Accepted forpublicationin the Astrophysical Journal PreprinttypesetusingLATEXstyleemulateapjv.6/22/04 THE MORPHOLOGY–DENSITY RELATION IN Z 1 CLUSTERS ∼ M. Postman1, M. Franx2, N.J.G. Cross3, B. Holden4, H.C. Ford3, G.D. Illingworth4, T. Goto3, R. Demarco3, P. Rosati5, J.P. Blakeslee3, K.-V. Tran6, N. Ben´ıtez3, M. Clampin7, G.F. Hartig1, N. Homeier3, D.R. Ardila3, F. Bartko8, R.J. Bouwens4, L.D. Bradley3, T.J. Broadhurst9, R.A. Brown1, C.J. Burrows10, E.S. Cheng11, P.D. Feldman3, D.A. Golimowski3, C. Gronwall12, L. Infante13, R.A. Kimble7, J.E. Krist1, M.P. Lesser14, A.R. Martel3, S. Mei3, F. Menanteau3, G.R. Meurer3, G.K. Miley2, V. Motta13, M. Sirianni1, W.B. Sparks1, H.D. Tran15, Z.I. Tsvetanov3, R.L. White1 & W. Zheng3 Accepted for publication in the Astrophysical Journal ABSTRACT 5 We measure the morphology–density relation (MDR) and morphology-radius relation (MRR) for 0 galaxies in seven z 1 clusters that have been observed with the Advanced Camera for Surveys on 0 ∼ boardthe Hubble Space Telescope. Simulations and independent comparisonsof our visually derived 2 morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies n downto z =24,correspondingto L/L∗=0.21and0.30atz =0.83andz =1.24,respectively. We 850 a adoptdensityandradiusestimationmethodsthatmatchthoseusedatlowerredshiftinordertostudy J the evolution of the MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and 4 that observed at z 0, consistent with recent work – specifically, the growth in the bulge-dominated 1 galaxy fraction, f ∼ , with increasing density proceeds less rapidly at z 1 than it does at z 0. E+S0 Atz 1andΣ 500galaxiesMpc−2,wefind=0.72 0.10. Atz∼ 0,anE+S0popula∼tion 2 ∼ ≥ ± ∼ fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is v confinedtodensitiesΣ>40galaxiesMpc−2 andappearstobeprimarilyduetoadeficitofS0galaxies 4 andanexcess ofSpiral+∼Irrgalaxiesrelativeto the localgalaxypopulation. Thef – density relation 2 E exhibits no significant evolution between z = 1 and z = 0. We find mild evidence to suggest that 2 the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications 1 for the evolution of the disk galaxy population in dense regions are discussed in the context of these 0 5 observations. 0 Subject headings: galaxies: clusters: general—galaxies: formation— galaxies: evolution— galaxies: / structure h p - 1. INTRODUCTION tivityasafunctionoftime,andconstraintherelativesig- o nificance of the effects of environmentalprocessesversus r The study of the origin and evolution of the morpho- t logical distribution of galaxies in different environments conditions at the epoch of their formation in establish- s inggalacticstructure. Instandardhierarchicalclustering a canrevealimportantinformationaboutinternalgalactic : stellar and gas dynamics, the state of star formation ac- models, galaxies in high density regions of the Universe, v suchas in the centralregionsof galaxyclusters, will col- i X 1SpaceTelescopeScienceInstitute,3700SanMartinDrive,Bal- lapse earlier and may evolve more rapidly than galax- timore,MD21218. ies in low density regions (Kauffmann 1995; Benson et ar 2 Leiden Observatory, Postbus 9513, 2300 RA Leiden, Nether- al. 2001, Heavens et al. 2004). In addition, galaxies in lands. dense environments are subject to a variety of external 3 Department of Physics and Astronomy, Johns Hopkins Uni- versity,3400N.CharlesStreet, Baltimore,MD21218. stresses,whichare,ingeneral,notconducivetothemain- 4 UCO/Lick Observatory, Universityof California,Santa Cruz, tenanceofspiralstructure. Theseprocessesincluderam- CA95064. pressure stripping of gas (Gunn & Gott 1972; Farouki 5EuropeanSouthernObservatory,Karl-Schwarzschild-Strasse2, & Shapiro 1980; Kent 1981; Fujita & Nagashima 1999; D-85748Garching,Germany. 6InstituteforAstronomy,ETHZu¨rich,Scheuchzerstrasse7,CH- Abadi, Moore & Bower 1999; Quilis, Moore & Bower 8093Zrich,Switzerland. 2000), galaxy harassment via high speed impulsive en- 7 NASA Goddard Space Flight Center, Code 681, Greenbelt, counters (Moore et al. 1996, 1999; Fujita 1998), cluster MD20771. 8 Bartko Science & Technology, 14520 Akron Street,Brighton, tidal forces (Byrd & Valtonen 1990; Valluri 1993; Fujita CO80602. 1998) which distort galaxies as they come close to the 9RacahInstituteofPhysics,TheHebrewUniversity,Jerusalem, center, interaction/merging (Icke 1985; Lavery & Henry Israel91904. 1988, Mamon 1992; Makino & Hut 1997; Bekki 1998), 10 MetaJivaScientific. 11 Conceptual Analytics, LLC, 8209 Woburn Abbey Road, andremovalandconsumptionofthegasdue totheclus- GlennDale,MD20769 terenvironment(Larson,Tinsley&Caldwell1980;Bekki 12 DepartmentofAstronomyandAstrophysics,ThePennsylva- et al. 2002). niaState University,525DaveyLab,UniversityPark,PA16802. Two of key relationships that must be understood in 13 DepartmentodeAstronom´ıayAstrof´ısica,PontificiaUniver- sidadCat´ølicadeChile,Casilla306,Santiago 22,Chile. the context of the above processes are the relative pop- 14 Steward Observatory, University of Arizona, Tucson, AZ ulation fraction of the different morphological classes 85721. as functions of the local galaxy density and their loca- 15 W. M. Keck Observatory, 65-1120 Mamalahoa Hwy., Ka- tion within the local gravitational potential well. The muela,HI96743 morphology – density relation (hereafter MDR) and 2 Postman et al. the morphology – radius relation (hereafter MRR) have usedtoestimatethelocalprojecteddensity,andsection5 been well studied at low-z (Dressler 1980 - hereafter presents our derived MDR and MRR. An assessment of D80; Postman & Geller 1984 - hereafter PG84; Whit- theimplicationsoftheseresultsisgiveninsection6anda more & Gilmore 1991; Goto et al. 2003a) and quantify summaryofthe essentialresultsis providedin section7. many long-standing observations showing a preference Twoappendices discuss details associatedwith the com- for spheroidalsystems to residein dense regions(or per- putationpopulationfractionsthataresuitablycorrected haps better stated as a significant lack of spiral galaxies for contamination and incompleteness, the robustness of in dense regions). A full understanding of how such a our density estimators, and the validity of using com- cosmic arrangement came to be requires measuring the positesamplestoenhancethesignal-to-noiseratiointhe evolution of the MDR and MRR. Such an evolutionary derived MDR and MRR. We adopt H = 70 km s−1 o study is only possible using the high angular resolution Mpc−1, Ω =0.3, and Ω =0.7 for the computation of m Λ provided by the Hubble Space Telescope (HST). Several allintrinsicquantitiesunlessspecificallyindicatedother- pioneering works have now shed light on this evolution wise. (Dressler et al. 1997– hereafter D97; Fasano et al. 2000; 2. OBSERVATIONS Treu et al. 2003; Smith et al. 2004 – hereafter Sm04). D97 andFasanoetal.(2000)finda significantdecline in The clusters included in this study, along with a sum- the fraction of lenticular galaxies (f ) when one looks maryoftheACSobservations,arelistedinTable1. The S0 back from the present epoch to an epoch 4 5 Gyr ago. average cluster redshift, based on all available spectro- The results presented by Treu et al. 2003 a−nd Sm04 are scopicallyconfirmedclustermembers,isgivenincolumn perhaps the most enlightening - they find a smaller in- 2 of this table. The number of redshifts acquired for crease in the bulge-dominated galaxy (E+S0) fraction galaxiesinthe regionofeachcluster(bothmembersand (f ) with increasing density at z > 0.4 than is seen non-members and including galaxies outside the ACS E+S0 at z < 0.1 but also find comparable∼f values for mosaic boundaries)is listed in column 3. Column 4 lists E+S0 low-density regions (Σ<10 galaxies Mpc−2) at z > 0.4 the number of spectroscopically confirmed cluster mem- and the current epoch. Sm04 propose a simple mod∼el to bersthatalsoliewithintheACSmosaicboundaries. The explain these observations in which high density regions detailsoftheHSTACSobservationsaregivenincolumns atz 1wouldlargelybe comprisedofellipticalgalaxies 5 – 7. The sample selection process was limited by the with∼onlyatraceoflenticulars(e.g.,0 f <0.1). They small number of spectroscopically confirmed clusters at S0 consider various processes to transfor≤m spiral galaxies z > 0.8. However, we were able to include clusters with intolenticulargalaxiesinordertoincreasefS0 withtime a range of X-ray luminosities, from Lx,Bol<1044 erg s−1 to match the observed morphological population frac- to L = 1.9 1045 erg s−1. Table 2 is a compilation x,Bol × tions at z 0.5. However, the Sm04 z 1 f mea- of the derived X-ray properties and velocity dispersions S0 ∼ ∼ surement was inferred from f rather than measured of these clusters. Two of the seven clusters (the two at E+S0 directlyasSm04chose(perhapswisely)nottoattemptto R.A. = 16 hr) are optically selected systems, the rest distinguish betweenS0 and E galaxiesfromthe WFPC2 are X-ray selected although RXJ0849+4452 is a binary data used in their study. cluster system in which the less massive component (CL The deployment of the Advanced Camera for Surveys J0848+4453)was IR-selected (Stanford et al. 1997). (ACS; Ford et al. 2003) on the HST has provided us 2.1. ACS Observations with an opportunity to greatly expand our understanding of the physics behind the morphologicalevolution of We used the WFC on the ACS to image each clus- galaxies in a wide variety of environments. The higher ter. For MS1054-0321,RXJ0152-1357,RDCS1252-2927, sensitivityandbetterangularsamplingoftheWideField andRXJ0849+4452multiplepointingswereusedtocon- Camera (WFC) on ACS relative to WFPC2 enables the struct larger mosaics covering 35 square arcminutes. acquisition of high S/N morphological information for For the first three of these clust∼ers, the pointings form a sub-L∗ galaxiesoverprojectedareasofupto 10Mpc2 in 2 2patternwithall4pointingsoverlappingthecentral z 1 clusters, in a modest allocation of telescope time. 1×arcminute region of each cluster - hence the exposure ∼ Thisisasignificantadvantageoverpriorcapabilitiesand time in the centralregions of these systems is 4 times as enables us to sample >3 decades in localgalaxydensity long as the values givenin Table 1. For RXJ0849+4452, using the same homogeneous data samples. weuseda3 1patterninordertoobtainimagesofboth Aspartofanextensiveprogramtostudytheformation components×of this binary cluster system. All the re- and evolution of clusters and their galaxy populations, mainingtargetswereobservedusingasingleWFCpoint- theACSInvestigationDefinitionTeam(IDT)hasimple- ing centered on the cluster. menteda128orbitprogramtoobserve7distantclusters The filters are chosen to approximately straddle the in the redshift range 0.83 z 1.27. In this paper, rest-frame4000˚Abreakinordertofacilitate the identifi- ≤ ≤ we presentnew constraints on the form and evolutionof cationofbulge-dominatedgalaxiesintheredsequenceof both the MDR and the MRR in these clusters and their the clusters. This sequence, which is populated mostly surroundingsbasedonmorphologiesdeterminedfromthe by elliptical and lenticular galaxies with a strong 4000˚A ACS/WFCimagerycoupledwithextensivespectroscopic break, is well separated from the CM relation for late- data and X-ray observations. This paper is organizedas type cluster galaxies as well as that for most field galax- follows: section 2 contains a brief summary of the space ies. In all cases, we have at least one filter that samples and ground-based observations used in this study, sec- part of the rest-frame B band. We use the ACS im- − tion3presentsadetaileddiscussionofourmorphological ages taken in those filters to perform our morphological classificationprocedureandanassessmentofthereliabil- classifications so that we can readily compare our re- ityoftheseclassifications,section4presentsthemethods sults with morphological information obtained at lower The Morphology–Density Relation in z 1 Clusters 3 ∼ redshifts (e.g., Fabricant, Franx, & van Dokkum 2000). in(z z )/(1+z ), σ ,is0.05. ForRDCS1252- spec ph spec ph − The filters used are explicitly listed in Table 1. Here- 2927, σ is 0.10. Only objects that have a BPZ con- ph after, we will use V to denote the F606W bandpass, fidence level of 0.90 or greater are selected for analysis. 606 r to denote F625W, i to denote F775W, I to Figure 1 shows the distribution of galaxies with photo- 625 775 814 denote F814W, and z to denote F850LP. metric redshifts within 2σ of the mean cluster redshift 850 ph and galaxies with 2σ < z z /(1+z )< 6σ . ph CL ph CL ph 2.2. Object Photometry and Classification The cluster overdensity is|clear−ly see|n only when we se- lectgalaxiesclosetotheactualmeanclusterredshift,in- Object detection and analysis is performed using the dicatingourphoto-z’sareusefulinsignificantlysuppress- ACSIDTPipeline(a.k.a. APSIS;Blakesleeetal.2003a). ingfore/backgroundcontaminationandisolatingmostof APSISphotometryisonthe AB systemandis corrected the actual cluster members. for Galactic extinction using the Schlegel et al. (1998) 100µmap. Object detection andstar-galaxydiscrimina- 3. MORPHOLOGICALCLASSIFICATION tion is done using the dual image mode in SExtractor We visually classified the morphologies of all galaxies (Bertin & Arnouts 1996). The detection image is an in each field with i 23.5 (for the z < 1 clusters) inverse variance weighted combination of the ACS ex- 775 ≤ or z 24 (for the z > 1 clusters) regardless of their posures from all available passbands. The inverse vari- 850 ≤ position or color. For reference, the characteristic mag- ance weighting preserves information about the struc- tural characteristics of the galaxies (see Benítez et al. nitude,m∗,forclustergalaxiesisi775 =22.3atz =0.83 (Gotoetal.2004)andz =22.7atz =1.24(Blakeslee 2004 for details). In this paper, we count as galaxies all 850 objectswithSExtractorstellarityparameterCLASS STAR et al. 2003b). For all our cluster observations, we have at least one filter that samples part of the rest-frame 0.50. The automated image structure analysis of de- ≤ B band (see Table 1) so that morphological classifica- tected objects in our ACS data includes the determina- − tions can be readily compared with those at lower red- tion of the luminosity-weighted moments, the ellipticity, shifts. We classify galaxies using the common Hubble and the 180◦ rotational asymmetry and image concen- sequence: E, E/S0, S0, S0/Sa, Sa, Sb, Sb/Sc, Sc/Sd, trationparameters(e.g., Abrahametal. 1994;Conselice Irr. However, for the purposes of the present analyses, etal.2000). Allmagnitudescitedinthisstudyarebased on the SExtractor MAG AUTO magnitude as it provides a we bin these finer classifications into just 3 broad morphological categories: E (elliptical; 5 T 3), S0 reasonable estimate of the total flux. − ≤ ≤ − (lenticular; 2 T 0),andSp+Irr(Spiral+Irregular; − ≤ ≤ 1 T 10). The FWHM of the point spread function 2.3. Spectroscopic Observations and Photometric ≤ ≤ in our co-added ACS images is 0.09 arcsec (1.8 WFC Redshifts ∼ pixels), corresponding to a projected proper distance of Spectroscopicredshiftshavebeenobtainedfortheclus- 684pc atz =0.831and752pc atz =1.27. We arethus ters in our survey, by us and others, using multiob- able to resolve sub-kpc structure in all cluster members. ject spectrographs on the Keck, VLT, or the Magel- At i = 23.5, the typical galaxy subtends an isophotal 775 lan observatories. The total number of redshifts and area of 400 WFC pixels or 125 (FWHM)2, making the number of confirmed cluster members within each visualcl∼assification(or for thatma×tter anyclassification ACS mosaic are listed in Table 1. The publications con- method) quite feasible. At fainter magnitudes, however, taining some or all of the redshift data include Tran classificationrapidlybecomesdifficultandsystematicer- et al. (2005) for MS1054-0321, Demarco et al. (2004) rorsincreasebothbecausegalaxiesarebecomingsmaller for RXJ0152-1357, Postman et al. (1998a, 2001) and (e.g., Roche et al. 1998; Bouwens et al. 1998; Ferguson Gal & Lubin (2004) for the CL1604+43 system, Stan- et al. 2004; Trujillo et al. 2004) and because there is in- ford et al. (2002) for RDCS0910+5422, Demarco et al. sufficientareaoverwhichthe integratedS/Nissufficient (2005) for RDCS1252-2927, and Rosati et al. (1999) for for accurate classification. Examples of the ACS image RXJ0849+4452. Thetargetselectioncriteriaforthered- quality and our corresponding classifications are shown shift surveys of MS1054-0321 and the CL1604+43 clus- in Figures 2 and 3. ters were based on a single red flux limit. The target Themorphologicalclassificationwasperformedonthe selection for RXJ0152-1357include a color-selection cri- full sample of 4750 galaxies (in 7 clusters) by one of us terion (see Demarco et al. 2004). The spectra are of (MP). Three other team members (NC, MF, BH) clas- moderateresolution(R 500 1200)andmosthavesuf- sified a subset of 400 of these galaxies to provide an ficient S/N to measure∼the pr−ominent spectral features estimateoftheunc∼ertaintyintheclassifications. Allclas- (e.g., [OII] line widths). sifiersusedacommonreferencesetofmorphologiesfrom Our photometric redshifts are derived using the a low redshift B band galaxy sample as a guide. Ex- Bayesian method (a.k.a. BPZ) described in Benítez actormajoritya−greementbetweenall4classifiersinthe (2000) and are based on a minimum of 3 passbands, in- overlapsamplewastypically achievedfor 75%ofthe ob- cluding all available ACS photometry. We have reliable jects brighter than i =23.5. Furthermore, there was 775 photo-z’s for RXJ0152-1357 (z = 0.837), MS1054-0321 no significant systematic offset between the mean classi- (z =0.831),andRDCS1252-2927(z =1.235). Photo-z’s fication for the 3 independent classifiers (as determined for RXJ0152-1357 are based on the ACS r , i , and using the voting scheme from Fabricant et al. 2000) and 625 775 z photometry. Photo-z’s for MS1054-0321 are based the classification by MP giving confidence that the full 850 on the ACS V , i , and z photometry. Photo-z’s sample was classified in a consistent manner. 606 775 850 for RDCS1252-2927 are based on ACS i , z pho- The overall population fractions between the 4 inde- 775 850 tometry and ground-based BVJK photometry (Toft et pendent classifiers exhibit only a relatively small vari- al. 2004). For the two z =0.83 clusters, the rms scatter ance. Figure 4 shows the E+S0 fraction for each classi- 4 Postman et al. 1 0 -1 1 0 -1 1 0 -1 -1 0 1 -1 0 1 Fig. 1.— Theprojecteddistributionofgalaxieswithphotometricredshiftsthatliewithin 2σph ofthemeanclusterredshiftareshown in the right-hand panels for RXJ0152-1357, MS1054-0321, and RDCS1252-2927. The distrib±ution of galaxies with photometric redshifts in the range 2σph < zCL zph/(1+zCL) < 6σph are shown in the left-hand panels. Different symbols denote different morphological classifications: black|dots−are el|lipticals, grey dots are S0’s, and stars are Sp+Irr. The dashed lines denote the boundaries of the ACS mosaics. TheRDCS1252-2927 photo-z’s areavailable over less areathan the fullACSmosaicbecause they relyonNIR photometry that coversasmallerregion. Theoverdensitiesassociatedwiththeclustersareeasilyseenintheright-handpanelsandaredominatedbyEand S0galaxies. fier as a function of z magnitude for a color-selected portantly, there are no significant systematic differences 850 ((i - z ) 0.5)subsetofgalaxiesinthe RDCS1252- between the classifiers. The scatter in the S0 popula- 775 850 ≥ 2927 field. The average rms scatter in the E+S0 pop- tionfractioniscomparablewith√N uncertaintiesinany ulation fraction between classifiers is 0.06. The average given value and while counting statistics do not suggest rms scatter in the S0 population fraction between clas- theminimumscatteronemightexpectbetweendifferent sifiers is about 2 times higher, 0.11. The rms in the E classifiers, this level of scatter indicates that our classi- populationfractionisthesameasthatfortheS0’s,0.11. fication errors are small enough for the task at hand - Inotherwords,thepopulationfractionsarefairlyrobust providing a uniform set of classifications. and the variance in these fractions is significantly less If our visual morphological classifications are robust, than the 20 25% disagreement level between classi- thereshouldbenoticeabledifferencesinthedistributions ∼ − fiersonthemorphologicalclassificationofanyindividual ofthe objectivelyderived “form”parameters(ellipticity, galaxy. Not surprisingly, the combined E+S0 fraction asymmetry,concentration)fortheE,S0,andSp+Irrcat- is more robust than either the E or the S0 fractions – egories. Figure 5 shows the histograms of these 3 form aquantitativedemonstrationthatdetectingspiralstruc- parameters for each of the 3 morphological bins. Clear tureisamorerobustskillthandetectingdisks. Mostim- differencesbetweenthe distributionsexist. Forexample, The Morphology–Density Relation in z 1 Clusters 5 ∼ Fig. 2.— Color postage stamp cutouts of the brightest 49 spectroscopically confirmed members of CL1604+4321 (z = 0.92) and CL1604+4304 (z = 0.90) and their corresponding visually derived morphological classifications. The “Pos Disk” classification stands for “possible disk” galaxy. It is given to objects that appear to have a disk structure but the precise nature of that disk could not established. Galaxies with the “Pos Disk” classification are counted as Spirals in our derivation of the MDR. The first 29 cells show galaxies from CL1604+4321 and the rest contain galaxies from CL1604+4304. The galaxies from each cluster are shown in increasing apparentmagnitudeorderandspantherange20.91 i775 23.50. Eachcutout subtends a6.4 6.4arcsecondarea. ≤ ≤ × the ellipticity distribution of visually classified elliptical shorterrest-framewavelengthbeingimagedwithincreas- galaxiesdiffers from that for visually classifiedS0 galax- ing redshift. The latter effect, sometimes referred to as iesatgreaterthanthe99.999%confidencelevel. We will the“morphologicalk-correction,”hasbeenstudiedfairly use this factlateronasa keypartofouranalysis. Ellip- well(Bunkeretal.2000,Abraham&vandenBergh2001, ticals as a class have, as expected, a lower median ellip- Windhorstetal.2002,Papovichet al.2003). Ingeneral, ticityandasymmetryandahighermedianconcentration over the redshift range being studied here, the morpho- thantheothertwomorphologicalclasses. Ellipticalsalso logicalk-correctionhasbeenshowntobeimportantonly exhibit less scatter about the mean values of these form inasmallfraction(<20%)ofthegalaxypopulationand parameters. The Sp+Irr classexhibits a higher meanel- inthosecases,itiso∼ftenmoreofanissueofhowonechar- lipticity and asymmetry than either the E or S0 class. acterizes any existing spiral structure and not usually a The S0 galaxies have form characteristics that are, on caseofmissingspiralstructurealtogether(seereferences average, intermediate between the E and Sp+Irr distri- above for details). Furthermore, the bluest wavelengths butions. weuseforourmorphologicalclassificationscorrespondto Two key concerns when performing galaxy morpho- the blue end of the rest-frame B band, which is where logicalclassification, visually or via machine algorithms, many lower redshift studies have−been done. Our classi- are the effect of surface brightness dimming and the 6 Postman et al. Fig. 3.— Color postage stamp cutouts of the brightest 49 spectroscopically confirmed members of MS1054-0321 (z = 0.831). The galaxiesaredisplayedinincreasingapparentmagnitudeorderandspantherange20.14 i775 22.15. ≤ ≤ ficationsarenever performedin the rest-frameU band. classificationstominimize“memory”oftheinitialclassi- − The effects of SB dimming are potentially of greater fications by the classifier. Some examples of the original concern as the ability to distinguish between adjacent and redshifted MS1054-0321 galaxies are shown in Fig- categories(e.g., E vs S0, S0 vs Sa) maybe compromised ure 6. The results of the comparisons between the mor- at higher redshift. To test how sensitive our visual clas- phological classifications of the original and redshifted sification scheme is to SB-dimming, we performed two objects are shown in Figure 7. The population fractions simulations. In the first test, we “redshift” our ACS im- obtainedusingtheredshiftedimagesarecompletelycon- ageofthez =0.33clusterMS1358+6245toz =0.83and sistentwiththoseintheoriginalimages–thedifferences perform visual classifications on both the original and are comparable to or less than the √N uncertainties. redshiftedversions. Inthe secondtest,weredshiftedour Thus, our increased exposure times for the more distant ACS image of MS1054-0321 to z = 1.24 and compared clusterscoupledwiththehighangularsamplingandsen- the classifications derived for the originaland redshifted sitivityoftheACS/WFCsuccessfullymitigatetheeffects versions. The “redshifting” involved dimming the ob- of SB-dimming and allow us to distinguish between our jectsappropriately,resamplingtheimagestoaccountfor 3 primary morphological categories (E, S0, and Sp+Irr) the smaller angular scale, and adjusting the noise lev- uniformly across the redshift range under study. els to correspond to those appropriate for our exposure times used in the more distant cluster observations. Re- 3.1. Classification of Mergers & External Morphology classification of the redshifted galaxy images was done Comparisons in a randomorder andatleast3 months after the initial The Morphology–Density Relation in z 1 Clusters 7 ∼ 22 22.5 23 23.5 24 24.5 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.15 RMS in S0 Frac 0.1 RMS in E+S0 Frac 0.05 0 22 22.5 23 23.5 24 24.5 Fig. 4.— Upper panel: TheE+S0populationfractionasafunctionofz850 magnitudeforeachofthe4independentvisualclassifiers. Thesampleusedinthis comparisonisacolor-selectedsamplethat favorsinclusionofbulge-dominated galaxies,hence therelativelyhigh overall early-type population fraction. Thevalues foreach classifier areoffset slightlyfromone another for clarity. The errorbars shown represent the √N uncertainties. The dashed line shows the mean E+S0 population fraction averaged over all classifiers. Lower panel: The RMS scatter in the E+S0 (stars) and S0 (diamonds) population fractions as a function of z850 magnitude between the 4 classifiers. ThedashedlinesrepresenttheaverageRMSscatter. As we wish to measure the MDR and MRR in forms bustness of our morphological classifications. There are that can be compared to previous work, we attempt to atotalof79galaxiesincommonthathavemorphological provide a standard Hubble type classification (E, S0, or classificationsbyusandbyvD2000. Ofthese,16areclas- Sp+Ir) for all galaxies above our flux limits. We do sifiedasM/P (merger/peculiar)by vD2000. We classify make a separate note on whether the object appears 13ofthemasbulge-dominatedsystems(E,S0,orS0/Sa) to be undergoing a merger or tidal disruption and the and 3 as later-type spiral galaxies. The ACS cutouts of analyses of the distribution and frequency of such sys- these 16 objects are displayed in Figure 8. As can be tems will be presentedina separatepaper(Bartkoetal. seenfromthis figure,the morphologyofmostofthe sys- 2005). However, for the present work, we do not clas- tems classified as mergers/peculiar by vD2000 can also sify galaxiesas merger/peculiarsystems (as doneby van reasonably be placed into one of the E/S0/Sp+Irr bins. Dokkum et al. 2000; hereafter referred to as vD2000) if WhilethisconfirmsthevD2000conclusionthatMS1054- one of the above standard morphological categories can 0321hosts a significantfractionofearly-typemergers,it indeed be applied to the individual objects involved in doesyieldonedifference (albeitperhapsa semanticone) the merger. Nonetheless, the WFPC2 study of MS1054- in the conclusions reached regarding the overall early- 0321 by vD2000 provides an additional check on the ro- type populations in MS1054-0321. By counting merg- 8 Postman et al. Sp+Irr S0 E Fig. 5.— The image concentration, rotational asymmetry, and ellipticity distributions for galaxies visually classified as elliptical, S0, and Sp+Irr. The top row shows the distributions for Sp+Irr. The central row shows the S0 distributions. The bottom row shows the distributionsforellipticals. ers/peculiarsystemsasaseparatecategory,vD2000con- mately consistent, however, if one accounts for the ob- cluded that early-typesystems comprise a lowerpopula- servationthatthemajorityoftheclosepairsinMS1054- tion fraction (44%) than in comparably rich clusters at 0321 include at least one bulge-dominated member. lowerredshift. We conclude thatthe early-typefraction, We compare our Hubble classifications with those f , inMS1054-0321is higher – about73%,when one galaxies that vD2000 did classify as E, S0, or Spiral/Irr E+S0 attempts to classify cluster members (including merger (i.e., excluding the M/P objects). We find exact agree- components) as E, S0, or Sp+Irr. Of course, the factor ment with their Hubble classification (when binned into isafunctionoflocaldensityandthe abovevalueisaver- these 3 categories) 71% of the time. We swapped E or aged over densities in the range 15 < Σ 1000 galaxies S0 classifications 11% of the time (i.e., they called it E Mpc−2. Giventhatthemerger/peculiarc≤ategoryhasnot andwecalleditS0orviceversa)andweswappedS0and routinelybeenusedintheclassificationoflow-z clusters, Spiralclassifications10%ofthetime. Theremaining8% wefeelitisanimportantexercisetoprovideasetofmor- were cases where either we or they could not make a phologicalclassificationsthatareassimilaraspossibleto definitiveclassification. Thistranslatestoa 0.1scatter ± the low-z studies. We also agree, however, that quanti- between our respective f or f values, which is con- E S0 fying the frequency of mergers at higher redshift reveals sistent with the scatter estimated from our comparisons fundamental information about the evolution of cluster between our ACS team classifiers. galaxies. The assessment of the early-type population As a further external validation of our morphologi- component of MS1054-0321 by us and vD2000 are ulti- cal classifications, MP classified all galaxies from our The Morphology–Density Relation in z 1 Clusters 9 ∼ Fig. 6.— Imagecutoutsof21original(z=0.83)andredshifted(z=1.24)MS1054-0321galaxiesandtheircorrespondingmorphological classifications. Theoriginalimageisontheleft. Thegalaxiesshownherearespectroscopicallyconfirmedclustermembers. Theredshifted imagesareconstructed tomatchtheexposurelevelusedforourz850 mosaicofRDCS1252-2927(seeTable1). ACS exposure of MS1358+6245 (z = 0.33) that were ing cluster members. The grey bands show the typical in common with the extensive study of this system per- range in low-density population fractions derived from formed by Fabricant et al. (2000). Agreement between local galaxy redshift surveys (e.g., PG84, D97, Goto et theMPclassificationsandthosefromFabricantetal.was al.2003a). Thelocal(z <0.1)anddistant(0.5 < z < 1) achieved 80%ofthe time withnosystematicbiasseen field(i.e.,low-density)galaxypopulationsappea∼rto∼have ∼ in the discrepant classifications (see Figure 7). We thus similar fractions of E, S0, and Sp+Irr systems. concludethatourE,S0,andSp+Irrclassificationscheme is robust and produces Hubble types that are compara- 4. LOCALPROJECTEDGALAXYDENSITYESTIMATION bleinaccuracytovisualmorphologicaldatausedinother We compute the local projected galaxy density in two studies. differentwaystoensurerobustness-thenearestNneigh- bors approach used by D80 and D97 and a friends-of- 3.2. Field Morphological Population Fractions friends algorithm. Both methods yield consistentresults Figure 9 shows the E, S0, and Sp+Irr fractions as a andwethereforepresentourresultsintermsofthenear- function of z for galaxies either in the field. The data estNneighborsbaseddensityunless otherwisespecified. 850 points are the fractions derived from our ACS data and AppendixAprovidesademonstrationoftheconsistency our visual classifications. The results in this figure are of these two density estimation techniques. In the near- based exclusively on galaxies with spectroscopic or pho- estneighbormethod,onecomputestheareaoftheregion tometric redshifts that are incompatible with their be- containing the N nearest neighbors and then derives the 10 Postman et al. 20 20.5 21 21.5 22 22.5 20.5 21 21.5 22 22.5 23 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 18.5 19 19.5 20 20.5 19.5 20 20.5 21 21.5 22 Fig. 7.— Left Panels: The population fractions of E and S0 galaxies as a function of i775 magnitude for the cluster MS1358+6245 at z = 0.33. The population fractions shown are from Fabricant et al. (2003) (based on their WFPC2 mosaic), and from this paper (based on the i775 ACS WFC image and a redshifted version of this image out to z = 0.83). There are no obvious systematic offsets between our fractionsand those inFabricant etal. norany systematic changes as weredshiftthe data out to z=0.83. The dashed lines show thepopulation fractions obtained byaveraging theresults ofallclassifiers. Right Panels: The populationfractions inthe original MS1054-0321ACSimageandinaversionredshiftedtoz=1.24. Thepopulationfractionsateachmagnitudeareoffsetfromoneanother byasmallamountforclarity. corresponding projected density at each galaxy location regioncontainingthe Nnearestneighbors(arectangular from the expression: regioninourimplementation),andD isproportionalto A the angular diameter distance to the cluster (essentially N+1 Σ = fcorr(MCL,MRef) (w(m ,c )−1) N the conversionbetween arcsec and projected Mpc). The i (ΩNDA2) k=1 k k − bkgd! correctionfactor is X (1) MRef Φ(M)dM wgahlearxey,Σficoirsr(tMheCpLr,oMjeRcteefd) igsaalacxoyrrdeecntsioitnyfaabctoourttahagtiveenn- fcorr(MCL,MRef)= R−−M∞∞CLΦ(M)dM (2) sures the density is always measured with respect to a common fiducial luminosity that corresponds to that where M is the absolute mRagnitude limit to which Ref used in low-redshift studies of the MDR, N is the num- we measure all densities, M is the available limit CL ber of nearestneighbors,w(m ,c ) is the selectionfunc- for the cluster being analyzed, and Φ(M) is the galaxy k k tion (that can depend on magnitude and color– see Ap- luminosity function (with M∗(z = 0) = 21.28 and V − pendix B for details), N is a backgroundcontamina- α = 1.22). We choose M to match that of the bkgd Ref − tion correction (if needed), Ω is the solid angle of the original D80 study at z = 0 (M = 19.27 for our N V −

The Morphology - Density Relation in z ~ 1 Clusters

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