Accepted forPublicationin ApJ PreprinttypesetusingLATEXstyleemulateapjv.10/09/06 THE MORPHOLOGICAL CONTENT OF TEN EDISCS CLUSTERS AT 0.5 < Z < 0.81,2 V. Desai3,4, J. J. Dalcanton4, A. Aragoń-Salamanca5, P. Jablonka6, B. Poggianti7, S. M. Gogarten4, L. Simard8, B. Milvang-Jensen9, G. Rudnick10, D. Zaritsky11, D. Clowe11, C. Halliday12, R. Pello´13, R. Saglia14, S. White15 Accepted for Publication inApJ ABSTRACT We describe Hubble Space Telescope (HST) imaging of 10 of the 20 ESO Distant Cluster Survey (EDisCS)fields. Each∼40squarearcminute fieldwasimagedinthe F814Wfilter with the Advanced Camera for Surveys Wide Field Camera. Based on these data, we present visual morphological classifications for the ∼920 sources per field that are brighter than I = 23 mag. We use these 7 auto classificationstoquantifythemorphologicalcontentof10intermediate-redshift(0.5 0.5) has 2 Desai et al. justbeguntobeexplored. Smith et al.(2005)usedHub- mag are presented in §4. In §5 we describe our method ble Space Telescope (HST) Wide-Field Planetary Cam- of quantifying the morphological content in each clus- era 2 (WFPC2) data for six clusters at 0.75 < z < 1.25 ter. Our results are presented in §6, and our conclusions to estimate the early-type fraction as a function of local regarding them can be found in §7. galaxy surface density. They extended their measure- Results will be presented for two sets of cosmological ments to low densities (∼1 Mpc−1) by including field parameters, to allow a direct comparison with previous galaxies in HST WFPC2 images of the cluster CL0024 work (Ω = 1, Λ = 0, H = 50 km s−1 Mpc−1) and 0 0 at z = 0.395. Comparing their results to similar studies to provide the corresponding estimate for the currently at lower redshifts (Dressler 1980b; Dressler et al. 1997; standardcosmology(Ω =0.3,Λ=0.7,H =70kms−1 0 0 Treu et al. 2003), they found that the early-type frac- Mpc−1). In the remainder of this paper we will refer to tion has increased steadily since z ∼ 1 in the densest these as the classic cosmology and the WMAP cosmol- regions(∼1000Mpc−2), hasincreasedonlysincez ∼0.5 ogy, respectively. in moderate density regions (∼100 Mpc−2), and has re- mainedconstantsincez ∼1inthelowestdensityregions 2. THEESODISTANTCLUSTERSURVEY(EDISCS) (<10Mpc−2). Postman et al.(2005)findcompatiblere- The ten clusters included in this study are a subset of sults in their study of the morphology-density relation the ESO Distant Cluster Survey, an extensive program using Advanced Camera for Surveys (ACS) HST imag- toobtainspectroscopyandmultiwavelengthphotometry ing of seven z ∼ 1 clusters. Smith et al. (2005) suggest for20galaxyclustersat0.422 mag, we adopt the for similaranalyses ofclusters spanning a broadredshift values of P(i) down to I814 = 22 mag. Based on the range. behavior of P(i) versus I814, this procedure likely over- Thefirstcomparisonsampleconsistsofz <0.5clusters estimates P(E), P(S0), and P(Sp) and underestimates with morphologicalfractions originally measured by dif- P(Sp+Irr) and P(Irr). These trends translate into un- ferentgroupsusing a varietyof methods, butreanalyzed derestimatesoftheE,S0,andSpmorphologicalfractions in a uniform manner by Fasano et al. (2000), hereafter andoverestimatesofthe Sp+IrrandIrrfractions. These F00. The F00 sample includes 55 low redshift clusters mis-estimateswillbemostsevereforcl1054-1245,cl1216- (Dressler 1980a; Dressler et al. 1997); the ten MORPHS 1201, and cl1354-1230,which have the faintest values of clusters at 0.37 < z < 0.5 (Dressler et al. 1997); three Iauto,lim. The morphological fractions computed using clusters at z ∼ 0.3 plus A2218 and A1689 at z = 0.18 statistical background subtraction and Equation 15 are (Couch et al. 1998); and nine clusters at 0.1 < z < 0.25 shown as hollow triangles in Figure 2. Examination of (Fasano et al. 2000). Using this large sample of uni- this figure does not reveal any obvious biases with re- formly analyzed clusters at z <0.5, Fasano et al. (2000) specttoothermethods thatcouldbe attributabletoour confirmed the MORPHs finding that the S0 fraction in P(i) estimates. clusters has doubled since z ∼ 0.5, while the late-type As in §5.3.1 and §5.3.2, errors were computed us- fraction has decreased by a similar factor. ing the Gehrels (1986) approximation and Nstat(i) and We also compare our results with that of Nstat(tot). Postman et al. (2005), who we will refer to as P05. They computed the morphological fractions within the 6. RESULTS virial radii of 7 clusters at 0.8 < z < 1.27. This is the We have estimated the morphological fractions of 10 only large study of clusters at these redshifts in which EDisCS clusters at 0.5 < z < 0.8 using four differ- S0 galaxies are separately classified. As previously ent background-subtraction techniques (absolute limits discussed, the S0 population is an important one to from spectroscopic redshifts, direct estimates from spec- track at high redshift. troscopicredshifts,photometric redshifts, andstatistical In Figure 3, we show the evolution at z < 0.8 of subtraction),within aperturesof different radii(600 kpc the morphological fractions within rich clusters, using and 0.6×R ), and for two different cosmologies (the EDisCS data in conjunction with the F00 sample de- 200 classic cosmology: Ω = 1, Λ = 0, H = 50 km s−1 scribedabove. Allpointswerecomputedwithin600kpc, 0 0 Mpc−1; and the WMAP cosmology: Ω = 0.3, Λ = 0.7, usingthe classiccosmology. We do notinclude measure- 0 H = 70 km s−1 Mpc−1 ). Figure 2 shows how the dif- ments made onthe Postmansample in this plotbecause 0 ferent methods compare for an aperture of 0.6 ×R they were computed within R using the WMAP cos- 200 200 in the WMAP cosmology. The direct estimate methods mology. As discussed previously, the F00 sample shows are consistent with the absolute limits derived from our a systematic decrease in the S0 fraction from 55% at spectroscopy,andwithoneanother. Inaddition,thereis z = 0 to 20% at z ∼ 0.5. Where the redshift range no systematic trend for one estimate to produce higher of the EDisCS clusters overlaps with that of F00, the or lower fractions than any other. For each cluster we morphological fractions derived from the two samples therefore adopt a single method: direct estimate from are generally consistent. However, the longer redshift spectroscopy, photometric redshifts, or statistical back- baseline affordedby the additionof the EDisCS data re- ground subtraction. The adopted method, which differs veals that the S0 fraction is actually flat over the range fromclustertocluster,ischoseninthefollowingway. For 0.4 0.8 clusters collapsed at earlier times. In the latter it could are required to distinguish between these two possibili- be due to a higher efficiency of morphological transfor- ties. As discussed above, P05 have analyzed the mor- mations in more massive clusters. phological fractions, including the S0 fraction, in clus- P05 found that the E, S0, and E+S0 fractions within ters at z > 0.8. In Figure 4, we show the morphological R of seven z ∼ 1 clusters increase with increasing 200 fractions of the EDisCS and P05 clusters as functions of bolometric cluster X-ray luminosity, although the corre- redshift. Allestimates weremadeusing the WMAP cos- lations are significant only at the .3σ level. However, mology,but the EDisCSfractionswerecomputedwithin they find no correlation between the E+S0 fraction and 0.6×R (see §5.2), while the P05 fractions were com- the X-raytemperature orthe cluster velocitydispersion, 200 puted within R . P05 find that the E and E+S0 frac- perhaps because of small number statistics. In Figure 5 200 tions decrease out to 0.6×R , while the S0 fraction we plot the morphological fractions of the EDisCS and 200 staysroughlyconstantandtheSp+Irrfractionincreases. P05 samples against cluster velocity dispersion. From Between 0.6×R and R , all the fractions are flat. thisfigureitisapparentthatclusterswithlargervelocity 200 200 According to Table 4 of P05, the E, S0, early, and late dispersions harbor a higher fraction of early type galax- type fractions computed within R are factors of ap- ies and fewer late type galaxies. Spearman and Kendall 200 proximately0.79,0.96,0.85,and1.15timesthefractions rankcorrelationtestsshow thatthese visualimpressions computed within 0.6 × R . However, no corrections are statistically significant at the .3σ level. 200 have been made to the points shown in Figure 4. Whileitistemptingtointerpretthiscorrelationasone Figure 4 shows that the strong evolution in morpho- betweenmorphologicalcontentandclustermass,the ve- logical fractions seen at z < 0.4 does not continue at locitydispersionofaclusterisdirectlyrelatedtoitsmass z > 0.4. Indeed, the E+S0 (Sp+Irr) fraction appears only if the cluster is virialized. The velocity dispersion to be larger (smaller) in the higher-redshift P05 sample mayoverpredictmassiftheclusterisexperiencingsignif- comparedto the EDisCSclusters. However,aSpearman icant merging events. The degree of substructure in five rank correlation analysis shows that there is no statisti- of the clusters in this work (cl1040-1155, cl1054-1146, cally significant evidence for any evolution over the en- cl1054-1245, cl1216-1201, and cl1232-1250) has been tire redshift interval 0.4 1200 km s−1), while are made up of two separate populations: “primordial” the z > 0.5 EDisCS+P05 sample does not. Figure 7 galaxies whose stars all formed at z > 2.5 and galax- shows the cluster velocity dispersions of the EDisCS, ies whose star formation lasted until later times but was F00, and P05 samples as a function of lookback time. ultimately “quenched” due to entering a cluster-like en- Theclusterswiththehighestvelocitydispersionsinthese vironment (> 1014M⊙). In this model, the primordial samples lie at low redshift. Unfortunately, velocity dis- galaxiesmaybeidentifiedwiththeellipticalgalaxiesthat persions are unavailable for a significant fraction of the make up ∼30% on average of the galaxy populations in lowredshiftsample. Theseareurgentlyneededtounder- clusters at z < 1, and perhaps some of the S0 galaxies standthe extentto whichsampleselectionis responsible (see below). This identification is consistent with both for the apparent evolution in the morphological content the lack of evolution in the elliptical fraction at z < 1 of clusters. and with the old ages inferred for elliptical stellar populations. It is tempting to identify the quenched galaxies 7. DISCUSSIONANDCONCLUSIONS with S0s. However, the fraction of quenched galaxies in Usinghighresolutionimagingaffordedbythe ACSin- the model increases at z < 0.8, while the S0 fraction strumentonboardthe Hubble Space Telescope,wehave only increases at z < 0.4. These timescales are consis- shown that the morphological content of rich clusters tent if it takes roughly a billion years for a galaxy to with velocity dispersions in the range σ = 200−1200 resemble an S0 after the cessationof star formation in a kms−1 showsno evidencefor evolutionoverthe redshift spiralgalaxy (Dressler et al. 1999; Poggiantiet al. 1999; range 0.4 < z < 1.25 (see Figures 3 and 4). Although Tran et al.2003). However,theformationoftheS0pop- the scatter is significant, typical morphologicalfractions ulation observed in clusters at 0.4 < z < 1 cannot be for clusters in this redshift range are 0.3, 0.15, and 0.55 explained in this way. Perhaps these are “primordial” for E, S0, and Sp+Irr galaxies,respectively. In contrast, S0s that assumed an S0 morphology upon or soon after studiesofclustersatlowerredshiftshaveshownthatthe formationat z >2.5. If so, the S0 galaxiesin clusters at elliptical fraction remains constant between z = 0 and z > 0.4 should be, on average, older and more massive z ∼0.5,whiletheS0fractiondecreasesbyafactorof∼2 thanclusterS0satz <0.4. Anotherpossibilityisthatat and the Sp+Irr fraction increases by a similar amount least some late types may transform into S0s in systems over the same redshift range (e.g. F00). Our data show less massive than 1014M⊙, perhaps by an entirely dif- thattheobservedevolutioninclusterS0andSp+Irrpop- ferent mechanism than operates in > 1014M⊙ systems. ulations does not continue beyond z ∼ 0.4, at least for The existence of S0s in groups (e.g. Hickson et al. 1989) the velocity dispersions probed in this study. suggests that this may be the case. How do our results concerning the global evolution of If a significant fraction of S0 galaxies in nearby clus- galaxy morphologies within rich clusters translate into ters werequenched, what did they look like beforehand? a statement regarding the evolution of the morphology- TheparalleldeclineintheSp+IrrfractionastheS0frac- density relation? Although the centers of cluster cores tion increases between z = 0.4 and z = 0 suggests that are regions of high local galaxy density, our morphologi- some subset of late type galaxies transformed into S0s calfractionswerecomputedwithinsizableapertures(see subsequent to quenching. For the EDisCS clusters, we §5.2), so the average environment we are probing is of computed the fractions of Sp and Irr galaxiesseparately moderate density. For example, the average galaxy sur- (see Tables 3 through 4). As with the other morpholog- face density within a radius of 0.6 × R (Ω = 0.3, ical fractions at z >0.4, the Sp and Irr fractions do not 200 0 Λ = 0.7, H = 70 km s−1 Mpc−1) for the EDisCS clus- varysystematicallywithredshift. Moreover,theIrrfrac- 0 ters used in this analysis ranges from ∼40–175 Mpc−2. tionsintheEDisCSclustersareverysmall. Weconclude The morphologicalfractions of the F00 andP05 clusters that Sp galaxies and not Irr galaxies are the precursors were likely computed using galaxies inhabiting environ- of the quenched S0s in local clusters. mentsofsimilaraveragedensity. Theseresultsmaypoint In the P06 model, both nature and nurture (within to a lackof evolutionin the morphologydensity relation clusters) play a role in the environmental dependence between z = 0.8 and z = 0.4, with a subsequent in- of star formation and therefore morphology. The nur- creaseinthe S0populationandadecreaseinthe Sp+Irr turecomponentwithinclustersisconsistentwiththeob- population. Larger samples in this redshift range are servation that the relation between star formation and needed to understand the scatter in the morphological environment in nearby clusters is not solely a reflec- fractions, and to rule out weak evolution. However, our tion of the relation between stellar age and environ- current data are consistent with recent studies of the ment, as would be expected in a pure nature scenario morphology-densityrelationatz ∼1 (Smith et al.2005; (Christlein & Zabludoff 2005). The P06 model is also Postman et al. 2005). compatible with observations which show that the SFR- Severalstudiesofnearbygalaxiesindicatethattheob- density relation out to z ∼ 1 extends to very low local served relation between morphology and environment is densities, comparable to those found at the virial radius a reflection of a more primary relation between star for- of clusters and even outside clusters (Lewis et al. 2002; mation rate (SFR) and environment (Kauffmann et al. Gómez et al.2003; Cooper et al.2006). The low density 2004; Christlein & Zabludoff 2005; Blanton et al. 2005). relations could be set up by the dependence of galaxy Thestarformationpropertiesofcluster,group,andfield properties on initial conditions, or perhaps a different galaxies in the EDisCS spectroscopic sample have been mechanism creates S0 galaxies in low-density environ- measured and compared to a low redshift sample from ments, or a combination. the Sloan Digital Sky Survey by Poggiantiet al. (2006) Although the model forwarded by P06 is an attrac- 10 Desai et al. tive framework within which to view our observational and cluster velocity dispersion is itself a function of red- results on the morphological content of rich clusters, it shift. Largerclustersampleswithrobustvelocitydisper- is not necessarily the only model which could explain sions and morphologies are needed to determine which all of the data accumulating on the evolution of galaxy oftheseoptionsismostlikely. Ifthemorphology-density properties with environment. Furthermore, it does not relationis drivenby environmentalprocessesinclusters, identify a mechanism responsible for the “quenching”, suchmeasurementsareessentialfordetermininghowthe although it does specify a timescale (3 Gyr) and a mass efficiencyofthesetransformationsdependsonclusterve- scale (1014M⊙) of a workable mechanism. locitydispersion,which,asdiscussedin§6.2isrelatedto In addition to analyzing the evolution of the morpho- both the cluster mass and its dynamical state. logicalcontentingalaxyclustersouttoz =1.25,wehave also studied the dependence of the morphological con- tentonthe clustervelocitydispersion(see Figures5and 6). We find that the early and late-type fractions in the We greatly appreciate the timely and cheerful assis- EDisCS and P05 cluster samples correlate with cluster tance of Galina Soutchkova in planning the HST obser- velocitydispersionatastatisticallysignificantlevel. This vations. We also thank the referee, Manolis Plionis, for correlation highlights the importance of understanding feedback which improved the paper. VD acknowledges global cluster properties in samples where evolution is funding from the Graduate Student Researchers Pro- observed, such as the F00 sample. Unfortunately, only gram. This work was supported by NASA grant HST- limitedvelocitydispersioninformationisavailableforthe GO-09476.01. JJDwaspartiallysupportedbytheAlfred F00clusters. The existinginformationfor14clustersin- P. Sloan Foundation. The Medium Deep Survey catalog dicates that they have a higher average velocity disper- is based on observations with the NASA/ESA Hubble sionthaneithertheEDisCSorP05clusters. Inaddition, SpaceTelescope,obtainedattheSpaceTelescopeScience the morphological fractions of these 14 clusters do not Institute, which is operated by the Association of Uni- correlate strongly with cluster velocity dispersion. 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The Morphological Content of Ten EDisCS Clusters at 0.5 < z < 0.8

Similar The Morphological Content of Ten EDisCS Clusters at 0.5 < z < 0.8