Large- and Small-Scale Feedback in the Starburst Canters of Galaxies

Howard: And now the Kung Pao Chicken. 这是宫保鸡丁。

-Leonard: Ah, yeah. Wow. 啊，好，哇。

-Raj: Smooth. 厉害。

-Howard: And finally, my Moo Shu Pork. 最后，是我的木须肉。

-Raj: Whoo-hoo! 哇塞!-Howard: Oh, there you have it, gentlemen. Our entire dinner unpacked by robot.好了,先生们,你们都看到了机器人已经把所有饭菜取出来了。

-Raj: And it only took 28 minutes. 仅仅花了28分钟时间。

-Sheldon: Impressive, but we must be cautious.真不错啊，不过我们得小心点。

-Howard: Why? 为什么?-Sheldon: Today, it's a Chinese food retrieval robot. Tomorrow, it travels back in time and tries to kill Sarah Connor.今天，这是个中餐传递机器人，明天，它会及时地穿越时空，回去谋杀Sarah Connor(终结者外传女主人公)。

-Leonard: I don't think that's going to happen, Sheldon.Sheldon 我可不相信会发生这样的事情。

-Sheldon: No one ever does. That's why it happens.。

没人相信所以才会发生啊。

-Penny: Hey. Is the food here? Ooh. What's that?嘿，外卖都到了? 哇，那是什么?-Howard: That, dear lady, is the Wolowitz Programmable Hand, designedfor extravehicular repairs on the International Space Station.专门为国际空间站的舱外维修而设计的。

我们应该谈谈宇宙英文作文英文：The universe is a fascinating topic that has intrigued humans for centuries. From the stars in the sky to the planets in our solar system, there is so much to explore and discover. As for me, I have always been interested in astronomy and the mysteries of the universe.One of the most interesting things about the universe is its sheer size. It's difficult to even comprehend just how vast it is. For example, our own Milky Way galaxy is estimated to contain between 100 and 400 billion stars. And that's just one galaxy out of countless others in the universe.Another fascinating aspect of the universe is the concept of black holes. These are areas in space where the gravitational pull is so strong that nothing, not even light, can escape. It's a mind-boggling concept, but it'salso something that scientists are still trying to understand.Of course, there are also many practical applicationsof studying the universe. For example, understanding the movements of celestial bodies can help us predict thingslike eclipses and meteor showers. And the study of space travel is essential for the future of humanity, as we continue to explore and potentially colonize other planets.Overall, the universe is a topic that never fails to inspire awe and wonder. Whether you're a casual stargazeror a professional astronomer, there is always something new and exciting to learn.中文：宇宙是一个令人着迷的话题，数百年来一直吸引着人类。

DESY02-096ISSN0418-9833 July2002Search for Excited Electrons at HERAH1CollaborationAbstractA search for excited electron(e∗)production is described in which the electroweak decayse∗→eγ,e∗→eZ and e∗→νW are considered.The data used correspond to anintegrated luminosity of120pb−1taken in e±p collisions from1994to2000with the H1detector at HERA at centre-of-mass energies of300and318GeV.No evidence for a signalis found.Mass dependent exclusion limits are derived for the ratio of the couplings to thecompositeness scale,f/Λ.These limits extend the excluded region to higher masses thanhas been possible in previous direct searches for excited electrons.To be submitted toPhysics Letters BC.Adloff33,V.Andreev24,B.Andrieu27,T.Anthonis4,A.Astvatsatourov35,A.Babaev23,J.B¨a hr35,P.Baranov24,E.Barrelet28,W.Bartel10,S.Baumgartner36,J.Becker37,M.Beckingham21,A.Beglarian34,O.Behnke13,A.Belousov24,Ch.Berger1,T.Berndt14,ot26,J.B¨o hme10,V.Boudry27,W.Braunschweig1,V.Brisson26,H.-B.Br¨o ker2,D.P.Brown10,D.Bruncko16,F.W.B¨u sser11,A.Bunyatyan12,34,A.Burrage18,G.Buschhorn25, L.Bystritskaya23,A.J.Campbell10,S.Caron1,F.Cassol-Brunner22,D.Clarke5,C.Collard4, J.G.Contreras7,41,Y.R.Coppens3,J.A.Coughlan5,M.-C.Cousinou22,B.E.Cox21,G.Cozzika9,J.Cvach29,J.B.Dainton18,W.D.Dau15,K.Daum33,39,M.Davidsson20,B.Delcourt26,N.Delerue22,R.Demirchyan34,A.De Roeck10,43,E.A.De Wolf4,C.Diaconu22,J.Dingfelder13,P.Dixon19,V.Dodonov12,J.D.Dowell3,A.Droutskoi23,A.Dubak25,C.Duprel2,G.Eckerlin10,D.Eckstein35,V.Efremenko23,S.Egli32,R.Eichler32, F.Eisele13,E.Eisenhandler19,M.Ellerbrock13,E.Elsen10,M.Erdmann10,40,e,W.Erdmann36, P.J.W.Faulkner3,L.Favart4,A.Fedotov23,R.Felst10,J.Ferencei10,S.Ferron27,M.Fleischer10,P.Fleischmann10,Y.H.Fleming3,G.Fl¨u gge2,A.Fomenko24,I.Foresti37,J.Form´a nek30,G.Franke10,G.Frising1,E.Gabathuler18,K.Gabathuler32,J.Garvey3,J.Gassner32,J.Gayler10,R.Gerhards10,C.Gerlich13,S.Ghazaryan4,34,L.Goerlich6,N.Gogitidze24,C.Grab36,V.Grabski34,H.Gr¨a ssler2,T.Greenshaw18,G.Grindhammer25, T.Hadig13,D.Haidt10,L.Hajduk6,J.Haller13,B.Heinemann18,G.Heinzelmann11,R.C.W.Henderson17,S.Hengstmann37,H.Henschel35,R.Heremans4,G.Herrera7,44,I.Herynek29,M.Hildebrandt37,M.Hilgers36,K.H.Hiller35,J.Hladk´y29,P.H¨o ting2,D.Hoffmann22,R.Horisberger32,A.Hovhannisyan34,S.Hurling10,M.Ibbotson21,C¸.˙Is¸sever7, M.Jacquet26,M.Jaffre26,L.Janauschek25,X.Janssen4,V.Jemanov11,L.J¨o nsson20,C.Johnson3,D.P.Johnson4,M.A.S.Jones18,H.Jung20,10,D.Kant19,M.Kapichine8,M.Karlsson20,O.Karschnick11,J.Katzy10,F.Keil14,N.Keller37,J.Kennedy18,I.R.Kenyon3, C.Kiesling25,P.Kjellberg20,M.Klein35,C.Kleinwort10,T.Kluge1,G.Knies10,B.Koblitz25, S.D.Kolya21,V.Korbel10,P.Kostka35,S.K.Kotelnikov24,R.Koutouev12,A.Koutov8,J.Kroseberg37,K.Kr¨u ger10,T.Kuhr11,mb3,ndon19,nge35,ˇs toviˇc ka35,30,ycock18,E.Lebailly26,A.Lebedev24,B.Leißner1,R.Lemrani10,V.Lendermann10,S.Levonian10,B.List36,E.Lobodzinska10,6,B.Lobodzinski6,10,A.Loginov23,N.Loktionova24,V.Lubimov23,S.L¨u ders37,D.L¨u ke7,10,L.Lytkin12,N.Malden21,E.Malinovski24,S.Mangano36,R.Maraˇc ek25,P.Marage4,J.Marks13,R.Marshall21,H.-U.Martyn1,J.Martyniak6,S.J.Maxfield18,D.Meer36,A.Mehta18,K.Meier14,A.B.Meyer11,H.Meyer33,J.Meyer10,S.Michine24,S.Mikocki6,stead18, S.Mohrdieck11,M.N.Mondragon7,F.Moreau27,A.Morozov8,J.V.Morris5,K.M¨u ller37, P.Mur´ın16,42,V.Nagovizin23,B.Naroska11,J.Naumann7,Th.Naumann35,P.R.Newman3, F.Niebergall11,C.Niebuhr10,O.Nix14,G.Nowak6,M.Nozicka30,B.Olivier10,J.E.Olsson10, D.Ozerov23,V.Panassik8,C.Pascaud26,G.D.Patel18,M.Peez22,E.Perez9,A.Petrukhin35, J.P.Phillips18,D.Pitzl10,R.P¨o schl26,I.Potachnikova12,B.Povh12,J.Rauschenberger11,P.Reimer29,B.Reisert25,C.Risler25,E.Rizvi3,P.Robmann37,R.Roosen4,A.Rostovtsev23, S.Rusakov24,K.Rybicki6,D.P.C.Sankey5,S.Sch¨a tzel13,J.Scheins10,F.-P.Schilling10,P.Schleper10,D.Schmidt33,D.Schmidt10,S.Schmidt25,S.Schmitt10,M.Schneider22,L.Schoeffel9,A.Sch¨o ning36,T.Sch¨o rner25,V.Schr¨o der10,H.-C.Schultz-Coulon7,C.Schwanenberger10,K.Sedl´a k29,F.Sefkow37,V.Shekelyan25,I.Sheviakov24,1L.N.Shtarkov24,Y.Sirois27,T.Sloan17,P.Smirnov24,Y.Soloviev24,D.South21,V.Spaskov8, A.Specka27,H.Spitzer11,R.Stamen7,B.Stella31,J.Stiewe14,I.Strauch10,U.Straumann37, S.Tchetchelnitski23,G.Thompson19,P.D.Thompson3,F.Tomasz14,D.Traynor19,P.Tru¨o l37, G.Tsipolitis10,38,I.Tsurin35,J.Turnau6,J.E.Turney19,E.Tzamariudaki25,A.Uraev23,M.Urban37,ik24,S.Valk´a r30,A.Valk´a rov´a30,C.Vall´e e22,P.Van Mechelen4,A.Vargas Trevino7,S.Vassiliev8,Y.Vazdik24,C.Veelken18,A.Vest1,A.Vichnevski8,K.Wacker7,J.Wagner10,R.Wallny37,B.Waugh21,G.Weber11,D.Wegener7,C.Werner13,N.Werner37, M.Wessels1,G.White17,S.Wiesand33,T.Wilksen10,M.Winde35,G.-G.Winter10,Ch.Wissing7,M.Wobisch10,E.-E.Woehrling3,E.W¨u nsch10,A.C.Wyatt21,J.ˇZ´aˇc ek30,J.Z´a leˇs´a k30,Z.Zhang26,A.Zhokin23,F.Zomer26,and M.zur Nedden251I.Physikalisches Institut der RWTH,Aachen,Germany a2III.Physikalisches Institut der RWTH,Aachen,Germany a3School of Physics and Space Research,University of Birmingham,Birmingham,UK b4Inter-University Institute for High Energies ULB-VUB,Brussels;Universiteit Antwerpen (UIA),Antwerpen;Belgium c5Rutherford Appleton Laboratory,Chilton,Didcot,UK b6Institute for Nuclear Physics,Cracow,Poland d7Institut f¨u r Physik,Universit¨a t Dortmund,Dortmund,Germany a8Joint Institute for Nuclear Research,Dubna,Russia9CEA,DSM/DAPNIA,CE-Saclay,Gif-sur-Yvette,France10DESY,Hamburg,Germany11Institut f¨u r Experimentalphysik,Universit¨a t Hamburg,Hamburg,Germany a12Max-Planck-Institut f¨u r Kernphysik,Heidelberg,Germany13Physikalisches Institut,Universit¨a t Heidelberg,Heidelberg,Germany a14Kirchhoff-Institut f¨u r Physik,Universit¨a t Heidelberg,Heidelberg,Germany a15Institut f¨u r experimentelle und Angewandte Physik,Universit¨a t Kiel,Kiel,Germany16Institute of Experimental Physics,Slovak Academy of Sciences,Koˇs ice,Slovak Republic e,f 17School of Physics and Chemistry,University of Lancaster,Lancaster,UK b18Department of Physics,University of Liverpool,Liverpool,UK b19Queen Mary and Westfield College,London,UK b20Physics Department,University of Lund,Lund,Sweden g21Physics Department,University of Manchester,Manchester,UK b22CPPM,CNRS/IN2P3-Univ Mediterranee,Marseille-France23Institute for Theoretical and Experimental Physics,Moscow,Russia l24Lebedev Physical Institute,Moscow,Russia e25Max-Planck-Institut f¨u r Physik,M¨u nchen,Germany26LAL,Universit´e de Paris-Sud,IN2P3-CNRS,Orsay,France27LPNHE,Ecole Polytechnique,IN2P3-CNRS,Palaiseau,France28LPNHE,Universit´e s Paris VI and VII,IN2P3-CNRS,Paris,France29Institute of Physics,Academy of Sciences of the Czech Republic,Praha,Czech Republic e,i 30Faculty of Mathematics and Physics,Charles University,Praha,Czech Republic e,i31Dipartimento di Fisica Universit`a di Roma Tre and INFN Roma3,Roma,Italy232Paul Scherrer Institut,Villigen,Switzerland33Fachbereich Physik,Bergische Universit¨a t Gesamthochschule Wuppertal,Wuppertal, Germany34Yerevan Physics Institute,Yerevan,Armenia35DESY,Zeuthen,Germany36Institut f¨u r Teilchenphysik,ETH,Z¨u rich,Switzerland j37Physik-Institut der Universit¨a t Z¨u rich,Z¨u rich,Switzerland j38Also at Physics Department,National Technical University,Zografou Campus,GR-15773 Athens,Greece39Also at Rechenzentrum,Bergische Universit¨a t Gesamthochschule Wuppertal,Germany40Also at Institut f¨u r Experimentelle Kernphysik,Universit¨a t Karlsruhe,Karlsruhe,Germany 41Also at Dept.Fis.Ap.CINVESTAV,M´e rida,Yucat´a n,M´e xico k42Also at University of P.J.ˇSaf´a rik,Koˇs ice,Slovak Republic43Also at CERN,Geneva,Switzerland44Also at Dept.Fis.CINVESTAV,M´e xico City,M´e xico ka Supported by the Bundesministerium f¨u r Bildung und Forschung,FRG,under contract numbers05H11GUA/1,05H11PAA/1,05H11PAB/9,05H11PEA/6,05H11VHA/7and 05H11VHB/5b Supported by the UK Particle Physics and Astronomy Research Council,and formerly by the UK Science and Engineering Research Councilc Supported by FNRS-FWO-Vlaanderen,IISN-IIKW and IWTd Partially Supported by the Polish State Committee for Scientific Research,grant no.2P0310318and SPUB/DESY/P03/DZ-1/99and by the German Bundesministerium f¨u r Bildung und Forschunge Supported by the Deutsche Forschungsgemeinschaftf Supported by VEGA SR grant no.2/1169/2001g Supported by the Swedish Natural Science Research Councili Supported by the Ministry of Education of the Czech Republic under the projectsINGO-LA116/2000and LN00A006,by GAUK grant no173/2000j Supported by the Swiss National Science Foundationk Supported by CONACyTl Partially Supported by Russian Foundation for Basic Research,grant no.00-15-965843Among the unexplained features of the Standard Model(SM)of particle physics is the exis-tence of three distinct generations of fermions and the hierarchy of their masses.One possible explanation for this is fermion substructure,with the constituents of the known fermions being strongly bound together by a new,as yet undiscovered force[1,2].A natural consequence of these models would be the existence of excited states of the known leptons and quarks.Assum-ing a compositeness scale in the TeV region,one would naively expect that the excited fermions have masses in the same energy region.However,the dynamics of the constituent level are un-known,so the lowest excited states could have masses of the order of only a few hundred GeV. Electron1-proton interactions at very high energies provide an excellent environment in which to search for excited fermions of thefirst generation.These excited electrons(e∗)could be singly produced through t-channelγand Z boson exchange.Their production cross-section and par-tial decay widths have been calculated using an effective Lagrangian[3,4]which depends on a compositeness mass scaleΛand on weight factors f and f′describing the relative coupling strengths of the excited lepton to the SU(2)L and U(1)Y gauge bosons,respectively.In this model the excited electron can decay to an electron or a neutrino via the radiation of a gauge boson(γ,W,Z)with branching ratios determined by the e∗mass and the coupling parameters f and f′.In most analyses[5–7]the assumption is made that these coupling parameters are of comparable strength and only the relationships f=+f′or f=−f′are considered.If a relationship between f and f′is assumed,the production cross-section and partial decay widths depend on two parameters only,namely the e∗mass and the ratio f/Λ.In this paper excited electrons are searched for in three samples of data taken by the H1 experiment from1994to2000with a total integrated luminosity of120pb−1.Thefirst sample consists of e+p data accumulated from1994to1997at positron and proton beam energies of27.5GeV and820GeV respectively,and corresponds to an integrated luminosity of37pb−1.A search for excited electrons using this sample of data has been previously published[8].The strategy for the selection of events has been modified from the procedures described in[8]to optimize the sensitivity to higher e∗masses.The two other samples were taken from1998 to2000with an electron or positron beam energy of27.5GeV and a proton beam energy of 920GeV.The integrated luminosities of the e−p and e+p samples are15pb−1and68pb−1, pared to previous H1results[8]the analysis presented here benefits from an increase in luminosity by a factor of more than three and an increase of the centre-of-mass energy from300GeV to318GeV.We search for all electroweak decays e∗→eγ,e∗→eZ and e∗→νW,considering the subsequent Z and W hadronic decay modes only.This leads tofinal states containing an electron and a photon,an electron and jets or jets with missing transverse energy induced by the neutrinos escaping from the detector.The Standard Model backgrounds which could mimic such signatures are neutral current Deep Inelastic Scattering(NC DIS),charged current Deep Inelastic Scattering(CC DIS),QED Compton scattering(or Wide Angle Bremsstrahlung W AB),photoproduction processes(γp)and lepton pair production via the two photon fusion process(γγ).The determination of the contribution of NC DIS processes is performed using two MonteCarlo samples which employ different models of QCD radiation.Thefirst was produced withthe DJANGO[9]event generator which includes QEDfirst order radiative corrections based onHERACLES[10].QCD radiation is implemented using ARIADNE[11]based on the ColourDipole Model[12].This sample,with an integrated luminosity of more than10times the exper-imental luminosity,is chosen to estimate the NC DIS contribution in the e∗→eγanalysis.The second sample was generated with the program RAPGAP[13],in which QEDfirst order radia-tive corrections are implemented as described above.RAPGAP includes the leading order QCDmatrix element and higher order radiative corrections are modelled by leading-log parton show-ers.This sample is used to determine potential NC DIS background in the e∗→νW֒→q¯q and e∗→eZ֒→q¯q searches,as RAPGAP describes better this particular phase space domain[8]. For both samples the parton densities in the proton are taken from the MRST[14]parametriza-tion which includes constraints from DIS measurements at HERA up to squared momentum transfers Q2=5000GeV2[15–18].Hadronisation is performed in the Lund string fragmenta-tion scheme using JETSET[19].The modelling of the CC DIS process is performed using the DJANGO program with MRST structure functions.While inelastic W AB is treated using the DJANGO generator,elastic and quasi-elastic W AB is simulated with the W ABGEN[20]event generator.Direct and resolvedγp processes including prompt photon production are generated with the PYTHIA[21]event generator.Finally theγγprocess is produced using the LPAIR generator[22].For the calculation of the e∗production cross-section and to determine the efficiencies,events have been generated with the COMPOS[23]generator based on the cross-section for-mulae given in reference[3]and the partial decay widths stated in reference[4].Initial stateradiation of a photon from the incoming electron is included.This generator uses the narrowwidth approximation(NW A)for the calculation of the production cross section and takes intoaccount the natural width for the e∗decay.For all values of f/Λrelevant to this analysis thisassumption is valid even at high e∗masses where the natural width of the e∗is of the order ofthe experimental resolution.To give an example,for M e∗=250GeV this resolution is equal to7GeV,10GeV,and12GeV for the eγ,eZ andνW decay modes,respectively.All Monte Carlo samples are subjected to a detailed simulation of the response of the H1detector.The detector components of the H1experiment[24]most relevant for this analysis arebriefly described in the following.Surrounding the interaction region is a system of drift andproportional chambers which covers the polar angle2range7◦<θ<176◦.The tracking sys-tem is surrounded by afinely segmented liquid argon(LAr)calorimeter covering the polar angle√range4◦<θ<154◦[25]with energy resolutions ofσE/E≃12%/E⊕2%for hadrons,obtained in test beam measurements[26,27].The tracking system and calorimeters are surrounded by a superconducting solenoid and an iron yoke instrumented with streamer tubes.Backgrounds not related to e+p or e−p collisions are rejected by requiring that a primary interaction vertex be reconstructed within±35cm of the nominal vertex position,by usingfilters based on the event topology and by requiring an event time which is consistent with the interaction time.Electromagnetic clusters are required to havemore than95%of their energy in the electromagnetic part of the calorimeter and to be isolated from other particles[28].They are further differentiated into electron and photon candidates using the tracking chambers.Jets with a minimum transverse momentum of5GeV are recon-structed from the hadronicfinal state using a cone algorithm adapted from the LUCELL schemein the JETSET package[19].Missing transverse energy(E misst )is reconstructed using the vec-tor sum of energy depositions in the calorimeter cells.The analysis presented in this paper is described extensively in[29].The e∗→eγchannel is characterized by two electromagnetic clusters in thefinal state. The main sources of background are the W AB process,NC DIS with photon radiation or a high energyπ0in a jet and the production of electron pairs viaγγfusion.Candidate events are selected with two electromagnetic clusters in the LAr calorimeter of transverse energy greater than20GeV and15GeV,respectively,and with a polar angle between0.1and2.2radians. The sum of the energies of the two clusters has to be greater than100GeV.If this sum is below 200GeV,the background is further suppressed by rejecting events with a total transverse energy of the two electromagnetic clusters lower than75GeV or with more than two tracks spatially associated to one of the clusters.The numbers of events passing the analysis cuts for the SM background processes and for the data in each of the three samples are given in Table1.About half of the background originates from NC DIS events with most of the remainder being due to W AB events.The efficiency for selecting the signal varies from85%for an e∗mass of150GeV to72%for an e∗mass of250GeV.As in all other channels the efficiencies are derived using samples of1000e∗events generated at different e∗masses.The various sources of systematic error are discussed later.Distributions of the invariant mass of the candidate electron-photon pairs of the three data samples together and for the SM expectation are shown in Fig.1a.Sample e−p920GeVChannel SM background SM background SM background e∗→eγ7.2±1.0±0.1 4.0±0.7±0.215.6±1.7±0.4 e∗→eZ֒→q¯q7.1±2.1±2.8 5.6±0.4±1.225.3±1.9±5.5 e∗→νW֒→q¯q 2.4±0.2±0.7 3.9±0.2±0.7 6.1±0.4±1.5 Table1:Number of candidate events observed in the three decay channels with the correspond-ing SM expectation and the uncertainties on the expectation(statistical and systematic error).The e∗→eZ֒→q¯q channel is characterized by an electromagnetic cluster with an associated track and two high transverse energy jets.The analysis for this channel uses a sample of events with at least two jets with a transverse energy above17GeV and16GeV,respectively,and an electron candidate with a transverse energy E e t>20GeV.These two jets and the electron must have polar angles smaller than2.2radians.Furthermore,to avoid possible double counting of events from the e∗→eγchannel,events with two electromagnetic clusters with transverse energies above10GeV and a total energy of the two clusters greater than100GeV are removed. The main SM contribution is NC DIS as photoproduction events do not yield a significant rate of electron candidates with large E e t.For20GeV<E e t<65GeV,a cut is made on the602468101214161820invariant mass (GeV)e v e n t s0510152025invariant mass (GeV)e v e n t s 012345678910invariant mass (GeV)e v e n t s Figure 1:Invariant mass spectra of candidate (a)eγpairs for the e ∗→eγanalysis,(b)elec-tron and two jets for the e ∗→eZ analysis and (c)neutrino and two jets for the e ∗→νW anal-ysis.Solid points correspond to the data and the histogram to the total expectation from different SM processes.electron polar angle.This ranges from θe <1.35for E e t =20GeV to θe <2.2radians for E e t =65GeV and depends linearly on E e t .The dijet invariant mass has to be in the range −15<M jj −M Z <7GeV .If there are more than two jets,the pair with invariant mass closest to the nominal Z boson mass is chosen as the Z candidate.The two jets chosen are ordered such that E jet 1t >E jet 2t .In many SM events the direction of jet 2is close to the proton direction.To ensure that this jet is well measured,an additional cut on its polar angle,θjet 2>0.2radians,7is applied if its transverse momentum is lower than30GeV.For an electron transverse energy 65GeV<E e t<85GeV two jets with an invariant mass M jj>M Z−30GeV are required. At very high transverse energy,E e t>85GeV,the contribution from NC DIS is very low and no further cuts on M jj are needed.The number of events which remain in the data after these cuts are summarized in Table1and compared with the expected SM contribution(mostly NC DIS events).The efficiency for selecting the signal varies from44%for an e∗mass of150GeV to 62%for an e∗mass of250GeV.Distributions of the invariant mass of the electron and the two jets are shown in Fig.1b for data and for the SM expectation.The e∗→νW֒→q¯q channel is characterized by two jets and missing transverse energy E miss t. The main background originates from CC DIS events with a moderate contribution from photo-production,whereas the NC DIS contribution is suppressed for large E misst .The analysis startsfrom a sample of events with at least two jets with transverse energies above17GeV and16GeV,missing transverse energy E misst >20GeV and no isolated electromagnetic cluster withtransverse energy above10GeV.The jets must have a polar angle below2.2radians.Jets in which the most energetic track enters the boundary region between two calorimeter modules and central jets(θ>0.5radians)are required to contain more than two tracks.This cut re-moves NC DIS events in which the scattered electron is misidentified as a jet.Only events with S=V apevents in a mass interval that varies with the width of the expected signal.At M e ∗=150GeV ,a width of the mass interval of 30GeV is chosen for the e ∗→eγdecay mode and 60GeV is chosen for the decay channels with two jets.Systematic uncertainties are taken into account as in [8].The limits on the product of the e ∗production cross-section and the decay branching ratio are shown in Fig.2.As stated in the introduction,most experiments give f/Λlimits under the assumptions f =+f ′and f =−f ′.The H1limits for each decay channel and after combi-nation of all decay channels are given as a function of the e ∗mass in Fig.3a,for the assumption f =+f ′.With this hypothesis the main contribution comes from the e ∗→eγchannel.The values of the limits for f/Λvary between 5×10−4and 10−2GeV −1for an e ∗mass ranging from 130GeV to 275GeV .These results improve significantly the previously published H1limits for e +p [8]collisions.The LEP experiments [5,6]and the ZEUS collaboration [7]have also reported on excited electron searches.Their limits are shown in Fig.3b.The LEP 2experiments have shown results in two mass domains.In direct searches for excited electrons limits up to a mass of about 200GeV are given.Above 200GeV their results are derived from indirect searches only.The H1limit extends the excluded region to higher masses than reached in previous direct searches.Figure 2:Upper limits at 95%confi-dence level on the product of the pro-duction cross-section σand the decaybranching ratio BR for excited elec-trons e ∗in the various electroweakdecay channels,e ∗→eγ(dashed line),e ∗→eZ ֒→q ¯q (dotted-dashedline)and e ∗→νW ֒→q ¯q (dotted line)as a function of the excited electronmass.The signal efficiencies used tocompute these limits have been deter-mined with events generated under theassumption f =+f ′.1010110e * mass (GeV)σ*B R (p b )More generally,limits on f/Λas a function of f ′/f are shown in Fig.4for three e ∗masses (150,200and 250GeV ).It is worth noting that excited electrons have vanishing electromag-netic coupling for f =−f ′.In this case the e ∗is produced through pure Z boson exchange.As a consequence the production cross-section for excited electrons at HERA is much smaller in the f =−f ′case than in the f =+f ′case.For e ∗masses between 150and 250GeV the ratio910101010e* mass (GeV)f /Λ (G e V -1)10101010e* mass (GeV)f /Λ (G e V -1)Figure 3:Exclusion limits on the coupling f/Λat 95%confidence level as a function of the mass of excited electrons with the assumption f =+f ′.(a)Limit for each decay channele ∗→eγ(dashed line),e ∗→eZ ֒→q ¯q (thick dotted-dashed line),e ∗→νW ֒→q ¯q (dotted line)and for the combination of the three channels (full line).It must be noted that a part of the data included in the present result were used to obtain the previous H1limit [8].(b)Comparison of this analysis with ZEUS results [7](dashed line)and LEP 2results on direct searches [5](dotted-dashed line)and on indirect searches [6](dotted line).of the cross-sections for f =+f ′and f =−f ′varies between 170and 900.For high e ∗masses and some values of the couplings,no limits are given because the natural width of the e ∗would become extremely large.In summary,a search for excited electron production was performed using all the e +p and e −p data accumulated by H1between 1994and 2000.No indication of a signal was found.New limits have been established as a function of the couplings and the excited electron mass for the conventional relationship between the couplings f =+f ′.The dependence of the f/Λlimit on the ratio f ′/f has been shown for the first time at HERA.The data presented here restrict excited electrons to higher mass values than has been possible previously in direct searches.We are grateful to the HERA machine group whose outstanding efforts have made and continue to make this experiment possible.We thank the engineers and technicians for their work in constructing and now maintaining the H1detector,our funding agencies for financial support,the DESY technical staff for continual assistance and the DESY directorate for the hospitality which they extend to the non-DESY members of the collaboration.10Figure 4:Exclusion lim-its on the coupling f/Λat 95%confidence level as a function of the ratio f ′/f for three different masses of the e ∗:150GeV (full line),200GeV (dotted line)and 250GeV (dashed line).10101010f //ff / Λ (G e V -1)References[1]H.Harari,Phys.Rep.104(1984)159.[2]F.Boudjema,A.Djouadi and J.L.Kneur,Z.Phys.C 57(1993)425.[3]K.Hagiwara,D.Zeppenfeld and S.Komamiya,Z.Phys.C 29(1985)115.[4]U.Baur,M.Spira and P.M.Zerwas,Phys.Rev.D 42(1990)815.[5]G.Abbiendi et al.[OPAL Collaboration],CERN-EP/2002-043,arXiv:hep-ex/0206061.Submitted to Phys.Lett.B [6]P.Abreu et al.[DELPHI Collaboration],Eur.Phys.J.C 8(1999)41[hep-ex/9811005].[7]S.Chekanov et al.[ZEUS Collaboration],DESY-01-132arXiv:hep-ex/0109018.[8]C.Adloff et al.,[H1Collaboration],Eur.Phys.J.C 17(2000)567[hep-ex/0007035].[9]G.A.Schuler and H.Spiesberger,in:Proceedings of Physics at HERA ,Hamburg 1991,V ol.3,pp.1419-1432.[10]A.Kwiatkowski,H.Spiesberger and H.J.M¨o hring,mun.69(1992)155.11[11]L.L¨o nnblad,mun.71(1992)15.[12]B.Andersson,G.Gustafson,L.L¨o nnblad and U.Pettersson,Z.Phys.C43(1989)625.[13]H.Jung,mun.86(1995)147.[14]A.D.Martin,R.G.Roberts,W.J.Stirling and R.S.Thorne,Eur.Phys.J.C4(1998)463.[hep-ph/9803445].[15]S.Aid et al.,[H1Collaboration],Nucl.Phys.B470(1996)3[hep-ex/9603004].[16]C.Adloff et al.,[H1Collaboration],Nucl.Phys.B497(1997)3[hep-ex/9703012].[17]M.Derrick et al.,[ZEUS Collaboration],Z.Phys.C69(1996)607[hep-ex/9510009].[18]M.Derrick et al.,[ZEUS Collaboration],Z.Phys.C72(1996)399[hep-ex/9607002].[19]T.Sj¨o strand,hep-ph/9508391,LU-TP-95-20,CERN-TH-7112-93-REV,August1995.[20]C.Berger and P.Kandel,Proceedings of the Workshop Monte Carlo generators for HERAPhysics,Hamburg1998-1999,pp.596-600,[hep-ph/9906541].[21]T.Sj¨o strand,mun.82(1994)74.[22]S.P.Baranov,O.D¨u nger,H.Shooshtari and J.A.Vermaseren,Proceedings of Physics atHERA,Hamburg1991,vol.3,pp.1478-1482.[23]T.K¨o hler,Proceedings of Physics at HERA,Hamburg1991,vol.3,pp.1526-1541.[24]I.Abt et al.,[H1Collaboration],Nucl.Instrum.Meth.A386(1997)310;ibid.348.[25]B.Andrieu et al.,[H1Calorimeter Group],Nucl.Instrum.Meth.A336(1993)460.[26]B.Andrieu et al.,[H1Calorimeter Group],Nucl.Instrum.Meth.A350(1994)57.[27]B.Andrieu et al.[H1Calorimeter Group],Nucl.Instrum.Meth.A336(1993)499.[28]A.Sch¨o ning,Untersuchung von Prozessen mit virtuellen und reellen W±-Bosonen amH1-Detektor bei HERA,Ph.D.Thesis,University of Hamburg,1996.https://www-h1.desy.de/publications/theseslist.html[30]T.Carli,Proceedings of the Workshop Monte Carlo generators for HERA Physics1998-1999,pp.185-194,[hep-ph/9906541].[31]P.Bate,High Transverse Momentum2-jet and3-jet Cross-section Measurements in Pho-toproduction,Ph.D.Thesis,University of Manchester,1999.12。

TPO63阅读-2Structure and Composition of Comets原文 (1)译文 (3)题目 (4)答案 (7)背景知识 (8)原文Structure and Composition of Comets①Astronomers now have a fairly good idea of what a comet really is.When it is far from the Sun,it is a very small object only a few kilometers across.It consists mainly of ices(water,methane,ammonia)with bits of dust embedded in it-a kind of dirty ice ball.As it approaches the Sun,radiation from the Sun vaporizes the icy matter and releases some of the dust.This forms a gigantic halo around the ice ball. This halo-called the coma extends out tens of thousands of kilometers from the icy core,which is the nucleus of the comet.Sunlight reflected off the dust particles makes the coma visible to observers on Earth.Ultraviolet radiation from the Sun breaks down the vapor molecules into their constituents.These components can be excited by absorbing radiation from the Sun.In returning to lower-energy states, the excited atoms and ions emit light,contributing to the luminosity of the coma.②When the comet gets even closer to the Sun,one of its most spectacular parts begins to form-the tail.Actually,there are two kinds of tails:the dust tail and the ion tail.The dust tail is produced by the light from the Sun reflecting off the dust particles in the coma.A photon carries momentum.In bouncing off a dust particle, it imparts a tiny,but perceptible,momentum change to the dust particle,driving it away from the coma.As the comet sweeps along its orbit,it leaves a curving trail of dust behind in its path.This visible dust tail can extend for tens or hundreds of millions of kilometers out from the nucleus.The dust tail is characterized by its gently curving shape and its yellowish color.③A different mechanism is responsible for the ion tail.Near the Sun,ultraviolet radiation from the Sun(solar wind)ionizes and excites the atoms in the coma.Asthe solar wind sweeps through the coma,the high-velocity charged particles of the solar wind interact with the electrically charged excited ions in the coma,driving them away from the head of the comet.In returning to lower-energy states,these excited ions emit photons and form a luminous,bluish-colored tail extending out from the comet directly away from the Sun.Since both kinds of tails are produced by radiation streaming out from the Sun,they extend out from the coma in the general direction away from the Sun.A comet may exhibit several tails of each kind.④Although the nucleus is of the order of a few kilometers in size,the diameter of the coma may be tens or hundreds of thousands of kilometers;the tails typically extend out tens or hundreds of millions of kilometers away from the coma.⑤A comet leaves a trail of matter behind it as it moves through the inner solar system.Some of this debris may get strewn across Earth's orbit around the Sun. When Earth passes through this part of its annual path,it sweeps through the dust trail.The particles enter Earth's atmosphere at high velocity.The air friction can cause one of these bits of matter to produce a brief streak of light as it burns up in the atmosphere.⑥Since a comet loses matter on each pass by the Sun,eventually it will be depleted to the point where it is no longer ets that approach the Sun have finite lifetimes.Given the typical sizes of comets and the typical rates at which they lose matter,astronomers have concluded that the lifetimes of comets with orbits that bring them near enough to the Sun to be seen from Earth are very much shorter than the age of the solar system.Where do the new comets come from to replace the old ones that dissipate and vanish from view?⑦Dutch astronomer Jan Oort proposed that a giant cloud of matter left over from the formation of the solar system surrounds the Sun and extends out to about 50,000astronomical units.This cloud contains large chunks of matter like the nuclei of comets.The gravitational influence of a passing star can be sufficient to perturb the orbit of one of these chunks to send it toward the inner solar system and bring it near the Sun.译文彗星的结构和组成①天文学家现在对彗星的真正含义有了相当好的了解。

初三英语太空探索与宇宙科学阅读理解25题1<背景文章>Mars Exploration: Past, Present and FutureMars has always fascinated scientists and space enthusiasts alike. In the past, several missions have been launched to explore the Red Planet. The Viking missions in the 1970s were among the first to land on Mars and conduct scientific experiments.These early missions provided valuable insights into the Martian atmosphere, surface features, and potential for life. Since then, many more missions have been sent to Mars. The Mars Reconnaissance Orbiter, for example, has been studying the planet from orbit, mapping its surface and looking for signs of water.Current research on Mars is focused on understanding its geology, climate, and potential habitability. Scientists are also looking for evidence of past or present life on Mars. Future plans include sending humans to Mars. This would be a major milestone in space exploration.However, there are many challenges to overcome before humans can set foot on Mars. These include the long journey time, radiation exposure, and the need for sustainable life support systems.1. The Viking missions were launched in ____.A. the 1960sB. the 1970sC. the 1980sD. the 1990s答案：B。

extremely large的单词-回复"Extremely Large" is an intriguing topic that encompasses a wide range of aspects. In this article, we will explore various aspects related to extreme largeness, touching upon subjects such as celestial bodies, infrastructure, artworks, and human achievements. Let's dive into the astonishing world of the extraordinarily immense.To begin with, one cannot discuss extreme largeness without mentioning the universe. The universe, with its countless galaxies, stars, and planets, is an unimaginably vast expanse. Scientists estimate that the observable universe spans approximately 93 billion light-years in diameter. This astronomical size leaves one awe-inspired, questioning humanity's place in such a colossal cosmos.Zooming in on celestial objects within the universe, we encounter supermassive black holes. Found at the centers of galaxies, these monstrous entities possess incredible mass, hence the term "supermassive." One such example is the black hole in the M87 galaxy, which was captured by the Event Horizon Telescope in 2019. This black hole has a mass equivalent to 6.5 billion times that of oursun, an astounding size that defies comprehension.Shifting our focus to a terrestrial scenario, infrastructure projects often showcase extreme largeness. One notable example is the Great Wall of China. Spanning over 13,000 miles, this architectural marvel is an enduring symbol of human grandeur and dedication. Built for defensive purposes, the Great Wall exemplifies the colossal efforts undertaken by ancient civilizations.Another striking feat of engineering is the Three Gorges Dam in China. Completed in 2006, it is the world's largest hydroelectric power station. The dam stretches over 1.4 miles in length and stands approximately 600 feet tall. Its construction required the relocation of millions of people and the taming of the mighty Yangtze River. This project stands as a testament to humanity's ability to reshape the natural world on an unprecedented scale.Transitioning from infrastructure to art, we encounter theawe-inspiring dimensions found in various artistic creations. The Sistine Chapel's frescoes by Michelangelo provide an excellent example of artistry on an overwhelmingly large scale. Covering an area of approximately 5,000 square feet, the intricate details andvibrant colors of these frescoes are truly breathtaking.Similarly, the work of Christo and Jeanne-Claude, renowned for their large-scale environmental installations, captivates viewers through sheer size and audacity. One of their most famous projects was "The Floating Piers" on Italy's Lake Iseo. This installation consisted of bright orange walkways extending over three kilometers, inviting visitors to experience the sensation of walking on water. The expansiveness of their installations challenges our perception of physical boundaries and encourages contemplation of the relationship between art and its surroundings.Lastly, human achievements themselves can be seen as manifestations of extreme largeness. From monumental breakthroughs in scientific research to record-breaking athletic feats, individuals continue to push the boundaries of what is considered possible. Think of explorers like Edmund Hillary and Tenzing Norgay, whose summiting of Mount Everest in 1953 marked a momentous achievement in human history. Their perseverance and determination against the immense scale of Earth's highest peak inspire awe and admiration.In conclusion, extreme largeness manifests in various forms throughout our universe and human creations. From themind-boggling expanses of the cosmos to the monumental feats of engineering, art, and human accomplishments, we are constantly confronted with the astonishment that comes with confronting the extraordinary. These examples remind us of the remarkable potential, ingenuity, and resilience of humanity when confronted with seemingly insurmountable scales. The exploration of extreme largeness challenges our perceptions, fuels our imaginations, and compels us to continually aspire towards greatness.。

武汉市2024 届高中毕业生二月调研考试英语试卷养成良好的答题习惯，是决定高考英语成败的决定性因素之一。

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1.What are the speakers probably doing?A.Discussing at work.B.Talking on phoneC.Driving on the way2.What will the man do next?A.Have a dessert.B.Pay the check.C.Ask for a beer.3.What do we know about the hamburger?A.It might go bad.B.It's good-lookingC.It looked funny4.What are the speakers mainly talking about?A.The sceneryB.The transport.C.The weather.5.How does the woman sound in the end?A.Glad.B.Surprised.C.Impatient.第二节(共15小题；每小题1.5分，满分22.5分)听下面5段对话或独白。

a r X i v :a s t r o -p h /0012485v 1 22 D e c 2000The Astrophysical Journal,000:000–000,2001(MS-52562-ApJ)Preprint typeset using L A T E X style emulateapj2[O II]AS A TRACER OF STAR FORMATION the[O ii]/Hαratio on luminosity.The NFGS sample consists of196nearby galax-ies,objectively selected from the CfA redshift catalog(CfA I,Huchra et al.1983)to span a large range of−14to−22in absolute B magnitude without mor-phological bias.The sample should therefore be rep-resentative of the galaxy population in the local uni-verse,subject only to the biases inherent in B selec-tion and the surface brightness limit of the Zwickycatalog.Our data include nuclear and integratedspectrophotometry(covering the range3550–7250at∼6˚A resolution),supplemented by U,B,R surfacephotometry from which we obtained total magni-tudes and colors.For the details of the galaxy se-lection and for the U,B,R photometry,we refer thereader to Jansen et al.(2000a);for a full descriptionof the spectrophotometric data we refer to Jansenet al.(2000b).Of the191normal(non-AGN dominated)galax-ies in the NFGS sample,141show Hαemission in their integrated spectra.We will concentrate,how-ever,on the118galaxies with HαEW≤−10˚A for which the[O ii]/Hαratios are reliable,and on the 85galaxies that also have EW(Hβ)≤−5˚A for which we can derive accurate Balmer decrements to cor-rect the observed emission linefluxes for reddening. Throughout the paper we correct observed emission linefluxes for Galactic extinction,estimated from the H i maps of Burstein&Heiles(1984)and listed in the RC3(de Vaucouleurs et al.1991).The wave-length dependence of the extinction is assumed to follow the optical interstellar extinction law of Nandy et al.(1975)(as tabulated in Seaton1979,but adopt-ing R V=A V/E(B−V)=3.1instead of3.2).Ob-served Balmer emission linefluxes have also been cor-rected for stellar absorption lines.We partially compensated for the stellar absorp-tion underlying the Hβemission by placing the limits of the measurement window well inside the absorp-tion trough,closely bracketing the emission line.We evaluated the residual absorption using the spectra of galaxies with no detectable emission.On average,an additional correction of1˚A(EW)was required.Sim-ilarly,for Hαwe found that a correction of1.5˚A was needed.After correction,afit to the data in a Hβversus Hαplot passes through the origin.Because we restrict our analysis to galaxies with moderate to strong emission lines,we are insensitive to the details of this procedure.This paper is organized as follows.In section2 we evaluate the range in the observed and reddening corrected[O ii]/Hαratio and its dependence on lumi-nosity,and we demonstrate the effect of differences in the excitation state of the interstellar medium(ISM). In§3we relate the observed[O ii]and Hαfluxes Fig.1—The logarithm of the ratio of the observed[O ii] and Hαemission linefluxes versus total absolute B magni-tude.Only data for the118galaxies with strong emission [EW(Hα)<−10˚A]are shown to ensure reliable emission line ratios.The plotting symbols are coded according to morpho-logical type,as indicated.Overlayed is a linear least-squares fit to the data points.The[O ii]/Hαratio decreases systemat-ically from∼1.5at M B=−14to∼0.3at M B=−21.to the ionizingflux and show that these results are consistent with previous studies.We briefly discuss the implications for the use of Hβas SFR tracer in §4.We discuss how to improve SFR estimates and present empirical corrections in§5.The implications for evolutionary studies are discussed in§6.We con-clude with a short summary(§7).2.VARIATION IN THE[O ii]/HαRATIOInfigure1we plot the ratio of the observed[O ii] and Hαemission linefluxes vs.total absolute B mag-nitude for the118galaxies with EW(Hα)<−10˚A. The Balmer emission linefluxes were corrected for stellar absorption,but neither Hαnor[O ii]are cor-rected for internal interstellar reddening.The ob-served[O ii]/Hαratio decreases systematically with increasing galaxy luminosity,from∼1.5at M B=−14 to∼0.3at M B=−21,but the range is large.At M B=−18,only a magnitude fainter than the char-acteristic absolute magnitude of the local luminos-ity function(M∗∼−19.2,de Lapparent,Geller,& Huchra1989;M∗∼−18.8,Marzke et al.1994),the observed[O ii]/Hαratios vary by a factor of∼7.JANSEN,FRANX,&FABRICANT3Fig.2—(a )Logarithm of the observed [O ii ]/H αratio versus total absolute B magnitude.Here and in the following panels we only include the 85galaxies with EW(H β)<−5˚A ,allowing us to measure the Balmer decrement,H α/H β,reliably.The plotting symbols are coded according to morphological type as in Fig.1.(b )Logarithm of the [O ii ]/H αratio after correction for interstellar reddening versus absolute B magnitude.The dependence on galaxy luminosity is greatly reduced after correction for reddening,and both the total scatter and the scatter around the best linear fit through the data has decreased.(c )Logarithm of the observed [O ii ]/H αratio versus color excess E(B −V ),determined from the observed Balmer decrement.The [O ii ]/H αratio decreases as the reddening increases.The line shows the relation expected for the adopted reddening curve.(d )Color excess E(B −V )versus absolute B rger reddening values tend to correspond to higher galaxy luminosities,although the range in E(B −V )at a given galaxy luminosity is large.A linear least-squares fit to the data points is overlayed.2.1.Effects of Reddening by DustTo evaluate the contribution of extinction to the variation in the [O ii ]/H αratio and to the ratio’s de-pendence on galaxy luminosity,we compute the color excess,E(B −V ),from the observed Balmer decre-ment,H α/H β.We assume an intrinsic ratio of 2.85(the Balmer decrement for case B recombination at T=104K and n e ∼102–104cm −3;Osterbrock 1989)and adopt the optical interstellar extinction law of Nandy et al.(1975)(as tabulated in Seaton 1979,but adopting R V =A V /E(B −V )=3.1instead of 3.2).This procedure results in a lower limit to the actual reddening,as the observed Balmer decrement will be4[O II]AS A TRACER OF STAR FORMATIONweighted to the regions of lowest line-of-sight extinc-tion(Kennicutt1998a).Other studies suggest,how-ever,that this approach gives a reasonable estimate of the extinction(e.g.,Calzetti,Kinney,&Storchi-Bergmann1994;Buat&Xu1996),except for the dustiest regions in a galaxy.Figure2a is the same asfigure1except that we include only the85galaxies with EW(Hβ)<−5˚A, where we could measure the Balmer decrement parison offigures1and2a indicates that exclusion of the galaxies with fainter Hβemission is unlikely to bias our discussion.Infigure2b we present reddening corrected ratios of the[O ii]and Hαemission linefluxes versus ab-solute B magnitude on the same scale asfigure2a. The dependence of[O ii]/Hαon galaxy luminosity de-creases markedly after correction for interstellar red-dening:the slope of a linear least-squaresfit to the data changes from0.087dex/mag to0.035dex/mag and both the total scatter and the scatter around the fit decrease from0.18to0.09dex and from0.12to 0.08dex1,respectively.This implies that more than half of the observed scatter in[O ii]/Hαmust be due to dust.The total variation in[O ii]/Hαdecreases from a factor∼7to∼3.Ratios of(observed)equivalent widths behave much like extinction correctedflux ratios,but their dependence on the stellar continuum introduces a de-pendence on the star formation history of the galaxy. If we use EW ratios rather than reddening corrected flux ratios infigure2b the scatter around the bestfit increases(0.11versus0.08dex)and the slope steep-ens slightly(0.043versus0.035dex/mag).To show the effect of reddening on the relative strengths of[O ii]and Hαmore directly,infigure2c we plot the logarithm of the observed[O ii]/Hαratio versus color excess E(B−V).Wefind a clear trend toward lower[O ii]/Hαratios in galaxies with larger interstellar reddening.This trend spans most of the total range in[O ii]/Hα.For reference we overlay the relation expected for the adopted reddening law,as-suming an intrinsic[O ii]/Hαratio of1(i.e.,fixing the zeropoint to log([O ii]/Hα)=0for E(B−V)=0). Although the data points do not exactly follow the expected relation,the correspondence is good.Infigure2d we plot color excess E(B−V)versus absolute B magnitude.More luminous galaxies tend to show more reddening than low luminosity galaxies. The scatter on this trend is very large,however. Thesefigures show that galaxy luminosity,internal reddening and the observed[O ii]/Hαline ratios are linked.The trend of[O ii]/Hαwith reddening has the smaller scatter.Reddening,therefore,is an im-portant factor in the observed trend of[O ii]/Hαwith M B,but it is not uniquely related to galaxy luminos-ity.2.2.Effect of Excitation and MetallicityThe luminosity dependence of the[O ii]/Hαratio does not disappear completely after correction for reddening.Might the variation in the reddening cor-rected[O ii]/Hαratios result from differences in the excitation state and metallicity of the ISM?Differ-ences in excitation and metallicity affect the relative strengths of the[O ii][O iii]and the hydrogen re-combination lines.Figure3a shows the reddening corrected[O ii]/Hβversus[O iii]λλ4959,5007/Hβflux ratios for the85 galaxy subsample.Here,we used Hβrather than Hα(note that Hα=2.85Hβafter reddening correction) to allow a direct comparison of the data with the theoretical predictions of McCall,Rybski,&Shields (1985)for the line ratios of H ii regions in galaxies as a function of metallicity.Along the track,the metal-licity is high at the lower left(low excitation)and low at the upper right(high excitation).The small systematic deviations(∼0.1dex)from the model are in the same sense that McCall et al.(1985)reported for their comparison of the model and observations. The scatter of0.06dex of the data around the model track is consistent with the errors in the data points. The residuals from the model track are not corre-lated with either absolute B magnitude,reddening E(B−V),galaxy color,or Hαemission line strength. The tight coupling between[O ii]/Hαand metal-licity can be demonstrated with the metallicity sensi-tive ratio log{([O ii]+[O iii]λλ4959,5007)/Hβ},com-monly denoted as R23(Pagel et al.1979;Edmunds& Pagel1984),which we use as a quantitative indica-tor of the oxygen abundance(Pagel1997;Kennicutt et al.2000).Infigure3b we plot the reddening cor-rected[O ii]/Hαratios versus R23.R23is computed using reddening correctedflux ratios.The metallic-ity measurements are degenerate for values of R23= log{([O ii]+[O iii])/Hβ}∼>0.75(metallicities lower than∼0.6times solar when adopting the expansion in R23given by Zaritsky,Kennicutt,&Huchra1994). This is indicated in thefigure by a dotted line.The[O ii]/Hαratio is strongly correlated with metallicity.For R23<0.75(where the metallicity de-termination is non-degenerate)we can approximate the dependence on metallicity with a linear relation: log([O ii]/Hα)cor=(0.82±0.03)R23−(0.48±0.02) The scatter around this relation is only0.023dex, consistent with little or no intrinsic error.1Throughout this paper we will express scatter in terms of the Median Absolute Deviation(MAD),defined as1JANSEN,FRANX,&FABRICANT5Fig.3—(a )Reddening corrected [O ii ]/H βversus [O iii ]λλ4959,5007/H βflux ratios.The theoretical sequence of McCall et al.(1985)is overlayed.The scatter of the data around the model curve is consistent with the errors in the data points.Low excitations are found at the lower left of the model curve,high excitations at the upper right.(b )Reddening corrected [O ii ]/H αratio versus the metallicity sensitive index R 23=log {([O ii ]+[O iii ])/H β}.The metallicity determination is degenerate for R 23∼>0.75(indicated by the dotted line).The [O ii ]/H αratio and the oxygen abundance are strongly correlated.The data points for R 23<0.75follow a well defined sequence with little or no intrinsic scatter.(c )R 23versus total absolute B magnitude.The dotted line indicates the degeneracy limit R 23=0.75.Thus,we identify metallicity as the underlying cause of the observed range in [O ii ]/H α,affecting this ratio both indirectly through the differential extinc-tion of the [O ii ]and H αlines,and directly,as shown above.In figure 3c we plot the metallicity sensitive R 23index versus absolute B magnitude to explicitly show the correlation of metallicity with luminosity.ING [O ii ]AND H αAS TRACERS OF THE SFRIn the following discussion we use the reddening corrected H αflux,H αo ,to parameterize the SFR since this H αflux is directly proportional to the ion-izing flux,F ion .In figure 4a we plot the ratio of the observed [O ii ]flux and the ionizing flux (H αo )versus the absolute B magnitude.A clear trend with luminosity is seen:6[O II ]AS A TRACER OF STARFORMATIONFig.4—(a )Logarithm of the ratio of observed [O ii ]flux to ionizing flux versus total absolute B magnitude.The zeropoint on the ordinate is arbitrary (see text).The plotting symbols are coded according to morphological type as in figure 1.The total range in [O ii ]obs /H αo spans 1.5orders of magnitude.(b )Logarithm of the ratio of observed H αflux to ionizing flux versus M B .Imperfect correction for reddening is a much more serious problem when using [O ii ](see figure 2a )than when using H α.the lowest luminosity galaxies have the largest ratio of [O ii ]to ionizing flux.Near the characteristic absolute B magnitude of the local galaxy luminosity function,M ∗∼−19,the [O ii ]/H αo ratio varies by a factor of ∼10.The total range spans a factor 25.The SFR derived from observed [O ii ]fluxes,in the absence of other information,will be uncertain by a factor of ∼5,if the calibration of Kennicutt (1992)and the median absolute magnitude of his sample is used as a reference (see section 3.1).Using reddening cor-rected instead of observed [O ii ]fluxes (see figure 2b )greatly reduces both the luminosity dependence and the scatter at a given luminosity:from 0.147to 0.035dex/mag and from 0.19dex to 0.08dex,respectively.The total range of the reddening corrected [O ii ]/H αo ratio spans 0.53dex (0.49dex,if we exclude one out-lying data point).As expected,reddening is a more significant source of error if [O ii ]rather than H αflux is used as a SFR tracer.In figure 4b we plot the ratio of the observed H αflux and the ionizing flux versus the absolute B magnitude for comparison.The total range in the ratio of the uncorrected and corrected H αflux spans 0.65dex (a factor of ∼4.5),less than one-fifth that found for [O ii ].But here as well reddening produces a trend with galaxy luminosity of 0.060dex/mag.parison with the LiteratureWe compare our measurements for the NFGS sam-ple with those of Kennicutt (1992a)in order totie them to previous absolute calibrations of [O ii ]and H αas SFR tracers.In figure 5we plot ob-served ([O ii ]+[O iii ]λ5007)/H αflux ratios versus ob-served [O iii ]λ5007/[O ii ]flux ratios for both samples.Whereas [O iii ]λ5007/[O ii ]is an excitation sensitive ratio,([O ii ]+[O iii ]λ5007)/H αis sensitive mainly to the oxygen abundance.The galaxies in both sam-ples follow similar trends.Gallagher et al.(1989)did not publish [O iii ]λ5007measuments for their sample,so we are unable to make a similar comparison with their sample.We did however compare the observed [O ii ]/H βflux ratios versus absolute B magnitude for both the NFGS and the Gallagher et al.(1989)sam-ple and found them to be consistent with one another.Kennicutt (1992a)noted that his SFR calibration for the observed L ([O ii ])emission line luminosity ex-ceeded that of Gallagher et al.(1989)by a factor of 3.Adopting the same IMF and conversion factor from H αto SFR for both samples reduces this difference to a factor of 1.57(Kennicutt 1998b).The remain-ing discrepancy he ascribed to excitation differences between the two samples.The median M B of Kennicutt’s sample is −19.3±1.4mag,that of Gallagher et al.is −16.9±2.0.Tak-ing this magnitude difference at face value,we ex-pect (see §3)a difference in the logarithm of the [O ii ]/H αo ratio of 2.4mag ·0.147dex/mag =0.353dex,i.e.,a factor of 2.25.This factor has the correct sign,i.e.,the SFR inferred from a given [O ii ]/H αo ratio is higher for Kennicutt’s sample than for the sample of Gallagher et al.The predicted differenceJANSEN,FRANX,&FABRICANT7Fig.5—Logarithm of the observed ([O ii ]+[O iii ]λ5007)/H αflux ratio versus the logarithm of the observed [O iii ]λ5007/[O ii ]ratio in the sample of Kennicutt (1992a)(labeled K’92)and in the NFGS sample (including the 118galaxies with EW(H α)<−10˚A ).The abscissa is an excitation sensitive ratio,whereas the ordinate is sensitive mainly to the oxygen abun-dance.The data points in both samples follow similar trends.Those data points in the present sample with errors in excess of 0.10dex are indicated by upper limits.is,however,somewhat larger than that actually ob-served.This might be due to the large scatter in the trends with M B ,or to the fact that Kennicutt could not account for the systematic variations in absorp-tion.4.IMPLICATIONS FOR H βAS A SFR TRACERIn section 2we showed that the observed [O ii ]/H αratios strongly depend on the amount of interstellar reddening.H βwill be seriously affected by redden-ing as well.Here we test H βas a quantitative star formation indicator.In figure 6a we plot the ratio of observed H βflux and the ionizing flux versus total absolute B mag-nitude for the 85galaxies with reliable H βmeasure-ments.The H β/H αo ratio varies systematically with luminosity,although not as strongly as the [O ii ]/H αo ratio.The slope of a linear fit to the data is 0.090dex/mag and the scatter around the fit is 0.132dex.The H β/H αo ratio decreases from 0.37at M B =−14to ∼0.09at M B =−21.In most cases where H βcan be secured,the [O iii ]λλ4959,5007lines will be available as well.Fig-ure 6b shows the ratio of the observed H βflux andthe ionizing flux versus the observed [O iii ]/H βra-tio.It is remarkable that for high values of [O iii ]/H βthe absorption is generally very small.The SFR can therefore be estimated relatively well,with a conver-sion constant very different from that for L ∗galax-ies.The scatter increases drastically for lower values of [O iii ]/H β,and the extinction becomes much more significant.When no direct reddening measurement is available (e.g.,when none of the other Balmer lines can be measured reliably)the ionizing flux can be re-trieved to ±0.12dex using the observed H βand [O iii ]fluxes for log([O iii ]/H β)∼>0.2.At smaller observed [O iii ]/H βratios figure 6b will be useful to estimate the likely range in ionizing flux corresponding to a H βmeasurement.5.IMPROVED SFR ESTIMATION AND EMPIRICALCORRECTIONSIn this section our goal is to derive empirical correc-tions to relate the observed [O ii ]emission line flux to the ionizing flux in those cases where H βis not avail-able.We start with the situation where only [O ii ]fluxes are available.As we found above,the correla-tion between absolute magnitude and [O ii ]/H αo (fig-ure 4a )gives an indication of whether a high or low normalization of the [O ii ]–SFR calibration is likely,and may be used to give a relative weight to a set of several normalizations as published in the literature.For instance,at low luminosities the lower normal-ization of Gallagher et al.(1989)for blue irregular galaxies is likely to be more appropriate.Kennicutt’s (1992a)higher normalization,on the other hand,will be a better choice for luminous galaxies.The scat-ter at a given absolute magnitude is a measure of the errors involved in the choice of calibration constant.The [O ii ]equivalent width,EW([O ii ]),can be used to estimate the likely range in the [O ii ]/H αo ratio.In figure 7a we plot the logarithm of the ratio of the observed [O ii ]flux and the reddening corrected H αflux versus the logarithm of (the nega-tive of)the EW([O ii ]).For EW([O ii ])≤−45˚A [i.e.,log {EW([O ii ])}≥1.65]the scatter in [O ii ]/H αo at a given EW([O ii ])is only 0.064dex and the ion-izing flux (and therefore the SFR)can be deter-mined well.The [O ii ]/H αo ratios for these large EW([O ii ])are high (1.04±0.30),and the star forma-tion would be overestimated by a factor of (1.7±0.5)if the SFR normalization for luminous galaxies were used.EW([O ii ])≤−45˚A are found predominantly in the lower luminosity,lower metallicity galaxies.For smaller EW([O ii ])we find a trend toward lower [O ii ]/H αo ratios,but the scatter in [O ii ]/H αo in-creases to an order of magnitude.We note that the lack of galaxies with small EW([O ii ])and large8[O II ]AS A TRACER OF STARFORMATIONFig.6—(a )Logarithm of the ratio of observed H βflux to the ionizing flux (reddening corrected H αflux)versus absolute B magnitude (including the 85galaxies with reliable H βmeasurements).The plotting symbols are coded according to morphological type as in figure 1.Upper limits are used to indicate data points with errors larger than 0.15dex.A linear least-squares fit to the data points is overlayed.The dotted line indicates the intrinsic Balmer decrement,H α/H β=2.85.As was seen for the [O ii ]/H αo ratio (figure 4a ),reddening creates a dependence on galaxy luminosity.(b )Logarithm of the ratio of the observed H βflux to the ionizing flux versus the logarithm of the observed [O iii ]/H βratio.These data can be used to estimate the ionizing flux and its error when both H βand [O iii ]can be measured reliably,but a direct reddening measurement is not available.For log([O iii ]/H β)∼>0.2little reddening is seen and the ionizing flux can be estimated accurately.[O ii ]/H αo ratios may be due to a selection effect:these galaxies are expected to have small EW(H β)and will drop out of the sample with reliable redden-ing corrections.Therefore,the scatter in [O ii ]/H αo ratio for small EW([O ii ])may be underestimated in figure 7a .Broadband colors help to distinguish galaxies that are particularly dusty.In figures 7b and c we plot the logarithm of the ratio of the observed [O ii ]flux to the reddening corrected H αflux versus the effec-tive (U −B )and (B −R )color,respectively.Our ef-fective colors are the average colors measured within the half-light radius in B .Whereas the range in [O ii ]/H αo for galaxies bluer than (U −B )e ∼−0.3or (B −R )e ∼0.95is only ∼0.5dex (a scatter of ∼0.12dex),redder galaxies occupy nearly the full range of observed [O ii ]/H αo .Again,the galaxies dominated by star formation have the highest [O ii ]/H αo ratios.The trends shown here are purely empirical,and the correlations do not have direct physical causes.It remains to be seen if they hold at higher redshifts.6.IMPLICATIONS FOR HIGHER REDSHIFTSIntermediate and high redshift spectroscopic studies like the Hawaii Deep Survey (Cowie et al.1994;Songaila et al.1994;Cowie et al.1996)and the Canada-France Redshift Survey (CFRS;Lilly et al.1995;Hammer et al.1997)have shown that the fraction of relatively bright galaxies with EW([O ii ])<−15˚A increases strongly with redshift.Hammer et al.(1997)showed that converting [O ii ]into a SFR using Kennicutt’s (1992)calibration leads to a production of long-lived stars in excess of the number observed at the present ing the Gallagher et al.(1989)calibration for blue galaxies instead implies that 75%of the present-day mass in stars have been produced since z ∼1.The lumi-nosities sampled in the CFRS survey are comparable to those in the present sample.Hammer et al.(1997)speculate that excitation and/or dust play a role,and our results confirm this.Cowie et al.(1997)found remarkably little extinc-tion in the bulk of the blue star forming galaxies at redshifts z >0.8.The results by Pettini et al.(1998)for a small sample of z ∼3Lyman break galaxies are consistent with this general picture,as their UV selected galaxies generally show low absorption com-pared to our L ∗galaxies.The one possible excep-tion,DSF 2237+116C2,is the reddest,most massive galaxy in their sample.These results indicate that the correlation of reddening and excitation with lu-minosity may change with look-back time,but the colors and equivalent widths might well remain effec-tive indicators of reddening.JANSEN,FRANX,&FABRICANT9Fig.7—(a )Logarithm of the ratio of the observed [O ii ]flux to the ionizing flux versus the logarithm of (the negative of)the [O ii ]equivalent width.Plotting symbols are coded according to morphological type as in figure 1.Upper limits are used to indicate data points with errors larger than 0.15dex.The upper left region may be depleted due to selection effects.(b )Logarithm of the ratio of the observed [O ii ]flux to the ionizing flux versus effective rest-frame (U −B )color.(c )Same as (b )but using effective rest-frame (B −R )color.The uncertainties in the SFR derived from [O ii ]are of the order of a factor of 3if no other information is available.7.SUMMARYWe have used spectrophotometry for 85emission line galaxies from the Nearby Field Galaxy Survey sample to investigate the dependence of the [O ii ]/H αratio on galaxy luminosity.Reddening and excitation differences are the main cause of the anti-correlation between [O ii ]/H αand galaxy luminosity.Both are strongly correlated with absolute magnitude and are likely caused by systematic variation in metallicity as a function of galaxy luminosity.Excitation models match the dust corrected line ratios within the accu-racy of our data.The total variation in the ratio of the observed [O ii ]flux to the reddening corrected H αflux is a factor 25.The observed difference between the [O ii ]–SFR calibrations of Gallagher et al.(1989)and Kenni-cutt (1992a)is in the sense expected from the differ-ence in galaxy luminosity between the two samples.We confirm the conjecture of Hammer et al.(1997)that systematic variations in reddening by dust play an important role when interpreting emission line strengths in terms of SFRs.When corrections for metallicity and dust are not possible,the use of [O ii ]fluxes to measure star forma-tion rates may result in an overestimate of a factor of 3if local calibrations for luminous galaxies are used.We show that H βis a significantly better tracer of star formation than [O ii ],and we discuss some em-pirical trends which may be useful in the high redshift regime.ACKNOWLEDGEMENTSThis work was supported by grants from the Uni-versity of Groningen,the Leiden Kerkhoven-Bosscha Fund,the Netherlands Organisation for Scientific Re-search (NWO),and by the Smithsonian Institution.We thank the CfA TAC for generously allocating time for this project over three years.R. A.J.thanks the Harvard-Smithsonian Center for Astro-physics and the F.L.Whipple Observatory for hos-pitality during numerous visits,when all of the obser-vations and part of this work were carried out,and ESA’s ESTEC where this work was completed.We thank the referee,Dr.J.S.Gallagher,for his thought-ful comments that helped improve the manuscript.10[O II]AS A TRACER OF STAR FORMATIONREFERENCESBuat,V.,and Xu,C.1996,A&A,306,61Burstein,D.,&Heiles,C.1984,ApJS,54,33Calzetti, D.,Kinney,A.L.,and Storchi-Bergmann,T.1994, ApJ,429,582Cowie,L.L.,Gardner,J.P.,Hu,E.M.,Songaila,A.,Hodapp, K.-W.,&Wainscoat,R.J.1994,ApJ,434,114Cowie,L.L.,Songaila,A.,Hu,E.M.,&Cohen,J.G.1996, AJ,112,839Cowie,L.L.,Hu,E.M.,Songaila,A.,&Egami,E.1997,ApJ, 481,L9de Lapparent,V.,Geller,M.J.,&Huchra,J.P.1989,ApJ, 343,1de Vaucouleurs,G.,de Vaucouleurs,A.,Corwin,H.G.Jr., Buta,R.J.,Paturel,G.,&Fouqu´e,P.1991,Third Refer-ence Catalog of Bright Galaxies,Springer-Verlag,New York (RC3)Edmunds,M.G.,&Pagel,B.E.J.1984,MNRAS,211,507 Gallagher,J.S.,Bushouse,H.,&Hunter,D.A.1989,AJ,97, 700Hammer,F.,Flores,H.,Lilly,S.J.,Crampton,D.,Le F`e vre, O.,Rola,C.,Mallen-Ornelas,G.,Schade,D.,&Tresse,L. 1997,ApJ,481,49Huchra,J.P.,Davis,M.,Latham,D.,&Tonry,J.1983,ApJS, 52,89(CfA I)Jansen,R.A.,Franx,M.,Fabricant,D.G.,&Caldwell,N. 2000a,ApJS,126,271Jansen,R.A.,Fabricant,D.G.,Franx,M.,&Caldwell,N. 2000b,ApJS,126,331Kennicutt,R.C.,Jr.1992a,ApJ,388,310Kennicutt,R.C.,Jr.1992b,ApJS,79,255Kennicutt,R. C.1998a,in:”The Next Generation SpaceTelescope:Science Drivers and Technological Challenges”(ESA Conference Publications,34th Li`e ge Astrophysics Col-loquium),81Kennicutt,R.C.1998b,ARA&A,36,189Kennicutt,R.C.,Jr.,Bresolin,F.,French,H.,&Martin,P. 2000,ApJ,537,589Lilly,S.J.,Le F`e vre,O.,Crampton,D.,Hammer,F.,&Tresse, L.1995,ApJ,455,50Marzke,R.O.,Huchra,J.P.,&Geller,M.J.1994,ApJ,428, 43McCall,M.L.,Rybski,P.M.,&Shields,G.A.1985,ApJS, 57,1Nandy,K.,Thompson,G.I.,Jamar,C.,Monfils,A.,&Wilson, R.1975,A&A,44,195Osterbrock,D.E.1989,Astrophysics of Gaseous Nebulae and Active Galactic Nuclei(Mill Valley,CA:University Science Books)Pagel,B.E.J.,Edmunds,M.G.,Blackwell,D.E.,Chun,M. S.,&Smith,G.1979,MNRAS,189,95Pagel,B.E.J.1997,Nucleosynthesis&Chemical Evolution of Galaxies(Cambridge:Cambridge University Press) Pettini,M.,Kellogg,M.,Steidel,C.C.,Dickinson,M.,Adel-berger,K.L.,&Giavalisco,M.1998,ApJ508,539 Seaton,M.J.1979,MNRAS,187P,73Songaila,A.,Cowie,L.L.,Hu,E.M.,&Gardner,J.P.1994, ApJS94,461Tresse,L.,Maddox,S.,Loveday,J.,&Singleton,C.1999,MN-RAS310,262Zaritsky,D.,Kennicutt,R.C.,Jr.,&Huchra,J.P.1994,ApJ, 420,87。