A New SX Phe Star in the Globular Cluster M15

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。

中考英语经典科学实验与科学理论深度剖析阅读理解20题1<背景文章>Isaac Newton is one of the most famous scientists in history. He is known for his discovery of the law of universal gravitation. Newton was sitting under an apple tree when an apple fell on his head. This event led him to think about why objects fall to the ground. He began to wonder if there was a force that acted on all objects.Newton spent many years studying and thinking about this problem. He realized that the force that causes apples to fall to the ground is the same force that keeps the moon in orbit around the earth. He called this force gravity.The discovery of the law of universal gravitation had a huge impact on science. It helped explain many phenomena that had previously been mysteries. For example, it explained why planets orbit the sun and why objects fall to the ground.1. Newton was sitting under a(n) ___ tree when he had the idea of gravity.A. orangeB. appleC. pearD. banana答案：B。

Unraveling the Purpose of This AncientMonumentThroughout history, ancient monuments have captivated the imagination of people around the world. Their enigmatic presence serves as a testament to the ingenuity and creativity of our ancestors, leaving us with a sense of wonder and curiosity about their purpose. One such monument that continues to baffle historians and archaeologists is the Great Sphinx of Giza. Standing guard over the pyramids of Egypt, this colossal statue has sparked countless theories and speculations about its true purpose. As we delve into the mystery of the Great Sphinx, we are confronted with a myriad of perspectives that seek to unravel its enigmatic purpose. From an archaeological standpoint, the Great Sphinx presents a tantalizing puzzle that has eluded definitive answers for centuries. The sheer scale and grandeur of the monument suggest that it held immense significance to the ancient Egyptians, but the exact nature of this significance remains shrouded in mystery. Some scholars believe that the Great Sphinx served as a guardian of the Giza complex, symbolizing the divine protection of the pharaohs and their eternal resting places. This interpretation is supported by the positioning of the statue, which directly faces the rising sun, a celestial symbol of rebirth and renewal in ancient Egyptian religion. Additionally, the inscriptions and reliefs found in the vicinity of the Great Sphinx depict the pharaoh as a divine ruler, further reinforcing the notion of the monument as a symbol of royal authority and protection. However, not all experts are convinced by this interpretation. Alternative theories propose that the Great Sphinx may have had a more practical function, such as serving as a marker for astronomical events or as a part of a larger complex of structures dedicated to religious or ceremonial purposes. The alignment of the Great Sphinx with the constellation of Leo during the time of its construction has led some researchers to speculate that it was intended as an astronomical observatory, allowing the ancient Egyptians to track the movements of the stars and planets. Moreover, the discovery of hidden chambers and tunnels beneath the statue has fueled speculation about the existence of undiscovered chambers or passages that may shed light on its purpose. Beyond the realm ofacademia, the Great Sphinx has also captured the imagination of storytellers, artists, and mystics who have woven a rich tapestry of myths and legends aroundthe monument. In popular culture, the Great Sphinx is often depicted as arepository of ancient knowledge or as the guardian of hidden treasures, fuelingthe romantic notion of a secret chamber concealed within its massive stone form. This portrayal has been further popularized by the enduring allure of conspiracy theories and the enduring fascination with the mysteries of ancient civilizations. As we contemplate the purpose of the Great Sphinx, it is impossible to ignore the emotional and spiritual resonance that it holds for people around the world. The sheer magnitude of the monument, coupled with its enigmatic smile and inscrutable gaze, evokes a sense of awe and reverence that transcends time and culture. For many, the Great Sphinx represents a tangible link to the past, a connection to the wisdom and achievements of the ancient world. Its enduring presence serves as a reminder of the enduring human quest for meaning and purpose, prompting us to ponder the mysteries of existence and our place in the cosmos. In conclusion, the enigma of the Great Sphinx continues to inspire wonder and fascination, drawing us into a timeless quest to unravel its purpose. From the vantage point of archaeology, the monument stands as a testament to the ingenuity and spiritual beliefs of the ancient Egyptians, symbolizing the enduring power of royalauthority and divine protection. Yet, alternative perspectives suggest that the Great Sphinx may have served a more practical or esoteric function, inviting us to contemplate its role as an astronomical observatory or a gateway to hidden knowledge. Regardless of the specific interpretation, the emotional and spiritual impact of the Great Sphinx is undeniable, resonating with people across the ages and inviting us to ponder the enduring mysteries of the past.。

Adventure is the spirit that has always driven humanity to explore the unknown and push the boundaries of what is possible.The phrase braving the journey to the ends of the earth encapsulates this daring attitude,inspiring us to venture beyond our comfort zones and seek out new experiences.Embarking on such a journey requires courage and determination.It means leaving behind the familiar and stepping into the unknown,where every step is a challenge and every discovery a reward.This spirit of adventure is not just about physical exploration it also applies to intellectual and emotional growth.One of the most famous examples of this spirit is the story of Christopher Columbus,who, in1492,set sail from Spain with the intention of finding a new route to Asia.Despite the many obstacles he faced,including skepticism from others and treacherous seas, Columbus persevered and ultimately discovered the Americas,opening up a new world to exploration and colonization.In modern times,the spirit of adventure continues to inspire people to push their limits. Climbers like Sir Edmund Hillary and Tenzing Norgay,who were the first to reach the summit of Mount Everest in1953,exemplify the drive to conquer the highest peaks. Similarly,astronauts like Neil Armstrong,who took the first steps on the moon in1969, demonstrate the human desire to explore even the most distant and inhospitable environments.Adventure also fosters personal growth.When we step out of our comfort zones,we learn about ourselves and develop resilience,adaptability,and problemsolving skills.These qualities are invaluable not only in the context of an adventurous journey but also in everyday life.Moreover,the spirit of adventure encourages us to appreciate the beauty and diversity of our world.From the breathtaking landscapes of the Amazon rainforest to the aweinspiring vistas of the Grand Canyon,the natural world offers countless opportunities for exploration and wonder.In conclusion,the journey to the ends of the earth is not just about physical exploration it is a metaphor for the pursuit of knowledge,selfdiscovery,and the expansion of our horizons.Whether its climbing a mountain,traveling to a new country,or learning a new skill,the spirit of adventure is a call to action,urging us to embrace the unknown and all the possibilities it holds.。

Plant and Animal Life of the Pacific IslandsParagraph 1There are both great similarities and considerable diversity in the ecosystems that evolved on the islands of Oceania in and around the Pacific Ocean. The islands, such as New Zealand, that were originally parts of continents still carry some small plant and animal remnants of their earlier biota (animal and plant life), and they also have been extensively modified by evolution, adaptation, and the arrival of new species. By contrast, the other islands, which emerged via geological processes such as volcanism, possessed no terrestrial life, but over long periods, winds, ocean currents, and the feet, feathers, and digestive tracts of birds brought the seeds of plants and a few species of animals. Only those species with ways of spreading to these islands were able to undertake the long journeys, and the various factors at play resulted in diverse combinations of new colonists on the islands. One estimate is that the distribution of plants was 75 percent by birds, 23 percent by floating, and only 2 percent by wind.1. The word “remnants” in the passage is closet in meaning toA. remaindersB. remindersC. reproductionsD. resemblances2. The passage supports which of the following statements about species on volcanic islands?A. Volcanic island species are unlike the species found in other Pacific Ocean locations.B. Volcanic islands lack the diversity of species found elsewhere in the Pacific.C. Volcanic Island species are all transplants from distant locations and exist in combinations not found elsewhere.D. Volcanic island species differ from those on other islands in that animal species show greater diversity than plant species do.3. According to paragraph 1, how did the majority of plant species arrive on islands created by geological processes such as volcanism?A. They were transported by ocean currents.B. They were carried to the islands by birds.C. They were brought to the islands by humans.D. They were transported by winds.Paragraph 2The migration of Oceanic biota was generally from west to east, with four major factors influencing their distribution and establishment. The first was the size and fertility of the islands on which they landed, with larger islands able to provide hospitality for a wider range of species. Second, the further east the islands, generallythe less the species diversity, largely because of the distance that had to be crossed and because the eastern islands tended to be smaller, more scattered, and remote. This e asterly decline in species diversity is well demonstrated by birds and coral fish. It ise stimated that there were over 550 species of birds in New Guinea, 127 in theS olomon Islands, 54 in Fiji, and 17 in the Society Islands. From the west across theP acific, the Bismarck Archipelago and the Solomon Islands have more than 90f amilies of shore fish (with many species within the families), Fiji has 50 families,and the Society Islands have 30. Third, the latitude of the islands also influenced the biotic mix, as those islands in relatively cooler latitudes, notably New Zealand, were unsuited to supporting some of the tropical plants with which Pacific islands are generally associated.4. The word “remote” in the passage is closet in meaning toA.unknownB. isolatedC. hostileD. infertile5. In paragraph 2, what is the author’s purpose in mentioning the estimated numbers o f birds and coral fish species on various Oceanic islands?A. To give examples of the wide range of species that can be found on Oceanic islandsB. To demonstrate how much knowledge about Oceanic species has been accumulatedC. To illustrate the decline in species diversity from west to east on Oceanic islandsD. To identify the influence of latitude upon Oceanic plants and animals6. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.A. Because of its latitude, New Zealand had a relatively cooler climate than other Pacific islands.B. New Zealand, like other Pacific islands, showed the effects of latitudes on its richt ropical plants.C. Because the latitudinal position of an island also affected its biotic mix, islands in cooler latitudes did not support some tropical species typical of the Pacific islands.D. Pacific islands were notable for their impressive biotic mix and association with t ropical plants.7. According to paragraph 2, all of the following types of islands are associated withhigher species diversity EXCEPTA. islands that are large in sizeB. islands located in cool latitudesC. islands located in the western part of OceaniaD. islands located near other landmassesParagraph 3Finally, a fourth major factor in species distribution, and indeed in the shaping ofP acific ecosystems, was wind. It takes little experience on Pacific islands to be aware t hat there are prevailing winds. To the north of the equator these are called n orth-easterlies, while to the south they are called south-easterlies. Further south, from about 30° south, the winds are generally from the west. As a result, on nearly e very island of significant size there is an ecological difference between its windward a nd leeward (away from the wind) sides. Apart from the wind action itself on plants and soils, wind has a major effect on rain distribution. The Big Island of Hawaii offersa prime example: one can leave Kona on the leeward side in brilliant sunshine and drive across to the windward side where the city of Hilo is blanketed in mist and rain.8. The Big Island of Hawaii discussed in the passage as an example ofA. the relationship between latitude and windB. how prevailing winds influence rainfall patternsC. the relationship between rainfall and species distributionD. the effects of wind action upon plants and soils9. What can be inferred from paragraph 3 about Kona and Hilo?A. The ecosystems of Kona and Hilo differ from each otherB. Kona and Hilo have approximately the same rainfall in a given year.C. Kona receives northeasterly winds while Hilo receives southeasterly winds.D. Both Kona and Hilo have plants and soils that are often damaged by winds. Paragraph 4While such localized plant life and climatic conditions are very noticeable, over Oceania as a whole there is relatively little biodiversity, and the smaller the island and the further east it lies, the less there is likely to be. When humans moved beyond the islands of Near Oceania (Australia, New Guinea, and the Solomon Islands), they encountered no indigenous mammals except for flying foxes, fruit bats, and seals on some islands. Other vertebrate species were restricted to flying animals and a few small reptiles. However, local adaptations and evolution over long periods of isolation promoted fascinating species adaptations to local conditions. Perhaps most notable, in the absence of mammals and other predators, are the many species of flightless and ground-nesting birds. Another consequence of evolution was that many small environments boasted their own endemic (native) species, often small in number, unused to serious predation, limited in range, and therefore vulnerable to disruption. In Hawaii, for example, the highly adapted 39 species and subspecies of honeycreepers, several hundred species of fruit flies, and more than 750 species of tree snails are often cited to epitomize the extent of localized Oceanic endemism (species being native to the area).10. The word “cited” in the passage is closet in meaning toA. expectedB. believedC. comparedD. mentioned11. According to paragraph 4, why have species of flightless andground-nesting birdsb ecome so numerous on Oceanic islands?A. They have no predators on the islands.B. They were some of the strongest species to arrive on the islands.C. They live closer to their food sources than other species do.D. They are affected less by climatic changes than other animals are.12. Which of the following is NOT mentioned in paragraph 4 about the species that live on Oceanic islands?A. Certain species are native only to particular islands.B. Species that are native to Oceanic islands include relatively few mammals.C. Populations of most species are small in number.D. Some species have evolved over time to become predators.Paragraph 1T here are both great similarities and considerable diversity in the ecosystems that e volved on the islands of Oceania in and around the Pacific Ocean. ■The islands, such a s New Zealand, that were originally parts of continents still carry some small planta nd animal remnants of their earlier biota (animal and plant life), and they also haveb een extensively modified by evolution, adaptation, and the arrival of new species. ■By contrast, the other islands, which emerged via geological processes such as volcanism, possessed no terrestrial life, but over long periods, winds, ocean currents, and the feet, feathers, and digestive tracts of birds brought the seeds of plants and a few species of animals. ■Only those species with ways of spreading to these islands were able to undertake the long journeys, and the various factors at play resulted in diverse combinations of new colonists on the islands. ■One estimate is that the distribution of plants was 75 percent by birds, 23 percent by floating, and only 2 percent by wind.13. Look at the four squares [■]that indicate where the following sentencecan be added to the passage.When varied ecosystems are present, they can be explained as resulting in part from the process that formed the islands.Where would the sentence best fit? Click on a square [■] to add the sentenceto the passage.14. Directions: An introductory sentence for a brief summary of the passage isprovided below.Complete the summary by selecting the THREE answer choices that express the mostimportant ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas inthe passage.This question is worth 2 points.Drag your choices to the spaces where they belong. To review the passage, click on View Text.Biodiversity on Oceanic islands is dependent on a number offactors. ● ● ●Answer ChoicesA. Unlike Oceanic islands that were once part of continental landmasses, islandsformed by such geological processes as volcanism contain only plants and animals that could be transported there.B. An island’s size is less important than its latitude in determining species diversity.C. Though biodiversity is low on many Oceanic islands, many native species haveevolved that are uniquely adapted to their local environments.D. Species distribution in Oceania is determined by the location of islands, their size,and the direction of the wind.E. Most Oceanic islands are similar to one another in latitude and contain plants andanimals typical of tropical islands.F. The absence of natural predators on the eastern Oceanic islands allowed manyspecies of large mammals to evolve that were capable of inhibiting a wide range of territory.。

Exploration is a fundamental human endeavor that has driven us to push the boundaries of our understanding and experience.The spirit of exploration is deeply ingrained in our nature,and it has led to countless discoveries and advancements throughout history.Here is an essay on the subject of challenging new frontiers and the exploration of the unknown.Title:Embracing the Spirit of ExplorationExploration is not merely a journey to distant lands or the depths of the ocean it is a mindset that propels us to venture into the unknown,to question the status quo,and to seek out new knowledge and experiences.It is an essential component of human progress, and it is this spirit that has led us from the earliest civilizations to the technological marvels of the modern age.The Historical Context of ExplorationHistorically,exploration has been a catalyst for change.The Age of Discovery,for instance,saw European explorers like Christopher Columbus and Vasco da Gama set sail in search of new trade routes and lands.Their voyages not only expanded geographical knowledge but also initiated cultural exchanges and economic growth.The spirit of exploration during this period was driven by curiosity,courage,and a desire for advancement.Exploration in Science and TechnologyIn the realm of science and technology,exploration is the engine of innovation.From the invention of the steam engine to the development of the internet,each breakthrough has been a result of exploring new ideas and pushing the limits of what was previously thought possible.The exploration of space is a prime example of this,with missions to the moon and beyond expanding our understanding of the universe and inspiring generations to dream of a future among the stars.Cultural and Social ExplorationExploration is not limited to the physical or scientific domains it also encompasses cultural and social realms.The exploration of different cultures,languages,and belief systems enriches our perspectives and fosters a more inclusive and empathetic society. Social exploration,such as the civil rights movement or the womens suffrage movement, has challenged and changed societal norms,leading to greater equality and justice.The Role of Exploration in Personal GrowthOn an individual level,exploration is crucial for personal growth.It encourages us to step out of our comfort zones,to take risks,and to learn from our experiences.Whether its traveling to a new place,learning a new skill,or engaging with ideas that challenge our beliefs,the process of exploration helps us to grow and adapt.Challenges and Rewards of ExplorationExploration is not without its challenges.It often involves facing the unknown, overcoming obstacles,and dealing with failure.However,the rewards are significant. The joy of discovery,the satisfaction of overcoming a challenge,and the enrichment of the human experience are all part of the exploration journey.ConclusionIn conclusion,the spirit of exploration is a vital force that has shaped our world and will continue to do so.It is a call to action,urging us to be curious,to be brave,and to be open to the endless possibilities that lie beyond our current horizons.As we continue to explore new frontiers,we not only expand our knowledge but also enrich our lives and contribute to the collective human story.This essay aims to inspire a continued pursuit of exploration,whether it be in science, technology,culture,or personal development,recognizing that each step into the unknown is a step forward for humanity.。

【英文读物】The WillowsChapter 1After leaving Vienna, and long before you come to Budapest, the Danube enters a region of singular loneliness and desolation, where its waters spread away on all sides regardless of a main channel, and the country becomes a swamp for miles upon miles, covered by a vast sea of low willow-bushes. On the big maps this deserted area is painted in a fluffy blue, growing fainter in color as it leaves the banks, and across it may be seen in large straggling letters the word Sumpfe, meaning marshes.In high flood this great acreage of sand, shingle-beds, and willow-grown islands is almost topped by the water, but in normal seasons the bushes bend and rustle in the free winds, showing their silver leaves to the sunshine in an ever-moving plain of bewildering beauty. These willows never attain to the dignity of trees; they have no rigid trunks; they remain humble bushes, with rounded tops and soft outline, swaying on slender stems that answer to the least pressure of the wind; supple as grasses, and so continually shifting that they somehow give the impression that the entire plain is moving and alive. For the wind sends waves rising and falling over the whole surface, waves of leaves instead of waves of water, green swells like the sea, too, until the branches turn and lift, and then silvery white as their underside turns to the sun.Happy to slip beyond the control of the stern banks, the Danube here wanders about at will among the intricate network of channels intersecting the islands everywhere with broad avenues down which the waters pour with a shouting sound; making whirlpools, eddies, and foaming rapids; tearing at the sandy banks; carrying away masses of shore and willow-clumps; and forming new islands innumerably which shift daily in size and shape and possess at best an impermanent life, since the flood-time obliterates their very existence.Properly speaking, this fascinating part of the river's life begins soon after leaving Pressburg, and we, in our Canadian canoe, with gipsy tent and frying-pan on board, reached it on the crest of a rising flood about mid-July. That very same morning, when the sky was reddening before sunrise, we had slipped swiftly through still-sleeping Vienna, leaving it a couple of hours later a mere patch of smoke against the blue hills of the Wienerwald on the horizon; we had breakfasted below Fischeramend under a grove of birch trees roaring in the wind; and had then swept on the tearing current past Orth, Hainburg, Petronell (the old Roman Carnuntum of Marcus Aurelius), and so under the frowning heights of Thelsen on a spur of the Carpathians, where the March steals in quietly from the left and the frontier is crossed between Austria and Hungary.Racing along at twelve kilometers an hour soon took us well into Hungary, and the muddy waters —sure sign of flood—sent us aground on many a shingle-bed, and twisted us like a cork in many a sudden belching whirlpool before the towers of Pressburg (Hungarian, Poszony) showed against the sky; and then the canoe, leaping like a spirited horse, flew at top speed under the grey walls, negotiated safely the sunken chain of the Fliegende Brucke ferry, turned the corner sharply to theleft, and plunged on yellow foam into the wilderness of islands, sandbanks, and swamp-land beyond—the land of the willows.The change came suddenly, as when a series of bioscope pictures snaps down on the streets of a town and shifts without warning into the scenery of lake and forest. We entered the land of desolation on wings, and in less than half an hour there was neither boat nor fishing-hut nor red roof, nor any single sign of human habitation and civilization within sight. The sense of remoteness from the world of humankind, the utter isolation, the fascination of this singular world of willows, winds, and waters, instantly laid its spell upon us both, so that we allowed laughingly to one another that we ought by rights to have held some special kind of passport to admit us, and that we had, somewhat audaciously, come without asking leave into a separate little kingdom of wonder and magic—a kingdom that was reserved for the use of others who had a right to it, with everywhere unwritten warnings to trespassers for those who had the imagination to discover them.Though still early in the afternoon, the ceaseless buffetings of a most tempestuous wind made us feel weary, and we at once began casting about for a suitable camping-ground for the night. But the bewildering character of the islands made landing difficult; the swirling flood carried us in shore and then swept us out again; the willow branches tore our hands as we seized them to stop the canoe, and we pulled many a yard of sandy bank into the water before at length we shot with a great sideways blow from the wind into a backwater and managed to beach the bows in a cloud of spray. Then we lay panting and laughing after our exertions on the hot yellow sand, sheltered from the wind, and in the full blaze of a scorching sun, a cloudless blue sky above, and an immense army of dancing, shouting willow bushes, closing in from all sides, shining with spray and clapping their thousand little hands as though to applaud the success of our efforts."What a river!" I said to my companion, thinking of all the way we had traveled from the source in the Black Forest, and how he had often been obliged to wade and push in the upper shallows at the beginning of June."Won't stand much nonsense now, will it?" he said, pulling the canoe a little farther into safety up the sand, and then composing himself for a nap.I lay by his side, happy and peaceful in the bath of the elements—water, wind, sand, and the great fire of the sun—thinking of the long journey that lay behind us, and of the great stretch before us to the Black Sea, and how lucky I was to have such a delightful and charming traveling companion as my friend, the Swede.We had made many similar journeys together, but the Danube, more than any other river I knew, impressed us from the very beginning with its aliveness. From its tiny bubbling entry into the world among the pinewood gardens of Donaueschingen, until this moment when it began to play the great river-game of losing itself among the deserted swamps, unobserved, unrestrained, it had seemed to us like following the grown of some living creature. Sleepy at first, but later developing violent desires as it became conscious of its deep soul, it rolled, like some huge fluidbeing, through all the countries we had passed, holding our little craft on its mighty shoulders, playing roughly with us sometimes, yet always friendly and well-meaning, till at length we had come inevitably to regard it as a Great Personage.How, indeed, could it be otherwise, since it told us so much of its secret life? At night we heard it singing to the moon as we lay in our tent, uttering that odd sibilant note peculiar to itself and said to be caused by the rapid tearing of the pebbles along its bed, so great is its hurrying speed. We knew, too, the voice of its gurgling whirlpools, suddenly bubbling up on a surface previously quite calm; the roar of its shallows and swift rapids; its constant steady thundering below all mere surface sounds; and that ceaseless tearing of its icy waters at the banks. How it stood up and shouted when the rains fell flat upon its face! And how its laughter roared out when the wind blew up-stream and tried to stop its growing speed! We knew all its sounds and voices, its tumblings and foamings, its unnecessary splashing against the bridges; that self-conscious chatter when there were hills to look on; the affected dignity of its speech when it passed through the little towns, far too important to laugh; and all these faint, sweet whisperings when the sun caught it fairly in some slow curve and poured down upon it till the steam rose.It was full of tricks, too, in its early life before the great world knew it. There were places in the upper reaches among the Swabian forests, when yet the first whispers of its destiny had not reached it, where it elected to disappear through holes in the ground, to appear again on the other side of the porous limestone hills and start a new river with another name; leaving, too, so little water in its own bed that we had to climb out and wade and push the canoe through miles of shallows.And a chief pleasure, in those early days of its irresponsible youth, was to lie low, like Brer Fox, just before the little turbulent tributaries came to join it from the Alps, and to refuse to acknowledge them when in, but to run for miles side by side, the dividing line well marked, the very levels different, the Danube utterly declining to recognize the newcomer. Below Passau, however, it gave up this particular trick, for there the Inn comes in with a thundering power impossible to ignore, and so pushes and incommodes the parent river that there is hardly room for them in the long twisting gorge that follows, and the Danube is shoved this way and that against the cliffs, and forced to hurry itself with great waves and much dashing to and fro in order to get through in time. And during the fight our canoe slipped down from its shoulder to its breast, and had the time of its life among the struggling waves. But the Inn taught the old river a lesson, and after Passau it no longer pretended to ignore new arrivals.This was many days back, of course, and since then we had come to know other aspects of the great creature, and across the Bavarian wheat plain of Straubing she wandered so slowly under the blazing June sun that we could well imagine only the surface inches were water, while below there moved, concealed as by a silken mantle, a whole army of Undines, passing silently and unseen down to the sea, and very leisurely too, lest they be discovered.Much, too, we forgave her because of her friendliness to the birds and animals that haunted the shores. Cormorants lined the banks in lonely places in rows like short black palings; grey crowscrowded the shingle-beds; storks stood fishing in the vistas of shallower water that opened up between the islands, and hawks, swans, and marsh birds of all sorts filled the air with glinting wings and singing, petulant cries. It was impossible to feel annoyed with the river's vagaries after seeing a deer leap with a splash into the water at sunrise and swim past the bows of the canoe; and often we saw fawns peering at us from the underbrush, or looked straight into the brown eyes of a stag as we charged full tilt round a corner and entered another reach of the river. Foxes, too, everywhere haunted the banks, tripping daintily among the driftwood and disappearing so suddenly that it was impossible to see how they managed it.But now, after leaving Pressburg, everything changed a little, and the Danube became more serious. It ceased trifling. It was half-way to the Black Sea, within seeming distance almost of other, stranger countries where no tricks would be permitted or understood. It became suddenly grown-up, and claimed our respect and even our awe. It broke out into three arms, for one thing, that only met again a hundred kilometers farther down, and for a canoe there were no indications which one was intended to be followed."If you take a side channel," said the Hungarian officer we met in the Pressburg shop while buying provisions, "you may find yourselves, when the flood subsides, forty miles from anywhere, high and dry, and you may easily starve. There are no people, no farms, no fishermen. I warn you not to continue. The river, too, is still rising, and this wind will increase."The rising river did not alarm us in the least, but the matter of being left high and dry by a sudden subsidence of the waters might be serious, and we had consequently laid in an extra stock of provisions. For the rest, the officer's prophecy held true, and the wind, blowing down a perfectly clear sky, increased steadily till it reached the dignity of a westerly gale.It was earlier than usual when we camped, for the sun was a good hour or two from the horizon, and leaving my friend still asleep on the hot sand, I wandered about in desultory examination of our hotel. The island, I found, was less than an acre in extent, a mere sandy bank standing some two or three feet above the level of the river. The far end, pointing into the sunset, was covered with flying spray which the tremendous wind drove off the crests of the broken waves. It was triangular in shape, with the apex up stream.I stood there for several minutes, watching the impetuous crimson flood bearing down with a shouting roar, dashing in waves against the bank as though to sweep it bodily away, and then swirling by in two foaming streams on either side. The ground seemed to shake with the shock and rush, while the furious movement of the willow bushes as the wind poured over them increased the curious illusion that the island itself actually moved. Above, for a mile or two, I could see the great river descending upon me; it was like looking up the slope of a sliding hill, white with foam, and leaping up everywhere to show itself to the sun.The rest of the island was too thickly grown with willows to make walking pleasant, but I made the tour, nevertheless. From the lower end the light, of course, changed, and the river looked dark and angry. Only the backs of the flying waves were visible, streaked with foam, and pushedforcibly by the great puffs of wind that fell upon them from behind. For a short mile it was visible, pouring in and out among the islands, and then disappearing with a huge sweep into the willows, which closed about it like a herd of monstrous antediluvian creatures crowding down to drink. They made me think of gigantic sponge-like growths that sucked the river up into themselves. They caused it to vanish from sight. They herded there together in such overpowering numbers.Altogether it was an impressive scene, with its utter loneliness, its bizarre suggestion; and as I gazed, long and curiously, a singular emotion began to stir somewhere in the depths of me. Midway in my delight of the wild beauty, there crept, unbidden and unexplained, a curious feeling of disquietude, almost of alarm.A rising river, perhaps, always suggests something of the ominous; many of the little islands I saw before me would probably have been swept away by the morning; this resistless, thundering flood of water touched the sense of awe. Yet I was aware that my uneasiness lay deeper far than the emotions of awe and wonder. It was not that I felt. Nor had it directly to do with the power of the driving wind—this shouting hurricane that might almost carry up a few acres of willows into the air and scatter them like so much chaff over the landscape. The wind was simply enjoying itself, for nothing rose out of the flat landscape to stop it, and I was conscious of sharing its great game with a kind of pleasurable excitement. Yet this novel emotion had nothing to do with the wind. Indeed, so vague was the sense of distress I experienced, that it was impossible to trace it to its source and deal with it accordingly, though I was aware somehow that it had to do with my realization of our utter insignificance before this unrestrained power of the elements about me. The huge-grown river had something to do with it too—a vague, unpleasant idea that we had somehow trifled with these great elemental forces in whose power we lay helpless every hour of the day and night. For here, indeed, they were gigantically at play together, and the sight appealed to the imagination.But my emotion, so far as I could understand it, seemed to attach itself more particularly to the willow bushes, to these acres and acres of willows, crowding, so thickly growing there, swarming everywhere the eye could reach, pressing upon the river as though to suffocate it, standing in dense array mile after mile beneath the sky, watching, waiting, listening. And, apart quite from the elements, the willows connected themselves subtly with my malaise, attacking the mind insidiously somehow by reason of their vast numbers, and contriving in some way or other to represent to the imagination a new and mighty power, a power, moreover, not altogether friendly to us.Great revelations of nature, of course, never fail to impress in one way or another, and I was no stranger to moods of the kind. Mountains overawe and oceans terrify, while the mystery of great forests exercises a spell peculiarly its own. But all these, at one point or another, somewhere link on intimately with human life and human experience. They stir comprehensible, even if alarming, emotions. They tend on the whole to exalt.With this multitude of willows, however, it was something far different, I felt. Some essence emanated from them that besieged the heart. A sense of awe awakened, true, but of awetouched somewhere by a vague terror. Their serried ranks, growing everywhere darker about me as the shadows deepened, moving furiously yet softly in the wind, woke in me the curious and unwelcome suggestion that we had trespassed here upon the borders of an alien world, a world where we were intruders, a world where we were not wanted or invited to remain—where we ran grave risks perhaps!The feeling, however, though it refused to yield its meaning entirely to analysis, did not at the time trouble me by passing into menace. Yet it never left me quite, even during the very practical business of putting up the tent in a hurricane of wind and building a fire for the stew-pot. It remained, just enough to bother and perplex, and to rob a most delightful camping-ground of a good portion of its charm. To my companion, however, I said nothing, for he was a man I considered devoid of imagination. In the first place, I could never have explained to him what I meant, and in the second, he would have laughed stupidly at me if I had.There was a slight depression in the center of the island, and here we pitched the tent. The surrounding willows broke the wind a bit."A poor camp," observed the imperturbable Swede when at last the tent stood upright, "no stones and precious little firewood. I'm for moving on early tomorrow—eh? This sand won't hold anything."But the experience of a collapsing tent at midnight had taught us many devices, and we made the cozy gipsy house as safe as possible, and then set about collecting a store of wood to last till bed-time. Willow bushes drop no branches, and driftwood was our only source of supply. We hunted the shores pretty thoroughly. Everywhere the banks were crumbling as the rising flood tore at them and carried away great portions with a splash and a gurgle."The island's much smaller than when we landed," said the accurate Swede. "It won't last long at this rate. We'd better drag the canoe close to the tent, and be ready to start at a moment's notice.I shall sleep in my clothes."He was a little distance off, climbing along the bank, and I heard his rather jolly laugh as he spoke."By Jove!" I heard him call, a moment later, and turned to see what had caused his exclamation. But for the moment he was hidden by the willows, and I could not find him."What in the world's this?" I heard him cry again, and this time his voice had become serious.I ran up quickly and joined him on the bank. He was looking over the river, pointing at something in the water."Good heavens, it's a man's body!" he cried excitedly. "Look!"A black thing, turning over and over in the foaming waves, swept rapidly past. It keptdisappearing and coming up to the surface again. It was about twenty feet from the shore, and just as it was opposite to where we stood it lurched round and looked straight at us. We saw its eyes reflecting the sunset, and gleaming an odd yellow as the body turned over. Then it gave a swift, gulping plunge, and dived out of sight in a flash."An otter, by gad!" we exclaimed in the same breath, laughing.It was an otter, alive, and out on the hunt; yet it had looked exactly like the body of a drowned man turning helplessly in the current. Far below it came to the surface once again, and we saw its black skin, wet and shining in the sunlight.Then, too, just as we turned back, our arms full of driftwood, another thing happened to recall us to the river bank. This time it really was a man, and what was more, a man in a boat. Now a small boat on the Danube was an unusual sight at any time, but here in this deserted region, and at flood time, it was so unexpected as to constitute a real event. We stood and stared.Whether it was due to the slanting sunlight, or the refraction from the wonderfully illumined water, I cannot say, but, whatever the cause, I found it difficult to focus my sight properly upon the flying apparition. It seemed, however, to be a man standing upright in a sort of flat-bottomed boat, steering with a long oar, and being carried down the opposite shore at a tremendous pace. He apparently was looking across in our direction, but the distance was too great and the light too uncertain for us to make out very plainly what he was about. It seemed to me that he was gesticulating and making signs at us. His voice came across the water to us shouting something furiously, but the wind drowned it so that no single word was audible. There was something curious about the whole appearance—man, boat, signs, voice—that made an impression on me out of all proportion to its cause."He's crossing himself!" I cried. "Look, he's making the sign of theCross!""I believe you're right," the Swede said, shading his eyes with his hand and watching the man out of sight. He seemed to be gone in a moment, melting away down there into the sea of willows where the sun caught them in the bend of the river and turned them into a great crimson wall of beauty. Mist, too, had begun to ruse, so that the air was hazy."But what in the world is he doing at nightfall on this flooded river?" I said, half to myself. "Where is he going at such a time, and what did he mean by his signs and shouting? D'you think he wished to warn us about something?""He saw our smoke, and thought we were spirits probably," laughed my companion. "These Hungarians believe in all sorts of rubbish; you remember the shopwoman at Pressburg warning us that no one ever landed here because it belonged to some sort of beings outside man's world!I suppose they believe in fairies and elementals, possibly demons, too. That peasant in the boat saw people on the islands for the first time in his life," he added, after a slight pause, "and it scared him, that's all."The Swede's tone of voice was not convincing, and his manner lacked something that was usually there. I noted the change instantly while he talked, though without being able to label it precisely."If they had enough imagination," I laughed loudly—I remember trying to make as much noise as I could—"they might well people a place like this with the old gods of antiquity. The Romans must have haunted all this region more or less with their shrines and sacred groves and elemental deities."The subject dropped and we returned to our stew-pot, for my friend was not given to imaginative conversation as a rule. Moreover, just then I remember feeling distinctly glad that he was not imaginative; his stolid, practical nature suddenly seemed to me welcome and comforting. It was an admirable temperament, I felt; he could steer down rapids like a red Indian, shoot dangerous bridges and whirlpools better than any white man I ever saw in a canoe. He was a grand fellow for an adventurous trip, a tower of strength when untoward things happened. I looked at his strong face and light curly hair as he staggered along under his pile of driftwood (twice the size of mine!), and I experienced a feeling of relief. Yes, I was distinctly glad just then that the Swede was —what he was, and that he never made remarks that suggested more than they said."The river's still rising, though," he added, as if following out some thoughts of his own, and dropping his load with a gasp. "This island will be under water in two days if it goes on.""I wish the wind would go down," I said. "I don't care a fig for the river."The flood, indeed, had no terrors for us; we could get off at ten minutes' notice, and the more water the better we liked it. It meant an increasing current and the obliteration of the treacherous shingle-beds that so often threatened to tear the bottom out of our canoe.Contrary to our expectations, the wind did not go down with the sun. It seemed to increase with the darkness, howling overhead and shaking the willows round us like straws. Curious sounds accompanied it sometimes, like the explosion of heavy guns, and it fell upon the water and the island in great flat blows of immense power. It made me think of the sounds a planet must make, could we only hear it, driving along through space.But the sky kept wholly clear of clouds, and soon after supper the full moon rose up in the east and covered the river and the plain of shouting willows with a light like the day.We lay on the sandy patch beside the fire, smoking, listening to the noises of the night round us, and talking happily of the journey we had already made, and of our plans ahead. The map lay spread in the door of the tent, but the high wind made it hard to study, and presently we lowered the curtain and extinguished the lantern. The firelight was enough to smoke and see each other's faces by, and the sparks flew about overhead like fireworks. A few yards beyond, the river gurgled and hissed, and from time to time a heavy splash announced the falling away of further portionsof the bank.Our talk, I noticed, had to do with the faraway scenes and incidents of our first camps in the Black Forest, or of other subjects altogether remote from the present setting, for neither of us spoke of the actual moment more than was necessary—almost as though we had agreed tacitly to avoid discussion of the camp and its incidents. Neither the otter nor the boatman, for instance, received the honor of a single mention, though ordinarily these would have furnished discussion for the greater part of the evening. They were, of course, distinct events in such a place.The scarcity of wood made it a business to keep the fire going, for the wind, that drove the smoke in our faces wherever we sat, helped at the same time to make a forced draught. We took it in turn to make some foraging expeditions into the darkness, and the quantity the Swede brought back always made me feel that he took an absurdly long time finding it; for the fact was I did not care much about being left alone, and yet it always seemed to be my turn to grub about among the bushes or scramble along the slippery banks in the moonlight. The long day's battle with wind and water—such wind and such water!—had tired us both, and an early bed was the obvious program. Yet neither of us made the move for the tent. We lay there, tending the fire, talking in desultory fashion, peering about us into the dense willow bushes, and listening to the thunder of wind and river. The loneliness of the place had entered our very bones, and silence seemed natural, for after a bit the sound of our voices became a trifle unreal and forced; whispering would have been the fitting mode of communication, I felt, and the human voice, always rather absurd amid the roar of the elements, now carried with it something almost illegitimate. It was like talking out loud in church, or in some place where it was not lawful, perhaps not quite safe, to be overheard.The eeriness of this lonely island, set among a million willows, swept by a hurricane, and surrounded by hurrying deep waters, touched us both, I fancy. Untrodden by man, almost unknown to man, it lay there beneath the moon, remote from human influence, on the frontier of another world, an alien world, a world tenanted by willows only and the souls of willows. And we, in our rashness, had dared to invade it, even to make use of it! Something more than the power of its mystery stirred in me as I lay on the sand, feet to fire, and peered up through the leaves at the stars. For the last time I rose to get firewood."When this has burnt up," I said firmly, "I shall turn in," and my companion watched me lazily as I moved off into the surrounding shadows.For an unimaginative man I thought he seemed unusually receptive that night, unusually open to suggestion of things other than sensory. He too was touched by the beauty and loneliness of the place. I was not altogether pleased, I remember, to recognize this slight change in him, and instead of immediately collecting sticks, I made my way to the far point of the island where the moonlight on plain and river could be seen to better advantage. The desire to be alone had come suddenly upon me; my former dread returned in force; there was a vague feeling in me I wished to face and probe to the bottom.。

关于世界未解之谜的中考英语作文全文共3篇示例，供读者参考篇1The Unsolved Mysteries That Keep Me Up At NightHave you ever laid awake at night, staring at the ceiling, wondering about the great mysteries of the universe? Maybe that's just me, but I can't stop thinking about all themind-bending, reality-defying mysteries that scientists haven't been able to explain yet. It's like the more we learn, the more questions we have!I first got hooked on unsolved mysteries a few years ago when my parents took me to visit the Bermuda Triangle. As our cruise ship passed through that infamous patch of ocean, the tour guide told us crazy stories about planes and ships that disappeared without a trace after entering the Triangle's boundaries. My eyes went wide as he described the many theories people have come up with to explain the mysterious disappearances, like supernatural forces, rogue waves, and compass anomalies caused by magnetic mysteries. To this day,nobody knows for sure what causes the Bermuda Triangle phenomena! I was officially obsessed.Once I made it home, I immediately Googled "greatest unsolved mysteries" and went down a rabbit hole of the strangest, creepiest, most baffling cases that continue to defy explanation even now, in the 21st century. I learned all about bizarre events like the Dyatlov Pass Incident, where nine experienced hikers were found dead under really weird circumstances in the Russian mountains in 1959. Some of their bodies had traumatic injuries, their camp was wrecked from the inside out, and their clothing contained radioactive contamination. It's so freaky and nobody knows what happened to them!Then there are the ancient archaeological wonders like the Voynich Manuscript, an intricate medieval document written in a language and alphabet that has never been identified, filled with bizarre illustrations of plants and naked people doing strange rituals. How could an entire language system exist that has never been deciphered, even in today's age of advanced cryptography? It boggles the mind!Or what about the dozens of elongated skulls that have been found in parts of Peru, with massively deformed, oblongcraniums that don't resemble anything human? Some researchers think the skulls provide evidence of ancient aliens visiting Earth and interbreeding with humans way back when. Others disagree, arguing that the skulls were just the result of a local cultural practice of binding babies' skulls as they grew. The debate rages on!One of the most bone-chilling unsolved cases is the creepy details surrounding the Lady of Dai, that famous mummified body of an aristocratic woman from ancient China. Even though she lived over 2000 years ago, her corpse is so incrediblywell-preserved that her skin is soft and moist to the touch, and her arms and legs can still bend! Scientists have examined her remains but cannot explain the mysterious chemical compound that seems to have halted her body's decay. It's basically real-life mummy magic.Then you've got more recent unsolved mysteries like the bright night marchers seen wandering along roads in Mawnan, England. Multiple people have reported seeing strange "phantoms" walking in military-style lines wearing strange glowing outfits. Are they ghosts? Aliens? Hallucinations brought on by swamp gas? Nothing has been proven yet. Or what about the Ararat Anomaly—a weird bright spot captured inphotographs taken near the peak of Mount Ararat in Turkey? Believers think it could be the remains of Noah's Ark, while skeptics dismiss it as a strange lensing effect or just weird lighting. We may never know the truth!The list goes on and on when you start digging into these kinds of strange cases. The Mothman creature allegedly spotted in Point Pleasant, West Virginia in the 1960s. The Nazca Lines found etched into the Peruvian desert—could they have been made by ancient aliens as some claim? There's just too many out there to even scratch the surface!Whenever I find myself losing sleep over all these unsolved riddles, I try to remind myself that it's not necessarily a bad thing that some mysteries remain unsolved and unexplained. If we ever DO find out the truth behind all of these cases, things might get boring pretty quickly. It's the not knowing that fuels our imaginations and sparks our curiosity to constantly ask new questions. The great unsolved mysteries are what make our universe such an endlessly fascinating and wondrous place to live in!Maybe one day, if I study hard and pursue a career in science or another investigative field, I could be the one to crack the case on the Zodiac Killer, the Tunguska Event, or the Lost City ofAtlantis! Who knows what other Strange, reality-bending phenomenon are still waiting to be uncovered... An infinite number of cosmic puzzles for a new generation of explorers, scientists, and problem-solvers to dive into. That's one unsolved mystery I can't wait to experience for myself!篇2The Unsolved Mysteries That Boggle My MindHi there! My name is Timmy and I'm 10 years old. Today I wanted to tell you about some of the unsolved mysteries of the world that really make me scratch my head. There's so much we still don't understand about our planet and the universe! It's both fascinating and a little spooky if you ask me.One mystery that has stumped scientists and historians for ages is the disappearance of the Roanoke Colony. Way back in 1587, a group of English settlers landed on Roanoke Island, which is off the coast of what is now North Carolina in the United States. They settled there with the plan of establishing the first permanent English colony in the New World.Everything seemed to be going well at first. But then in 1590, a couple years after they arrived, a supply ship reached the island only to find...nothing! All the colonists had vanished without atrace. Their houses and forts were still standing, but there wasn't a single person around. The only clue was the word "CROATAN" carved into a post, which may have referred to a nearby island or Native American tribe. But nobody knows for sure what happened! Did they get attacked? Did they move locations? It's an eerie 400-year-old cold case.Another perplexing mystery is the existence of the Bermuda Triangle. This is a stretchy of sea between Florida, Puerto Rico, and Bermuda where dozens of ships and planes have disappeared over the years in bizarre circumstances. Personally, I think that area must be haunted by the restless souls of pirates and sea monsters!Some people blame natural phenomena like methane gas bubbles making ships sink or compass problems leading planes astray. But others insist there must be some kind of paranormal or extraterrestrial explanation since so many strange things happen in that one specific area. Maybe it's a portal to another dimension? Maybe aliens use it as a galactic highway? I have no clue, but it sure is creepy!Then there are the Easter Island heads, which have baffled scientists and archaeologists for ages. These huge stone monuments, carved into the shape of giant human heads, werebuilt centuries ago on the remote Easter Island in the Pacific Ocean by people from a ancient civilization we know very little about.What boggles my mind is...how the heck did they build and move those massive, heavy statues using just basic tools and manpower? The largest ones weigh over 80 tons! Whenever I try to move my couch an inch my mom has to help, so I can't imagine how an entire ancient society transported stone heads that outweigh bulldozers. Maybe they had help from aliens? Or maybe, and I'm just spitballing here, they could walk around back then!Another mystery that ties my brain in knots is the existence of crop circles. Have you seen these things? They're giant patterns and designs that appear overnight pressed into farmers' fields, creating intricate circles, spirals, and pictures in the crops. The patterns are so complex and geometrically perfect that it seems impossible they could have been made by human hands and feet, at least not without tons of equipment and planning.Some people claim they've witnessed lights in the sky and heard strange noises around the fields before the crop circles appeared. A few folks have even said they saw actual aliens making them! More likely there's a rational explanation, likeovernight artists using ropes and boards as stencils or freak gusts of wind sculpting the crops. But nobody knows for sure. It sure does make you wonder, doesn't it?Those are just a few of the enduring unsolved mysteries from around the globe that give me the creeps and make me lie awake at night pondering the unknown. From the eerie vanishings of entire societies to strange occurrences in nature that seem to defy explanation, there's so much about our world that we have yet to fully understand.I try not to think about it too much though, or else I'll start seeing UFOs in every shadow! A little mystery keeps life interesting. As long as I have a nightlight on and a stash of fresh batteries for my flashlight, I'll be brave and keep searching for answers. The truth is out there...I think!篇3The Unsolved Mysteries That Boggle My MindHi there! I'm just a regular kid who loves learning about the world around me. But you know what really gets me going? The huge mysteries that nobody has been able to solve yet! There's so much we still don't understand, and it's both exciting and a little spooky to think about.One mystery that has puzzled humans for centuries is the existence of the Bermuda Triangle. This is a region of the Atlantic Ocean where dozens of ships and planes have gone missing without a trace over the years. Some people say it's because of crazy weather, while others blame it on supernatural forces or alien activity! Could you imagine if aliens really were snatching up our vehicles? I'd be equal parts terrified and thrilled.Speaking of aliens, there's the ongoing debate about whether we've been visited by extraterrestrials before. There have been so many reported UFO sightings and even supposed encounters with alien beings. My friend Jacob swears his uncle was abducted by aliens when he was camping in the woods! I'm not fully convinced, but I have to admit the thought of aliens walking among us is pretty cool. What would they look like? Where did they come from? I want to believe!Another mystery that gives me chills is the cases of people who have gone missing in national parks and forests. Often no trace of them is ever found, not even belongings or clothes. It's like they literally vanished into thin air! Park rangers have all sorts of theories, from animal attacks to getting lost and starving. But I prefer to imagine they were whisked away into a paralleluniverse or something equally bizarre. Hey, a guy can dream, right?Then we have all the ancient unsolved mysteries, like the pyramids, Stonehenge, and the Nazca Lines. How did ancient civilizations with such basic technology build monuments and structures that even modern scientists struggle to replicate? Were they getting outside assistance...maybe from aliens again? Or did they simply possess engineering knowledge that has been lost to history? I'd love to travel back in time and get the inside scoop.Cryptids, or creatures whose existence hasn't been proven, are another source of endless fascination for me. Bigfoot, the Loch Ness Monster, El Chupacabra...could they be more than just myths and legends? There are so many alleged sightings and even some fuzzy photos and footage. My skeptical older sister says it's all just people seeing what they want to see. But I choose to keep an open mind. Can you imagine how epic it would be to come face-to-face with a real live Bigfoot? I'd probably faint from excitement!Those are just a few of the endless mysteries that occupy my active imagination. From strange occurrences and paranormal phenomena to ancient riddles and possible undiscoveredcreatures, our world is full of unsolved cases that may never be definitively explained. And you know what? I kinda like it that way. Isn't it fun to use your creativity and let your mind wonder about the great unknowns?Maybe when I'm older and a world-famous scientist or explorer, I'll help solve some of these baffling mysteries myself. But for now, I'll just keep reading about them, watching documentaries, and dreaming up my own fantastical theories. Who knows? Perhaps the key to unraveling the Bermuda Triangle or finding Bigfoot is just waiting to be discovered by a curious kid like me. An unsolved mystery is simply a mystery that's waiting to be solved!。

ships in the desert课文翻译Unit 3 - Ships In The DesertShips In The DesertI was standing in the sun on the hot steel deck of a fishing ship capable of processing a fifty-ton catch on a good day. But it wasn't a good day.We were anchored in what used to be the most productive fishing site in all of central Asia, but as I looked out over the bow, the prospects of a good catch looked bleak.Where there should have been gentle blue-green waves lapping against the side of the ship, there was nothing but hot dry sand-as far as I could see in all directions.The other ships of the fleet were also at rest in the sand, scattered in the dunes that stretched all the way to the horizon.Ten years ago the Aral was the fourth largest inland sea in the world, comparable to the largest of North America's Great Lakes.Now it is disappearing because the water that used to feed it has been diverted in an ill-considered irrigation scheme to grow cotton in the desert.The new shoreline was almost forty kilometers across, the sand from where the fishing fleet was now permanently docked.Meanwhile, in the nearby town of Muynak the people were still canning fish -brought not from the Aral Sea but shipped by rail through Siberia from the Pacific Ocean, more than a thousand miles away.My search for the underlying causes of the environmental crisis has led me to travel around the world to examine and study many of these images of destruction.At the very bottom of the earth, high in the Trans-Antarctic Mountains, with the sun glaring at midnight through a hole in the sky, I stood in the unbelievable coldness and talked with a scientist in the late fall of 1988 about the tunnel he was digging through time.Slipping his parka back to reveal a badly burned face that was cracked and peeling, he pointed to the annual layers of ice in a core sample dug from the glacier on which we were standing.He moved his finger back in time to the ice of two decades ago. "Here's where the U.S. Congress passed the Clean Air Act," he said.At the bottom of the world, two continents away from Washington, D. C., even a small reduction in one country's emissions had changed the amount of pollution found in the remotest and least accessible place on earth.But the most significant change thus far in the earth's atmosphere is the one that began with the industrial revolution early in the last century and has picked up speed ever since.Industry meant coal, and later oil, and we began to burn lots of it-bringing rising levels of carbon dioxide (CO2), with its ability to trap more heat in the atmosphere and slowly warm the earth.Fewer than a hundred yards from the South Pole, upwind from the ice runway where the ski plane lands and keeps its engines running to prevent the metal parts from freeze-locking together, scientists monitor the air several times every day to chart the course of that inexorable change.During my visit, I watched one scientist draw the results of that day's measurements, pushing the end of a steep line still higher on the graph.He told me how easy it is-there at the end of the earth-to see that this enormous change in the global atmosphere is still picking up speed.Two and a half years later I slept under the midnight sun at the other end of our planet, in a small tent pitched on atwelve-foot-thick slab of ice floating in the frigid Arctic Ocean.After a hearty breakfast, my companions and I traveled by snowmobiles a few miles farther north to a rendezvous point wherethe ice was thinner -only three and a half feet thick-and a nuclear submarine hovered in the water below.After it crashed through the ice, took on its new passengers, and resubmerged, I talked with scientists who were trying to measure more accurately the thickness of the polar ice cap, which many believe is thinning as a result of global warming.I had just negotiated an agreement between ice scientists and the U. S. Navy to secure the release of previously top secret data from submarine sonar tracks, data that could help them learn what is happening to the north polar cap.Now, I wanted to see the pole itself, and some eight hours after we met the submarine, we were crashing through that ice, surfacing, and then I was standing in an eerily beautiful snowscape, windswept and sparkling white, with the horizon defined by little hummocks, or "pressure ridges" of ice that are pushed up like tiny mountain ranges when separate sheets collide.But here too, CO2 levels are rising just as rapidly, and ultimately temperature will rise with them-indeed, global warming is expected to push temperatures up much more rapidly in the polar regions than in the rest of the world.As the polar air warms, the ice here will thin; and since then, polar cap plays such a crucial role in the world's weather system, the consequences of a thinning cap could be disastrous.Considering such scenarios is not a purely speculative exercise.Six months after I returned from the North Pole, a team of scientists reported dramatic changes in the pattern of ice distribution in the Arctic, and a second team reported a still controversial claim (which a variety of data now suggest) that, overall, the north polar cap has thinned by 2 percent in just the last decade.Moreover, scientists established several years ago that in many land areas north of the Arctic Circle, the spring snowmelt now comes earlier every year, and deep in the tundra below, the temperature of the earth is steadily rising.As it happens, some of the most disturbing images of environmental destruction can be found exactly halfway between the North and South poles -precisely at the equator in Brazil-where billowing clouds of smoke regularly blacken the sky above the immense but now threatened Amazon rain forest.Acre by acre, the rain forest is being burned to create fast pasture for fast-food beef; as I learned when I went there in early 1989, the fires are set earlier and earlier in the dry season now,with more than one Tennessee's worth of rain forest being slashed and burned each year.According to our guide, the biologist Tom Lovejoy, there are more different species of birds in each square mile of the Amazon than exist in all of North America-which means we are silencing thousands of songs we have never even heard.But one doesn't have to travel around the world to witness humankind's assault on the earth. Images that signal the distress of our global environment are now commonly seen almost anywhere.On some nights, in high northern latitudes, the sky itself offers another ghostly image that signals the loss of ecological balance now in progress.If the sky is clear after sunset-and if you are watching from a place where pollution hasn't blotted out the night sky altogether-you can sometimes see a strange kind of cloud high in the sky.This "noctilucent cloud" occasionally appears when the earth is first cloaked in the evening darkness; shimmering above us with a translucent whiteness, these clouds seem quite unnatural: And they should:noctilucent clouds have begun to appear more often because of a huge buildup of methane gas in the atmosphere.Also called natural gas, methane is released from landfills, from coal mines and rice paddies, from billions of termites that swarm through the freshly cut forestland, from the burning of biomass and from a variety of other human activities.Even though noctilucent clouds were sometimes seen in the past, all this extra methane carries more water vapor into the upper atmosphere, where it condenses at much higher altitudes to form more clouds that the sun's rays still strike long after sunset has brought the beginning of night to the surface far beneath them.What should we feel toward these ghosts in the sky? Simple wonder or the mix of emotions we feel at the zoo?Perhaps we should feel awe for our own power: just as men tear tusks from elephants' heads in such quantity as to threaten the beast with extinction, we are ripping matter from its place in the earth in such volume as to upset the balance between daylight and darkness.In the process, we are once again adding to the threat of global warming, because methane has been one of the fastest-growing greenhouse gases, and is third only to carbon dioxide and water vapor in total volume, changing the chemistry of the upper atmosphere.But, without even considering that threat, shouldn't it startle us that we have now put these clouds in the evening sky which glisten with a spectral light?Or have our eyes adjusted so completely to the bright lights of civilization that we can't see these clouds for what they are-a physical manifestation of the violent collision between human civilization and the earth?Even though it is sometimes hard to see their meaning, we have by now all witnessed surprising experiences that signal the damage from our assault on the environment-whether it's the new frequency of days when the temperature exceeds 100 degrees, the new speed with which the sun burns our skin, or the new constancy of public debate over what to do with growing mountains of waste.But our response to these signals is puzzling. Why haven't we launched a massive effort to save our environment?To come at the question another way: Why do some images startle us into immediate action and focus our attention on ways to respond effectively?And why do other images, though sometimes equally dramatic, produce instead a kind of paralysis, focusing our attention not on ways to respond but rather on some convenient, less painful distraction?Still, there are so many distressing images of environmental destruction that sometimes it seems impossible to know how to absorb or comprehend them.Before considering the threats themselves, it may be helpful to classify them and thus begin to organize our thoughts and feelings so that we may be able to respond appropriately.A useful system comes from the military, which frequently places a conflict in one of three different categories, according to the theater in which it takes place.There are "local" skirmishes, “regional" battles, and "strategic" conflicts. This third category is reserved for struggles that can threaten a nation's survival and must be understood in a global context.Environmental threats can be considered in the same way. For example, most instances of water pollution, air pollution, and illegal waste dumping are essentially local in nature.Problems like acid rain, the contamination of underground aquifers, and large oil spills are fundamentally regional.In both of these categories, there may be so many similar instances of particular local and regional problems occurring simultaneously all over the world that the pattern appears to be global, but the problems themselves are still not truly strategicbecause the operation of the global environment is not affected and the survival of civilization is not at stake.However, a new class of environmental problems does affect the global ecological system, and these threats are fundamentally strategic.The 600 percent increase in the amount of chlorine in the atmosphere during the last forty years has taken place not just in those countries producing the chlorofluorocarbons responsible but in the air above every country, above Antarctica, above the North Pole and the Pacific Ocean-all the way from the surface of the earth to the top of the sky.参考译文——沙漠中的捕鱼船队沙漠中的捕鱼船队我在阳光中站在一艘渔轮的灼热的钢甲板上。

Vol.3, No.1, 65-68 (2011)doi:10.4236/ns.2011.31009Natural ScienceEntropy changes in the clustering of galaxies in an expanding universeNaseer Iqbal1,2*, Mohammad Shafi Khan1, Tabasum Masood11Department of Physics, University of Kashmir, Srinagar, India; *Corresponding Author:2Interuniversity Centre for Astronomy and Astrophysics, Pune, India.Received 19 October 2010; revised 23 November 2010; accepted 26 November 2010.ABSTRACTIn the present work the approach-thermody- namics and statistical mechanics of gravitating systems is applied to study the entropy change in gravitational clustering of galaxies in an ex-panding universe. We derive analytically the expressions for gravitational entropy in terms of temperature T and average density n of the par-ticles (galaxies) in the given phase space cell. It is found that during the initial stage of cluster-ing of galaxies, the entropy decreases and fi-nally seems to be increasing when the system attains virial equilibrium. The entropy changes are studied for different range of measuring correlation parameter b. We attempt to provide a clearer account of this phenomena. The entropy results for a system consisting of extended mass (non-point mass) particles show a similar behaviour with that of point mass particles clustering gravitationally in an expanding uni-verse.Keywords:Gravitational Clustering; Thermodynamics; Entropy; Cosmology1. INTRODUCTIONGalaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation [1]. They form the densest part of the large scale structure of the uni-verse. In models for the gravitational formation of struc-ture with cold dark matter, the smallest structures col-lapse first and eventually build the largest structures; clusters of galaxies are then formed relatively. The clus-ters themselves are often associated with larger groups called super-clusters. Clusters of galaxies are the most recent and most massive objects to have arisen in the hiearchical structure formation of the universe and the study of clusters tells one about the way galaxies form and evolve. The average density n and the temperature T of a gravitating system discuss some thermal history of cluster formation. For a better larger understanding of this thermal history it is important to study the entropy change resulting during the clustering phenomena be-cause the entropy is the quantity most directly changed by increasing or decreasing thermal energy of intraclus-ter gas. The purpose of the present paper is to show how entropy of the universe changes with time in a system of galaxies clustering under the influence of gravitational interaction.Entropy is a measure of how disorganised a system is. It forms an important part of second law of thermody-namics [2,3]. The concept of entropy is generally not well understood. For erupting stars, colloiding galaxies, collapsing black holes - the cosmos is a surprisingly or-derly place. Supermassive black holes, dark matter and stars are some of the contributors to the overall entropy of the universe. The microscopic explanation of entropy has been challenged both from the experimental and theoretical point of view [11,12]. Entropy is a mathe-matical formula. Standard calculations have shown that the entropy of our universe is dominated by black holes, whose entropy is of the order of their area in planck units [13]. An analysis by Chas Egan of the Australian National University in Canberra indicates that the col-lective entropy of all the supermassive black holes at the centers of galaxies is about 100 times higher than previ-ously calculated. Statistical entropy is logrithmic of the number of microstates consistent with the observed macroscopic properties of a system hence a measure of uncertainty about its precise state. Statistical mechanics explains entropy as the amount of uncertainty which remains about a system after its observable macroscopic properties have been taken into account. For a given set of macroscopic quantities like temperature and volume, the entropy is a function of the probability that the sys-tem is in various quantumn states. The more states avail-able to the system with higher probability, the greater theAll Rights Reserved.N. Iqbal et al. / Natural Science 3 (2011) 65-6866 disorder and thus greater the entropy [2]. In real experi-ments, it is quite difficult to measure the entropy of a system. The technique for doing so is based on the thermodynamic definition of entropy. We discuss the applicability of statistical mechanics and thermodynam-ics for gravitating systems and explain in what sense the entropy change S – S 0 shows a changing behaviour with respect to the measuring correlation parameter b = 0 – 1.2. THERMODYNAMIC DESCRIPTION OF GALAXY CLUSTERSA system of many point particles which interacts by Newtonian gravity is always unstable. The basic insta-bilities which may occur involve the overall contraction (or expansion) of the system, and the formation of clus-ters within the system. The rates and forms of these in-stabilities are governed by the distribution of kinetic and potential energy and the momentum among the particles. For example, a finite spherical system which approxi-mately satisfies the viral theorem, contracts slowlycompared to the crossing time ~ ()12G ρ- due to the evaporation of high energy particles [3] and the lack of equipartition among particles of different masses [4]. We consider here a thermodynamic description for the sys-tem (universe). The universe is considered to be an infi-nite gas in which each gas molecule is treated to be agalaxy. The gravitational force is a binary interaction and as a result a number of particles cluster together. We use the same approximation of binary interaction for our universe (system) consisting of large number of galaxies clustering together under the influence of gravitational force. It is important to mention here that the characteri-zation of this clustering is a problem of current interest. The physical validity of the application of thermody-namics in the clustering of galaxies and galaxy clusters has been discussed on the basis of N-body computer simulation results [5]. Equations of state for internal energy U and pressure P are of the form [6]:(3122NTU =-)b (1) (1NTP V=-)b (2) b defines the measuring correlation parameter and is dimensionless, given by [8]()202,23W nb Gm n T r K Tτξ∞=-=⎰,rdr (3)W is the potential energy and K the kinetic energy ofthe particles in a system. n N V = is the average num-ber density of the system of particles each of mass m, T is the temperature, V the volume, G is the universalgravitational constant. (),,n T r ξ is the two particle correlation function and r is the inter-particle distance. An overall study of (),n T r ξ has already been dis-cussed by [7]. For an ideal gas behaviour b = 0 and for non-ideal gas system b varies between 0 and 1. Previ-ously some workers [7,8] have derived b in the form of:331nT b nT ββ--=+ (4) Eq.4 indicates that b has a specific dependence on the combination 3nT -.3. ENTROPY CALCULATIONSThermodynamics and statistical mechanics have been found to be equal tools in describing entropy of a system. Thermodynamic entropy is a non-conserved state func-tion that is of great importance in science. Historically the concept of entropy evolved in order to explain why some processes are spontaneous and others are not; sys-tems tend to progress in the direction of increasing en-tropy [9]. Following statistical mechanics and the work carried out by [10], the grand canonical partition func-tion is given by()3213212,1!N N N N mkT Z T V V nT N πβ--⎛⎫⎡=+ ⎪⎣Λ⎝⎭⎤⎦(5)where N! is due to the distinguishability of particles. Λrepresents the volume of a phase space cell. N is the number of paricles (galaxies) with point mass approxi-mation. The Helmholtz free energy is given by:ln N A T Z =- (6)Thermodynamic description of entropy can be calcu-lated as:,N VA S T ∂⎛⎫=- ⎪∂⎝⎭ (7)The use of Eq.5 and Eq.6 in Eq.7 gives()3120ln ln 13S S n T b b -⎛⎫-=-- ⎪ ⎪⎝⎭- (8) where S 0 is an arbitary constant. From Eq.4 we write()31bn b T β-=- (9)Using Eq.9, Eq.8 becomes as3203ln S S b bT ⎡⎤-=-+⎢⎣⎦⎥ (10)Again from Eq.4All Rights Reserved.N. Iqbal et al. / Natural Science 3 (2011) 65-68 6767()13221n b T b β-⎡⎤=⎢⎣⎦⎥ (11)with the help of Eq.11, Eq.10 becomes as()011ln ln 1322S S n b b b ⎡-=-+-+⎡⎤⎣⎦⎢⎥⎣⎦⎤ (12) This is the expression for entropy of a system consist-ing of point mass particles, but actually galaxies have extended structures, therefore the point mass concept is only an approximation. For extended mass structures we make use of softening parameter ε whose value is taken between 0.01 and 0.05 (in the units of total radius). Following the same procedure, Eq.8 becomes as()320ln ln 13N S S N T N b Nb V εε⎡⎤-=---⎢⎥⎣⎦(13)For extended structures of galaxies, Eq.4 gets modi-fied to()()331nT R b nT R εβαεβαε--=+ (14)where α is a constant, R is the radius of a cell in a phase space in which number of particles (galaxies) is N and volume is V . The relation between b and b ε is given by: ()11b b b εαα=+- (15) b ε represents the correlation energy for extended mass particles clustering gravitationally in an expanding uni-verse. The above Eq.10 and Eq.12 take the form respec-tively as;()()3203ln 111bT b S S b b ααα⎡⎤⎢⎥-=-+⎢⎥+-+-⎢⎥⎣⎦1 (16) ()()()120113ln ln 2111b b b S S n b b ααα⎡⎤-⎡⎤⎢⎥⎣⎦-=-++⎢⎥+-+-⎢⎥⎣⎦1 (17)where2R R εεεα⎛⎫⎛⎫=⎪ ⎪⎝⎭⎝⎭(18)If ε = 0, α = 1 the entropy equations for extended mass galaxies are exactly same with that of a system of point mass galaxies approximation. Eq.10, Eq.12, Eq.16and Eq.17 are used here to study the entropy changes inthe cosmological many body problem. Various entropy change results S – S 0 for both the point mass approxima-tion and of extended mass approximation of particles (galaxies) are shown in (Figures 1and2). The resultshave been calculated analytically for different values ofFigure 1. (Color online) Comparison of isothermal entropy changes for non-point and point mass particles (galaxies) for an infinite gravitating system as a function of average relative temperature T and the parameter b . For non-point mass ε = 0.03 and R = 0.06 (left panel), ε = 0.04 and R = 0.04 (right panel).All Rights Reserved.N. Iqbal et al. / Natural Science 3 (2011) 65-68 68Figure 2. (Color online) Comparison of equi-density entropy changes for non-point and point mass particles (galaxies) for an infinite gravitating system as a function of average relative density n and the parameter b. For non-point mass ε= 0.03 and R = 0.04.R (cell size) corresponding to different values of soften-ing parameter ε. We study the variations of entropy changes S – S0with the changing parameter b for differ-ent values of n and T. Some graphical variations for S – S0with b for different values of n = 0, 1, 100 and aver-age temperature T = 1, 10 and 100 and by fixing value of cell size R = 0.04 and 0.06 are shown. The graphical analysis can be repeated for different values of R and by fixing values of εfor different sets like 0.04 and 0.05. From both the figures shown in 1 and 2, the dashed line represents variation for point mass particles and the solid line represents variation for extended (non-point mass) particles (galaxies) clustering together. It has been ob-served that the nature of the variation remains more or less same except with some minor difference.4. RESULTSThe formula for entropy calculated in this paper has provided a convenient way to study the entropy changes in gravitational galaxy clusters in an expanding universe. Gravity changes things that we have witnessed in this research. Clustering of galaxies in an expanding universe, which is like that of a self gravitating gas increases the gases volume which increases the entropy, but it also increases the potential energy and thus decreases the kinetic energy as particles must work against the attrac-tive gravitational field. So we expect expanding gases to cool down, and therefore there is a probability that the entropy has to decrease which gets confirmed from our theoretical calculations as shown in Figures 1 and 2. Entropy has remained an important contributor to our understanding in cosmology. Everything from gravita-tional clustering to supernova are contributors to entropy budget of the universe. A new calculation and study of entropy results given by Eqs.10, 12, 16 and 17 shows that the entropy of the universe decreases first with the clustering rate of the particles and then gradually in-creases as the system attains viral equilibrium. The gravitational entropy in this paper furthermore suggests that the universe is different than scientists had thought.5. ACKNOWLEDGEMENTSWe are thankful to Interuniversity centre for Astronomy and Astro-physics Pune India for providing a warm hospitality and facilities during the course of this work.REFERENCES[1]Voit, G.M. (2005) Tracing cosmic evolution with clus-ters of galaxies. Reviews of Modern Physics, 77, 207- 248.[2]Rief, F. (1965)Fundamentals of statistical and thermalphysics. McGraw-Hill, Tokyo.[3]Spitzer, L. and Saslaw, W.C. (1966) On the evolution ofgalactic nuclei. Astrophysical Journal, 143, 400-420.doi:10.1086/148523[4]Saslaw, W.C. and De Youngs, D.S. (1971) On the equi-partition in galactic nuclei and gravitating systems. As-trophysical Journal, 170, 423-429.doi:10.1086/151229[5]Itoh, M., Inagaki, S. and Saslaw, W.C. (1993) Gravita-tional clustering of galaxies. Astrophysical Journal, 403,476-496.doi:10.1086/172219[6]Hill, T.L. 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