or quarrels be-tween Chinese Malaysians and the local people. Through striving and hardship, Chinese Malaysians have made amazing achievements in many areas. There is a long tacit agreement in Malaysia that Chinese Malaysians would become the contractor for large projects. Although the Chinese Malaysians account for a small proportion of the whole population, they have accumulated tremendous wealth due to striving and diligence.The hard work by generations of Chinese Malaysians has made special contributions to the local economic development. They have also received recognition by the local society and gov-ernment. Labuan mayor Datuk Rozman Isli told our journalist that Labuan will play a leading role in Malaysia’s devel-opment and many people are competing to become part of the development. The BVA group led by Liu Fuwen is one of the best, because they have very good ideas and meet their promises to bring a lot of tourists and population growth to Labuan. The BVA has contributed a lot to Labuan’s financial center, to ensure the sustainability of the market.Labuan government attaches great importance to the construction of China Town, as it has attracted a huge amount of Chinese capital. Aquatic ac-tivities will be appealing to investors. When you go to Labuan, you will find that it is a paradise for tourists. Local life is colorful while the pace is slow. Men participate in aquatic activi-ties, kids play TIMBA and women go shopping at the outlets. All these would not be possible without the capital in-vestment by Chinese Malaysians. Labuan is a small but charming is-land. It has huge potential in international cooperation and offshore finance. Ac-cording to the governmental development plan, Labuan will develop into a new international city by the year 2030. Toachieve that goal Labuan will develop the logistics and tourism industry and achieve smart operation across the whole island. It is designed to become a shopping para-dise, an international sports activity center and international conference center. Labuan has been a pioneer in in-ternational finance center in Asia and it is promoting its reputation in the world. As a duty-free island, Labuan features many advantages and convenience fa-cilities. It has cheap rent for shopping stores and also huge commercial and investment opportunities. The raw ma-terials used for the manufacturing in-dustry will enjoy free import and export without tariffs. As a shopping paradise, Labuan has many outlets that offer 30% discount services for consumers. It also has many sand facilities that transform the island into a recreational center second only to Monte Carlo. As an ideal tourism desti-nation, Labuan will plant a huge area of tropical trees. People can go parasailing and jet skiing, and dance with the ocean waves. The nearby islands also have sunken a ship wreckage that was left during the World War II, making this water area is quite popular among divers. Labuan will become another Singapore. “It takes a 2-hour flight from La-buan to Hainan Province. We will de-velop a friendly relationship with China, to make Labuan a second Shenzhen,” said Rozman. The development of Labuan has become a testimony to the friendship be-tween China and Malaysia. As Rozman said, many places in Malaysia have fol-lowed the construction standards from Shenzhen, and adopted Beijing time as its timing standard. Since Mahathir became prime minister, Malaysia’s social security is good and its relationship with China remains normal. The slow-paced Labuan has joined the fast-paced Beijing, Shanghai and Shenzhen to reach new achievements under the framework of the Belt and Road Initiative. Labuan has attracted a huge amount of tourists into Malaysia. It has great ap-peal to foreign capital and won the federal government support. The recreational industry has provided opportunities for investors. It will be like a second home toChinese investors.。