Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed2February2008 (MNLATEXstylefilev2.2) . . Galaxy Groups at 0 3 ≤ z ≤ 0 55. II. Evolution to z ∼ 0 D. J. Wilman1,2,7, M. L. Balogh1,3, R. G. Bower1, J. S. Mulchaey4, A. Oemler Jr4, R. G. Carlberg5, V. R. Eke1, I. Lewis6, S. L. Morris1, R. J. Whitaker1 1Physics Department, Universityof Durham, South Road, Durham DH1 3LE, U.K. 5 2Max-Planck-Institut fu¨r extraterrestrische Physik, Giessenbachstraße, D-85748 Garching, Germany (present address) 0 3Department of Physics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 (present address). 0 4Observatoriesof the Carnegie Institution, 813 Santa Barbara Street, Pasadena, California, U.S.A. 2 5Department of Astronomy, Universityof Toronto, Toronto, ON, M5S 3H8 Canada. n 6Department of Physics, University of Oxford, KebleRoad, Oxford, UK a 7email:[email protected] J 1 1 2February2008 1 v ABSTRACT 3 8 We compare deep Magellan spectroscopy of 26 groups at 0.3≤z ≤0.55, selected 1 from the Canadian Network for Observational Cosmology 2 field survey, with a large 1 sample of nearby groups from the 2PIGG catalogue (Eke et al. 2004). We find that 0 5 the fraction of group galaxies with significant [Oii]λ3727 emission (≥5˚A) increases 0 strongly with redshift, from ∼ 29% in 2dFGRS to ∼ 58% in CNOC2, for all galaxies / brighter than ∼ M∗ +1.75. This trend is parallel to the evolution of field galaxies, h wheretheequivalentfractionofemissionlinegalaxiesincreasesfrom∼53%to∼75%. p The fraction of emission-line galaxies in groups is lower than in the field, across the - o fullredshiftrange,indicatingthatthehistoryofstarformationingroupsisinfluenced r by their environment. We show that the evolution required to explain the data is t s inconsistent with a quiescent model of galaxy evolution; instead, discrete events in a which galaxies cease forming stars (truncation events) are required. We constrain : v the probability of truncation (Ptrunc) and find that a high value is required in a Xi simpleevolutionaryscenarioneglectinggalaxymergers(Ptrunc >∼0.3Gyr−1).However, without assuming significant density evolution, Ptrunc is not required to be larger r in groups than in the field, suggesting that the environmental dependence of star a formation was embedded at redshifts z >∼0.45. Key words: galaxies:evolution– galaxies:stellar content 1 INTRODUCTION The properties of a galaxy population are known to be strongly correlated with their local environment (e.g. Star formation rates derived from high redshift UV sur- Dressler1980):galaxiesindenseenvironmentstypicallyhave veys and low redshift spectral analysis indicate that the bulge–dominatedmorphologies,lowstarformationratesand global star formation rate has declined since z ∼ 1.5 by HI gas content, and red colours. Large surveys such as the anuncertain factorofbetween 4.0and40.0 (e.g.Lilly etal. Sloan DigitalSkySurvey(SDSS)haveshownthatboth the 1996; Madau et al. 1998; Wilson et al. 2002; Panter et al. galaxy colour distribution (e.g. Blanton et al. 2003; Balogh 2003). This evolution is apparently associated with down- et al. 2004b) and star formation rate distribution (Lewis sizing (Cowie et al. 1999; Kauffmann et al. 2003; Poggianti etal.2002a;Mart´ınezetal.2002;G´omezetal.2003;Balogh et al. 2004), such that the characteristic mass of star form- etal.2004a; Kauffmannetal.2004) dependonlocal galaxy ing galaxies decreases with time. The precise cause of this densityoverawidedynamicrange.Similartrendshavealso decline, however, is unknown. It may be driven by internal been determined at redshifts up to z ∼ 0.5 (Dressler et al. (local) processes, leadingtotheexhaustionofthegasreser- 1997; Balogh et al. 1999; Kodama et al. 2001; Treu et al. voir, or by interactions with the local environment. This is 2003). Recently, De Propris et al. (2004) and Balogh et al. often referred to as the nature versus nurturedichotomy of (2004b)haveshownthat,whilethefractionofredgalaxiesin galaxy evolution. (cid:13)c 0000RAS 2 D. J. Wilman et al. thenearby Universeincreases with local density,the colour f inlarger groups.However,thestatistical limitations of blue distribution and median EW[Hα] of the blue, star-forming photometric data are significant, particularly through field galaxies is nearly independent of environment. A similar contamination. In addition, the radio selection might bias trend was observed in the Hα equivalent width (EW[Hα]) thechoiceofgroups.Itisthereforeimportanttorepeatthis distribution(Balogh etal.2004a). Apossibleinterpretation studyusingaredshift-spaceselectedsampleofspectroscopi- ofthesetrendsisthatdenseenvironmentstransform galax- callyconfirmedgroups.Higherredshiftcataloguesofgroups ies from blue to red on a relatively short timescale, <∼ 0.5 now exist (Cohen et al. 2000; Carlberg et al. 2001b) from Gyr. which galaxy properties have been analysed. For example, In a ΛCDM Universe, the growth of large scale struc- Carlbergetal.(2001a)studiedthepropertiesofgroupgalax- tureisaconsequenceofthehierarchicalclusteringprocess.It ies in the CNOC2 group sample at intermediate redshift. is therefore possible that this clustering process itself could Amongst otherthings, they discovered a trend in themean drivetheevolutionofglobalstarformation,asmoregalaxies galaxycolors,whichonaveragebecomeredderthanthefield are drawn into dense environments where their star forma- toward thegroup centers. InWilman et al. (2004, hereafter tion rates are somehow suppressed. The fraction of galax- Paper I), we present our deeper and more complete spec- ies located in galaxy clusters is only ∼ 10% even at the troscopy in the region of the intermediate redshift CNOC2 present epoch and, thus, such environments alone can not groups(Carlbergetal.2001b).Ourdatashowthattheprop- have a large influence on the global star formation rate. erties of galaxies in intermediate redshift groups are signif- However, perhaps over 50% of galaxies are today found in icantly different from those of coeval field galaxies, in that groupsofvarioussizes(Ekeetal.2004)andthustheseenvi- group galaxies are significantly less likely to have ongoing ronments may play a more significant role. Although some starformationthantheirfieldcounterparts,andgroupsalso proposedmechanismsfortransforminggalaxiesindenseen- contain a significant excess of bright galaxies (MbJ≤ −21). vironments, such as ram pressure stripping (Gunn & Gott These results are discussed in detail in that paper. 1972; Quilis etal. 2000) areunlikelytobeeffectiveinsmall In this paper, we contrast the properties of our inter- groups, many other effects, such as strangulation (Larson mediate redshift group sample (Carlberg et al. 2001b, Pa- etal.1980;Baloghetal.2000;Coleetal.2000;Diaferioetal. perI) with a large sample of galaxy groups at low redshift, 2001), tidal interactions (Byrd & Valtonen 1990; Gnedin selected from the 2dF Galaxy Redshift Survey (2dFGRS, 2003), or galaxy mergers and interactions (e.g Joseph & Ekeet al. 2004). Thisallows ustoexaminetheevolution of Wright1985;Mooreetal.1996)maybemorewidespread.In galaxiesinthegroupenvironmentwithpurelyspectroscopic particular,galaxyinteractionsarelikelytobemostcommon dataandoverasignificantrangeofredshift.InSection2we ingroups,wherethevelocitydispersionofthegroupsisnot introduceourgalaxyandgroupsamplesatintermediatered- much larger than that of the constituent galaxies (Barnes shift (CNOC2- seealso PaperI) and locally (2dFGRS).We 1985; Zabludoff & Mulchaey 1998; Hashimoto & Oemler then go on to ensure a fair comparison between these two 2000). populations and the surrounding field by examining the lu- Studies of nearby groups (e.g. Zabludoff & Mulchaey minosityfunctionsandEW[OII]distributions.InSection 3, 1998) showthattheirgalaxypopulationsvaryfromcluster- we present our results, in which we assess the environmen- like (mostly early types) to field-like (mostly late-types), talandevolutionarydependenciesofEW[OII]aswellasthe suggesting that a nurturing process of galaxy evolution dependenceon other parameters such as galaxy luminosity. may well be taking place (e.g. Zabludoff & Mulchaey 2000; WethendiscussthescientificimplicationsinSection4,and Hashimoto & Oemler 2000; Tran et al. 2001). However, presentsimplemodelsforthestarformationhistoryofthese galaxy groups are inevitably much more difficult to detect galaxypopulationstoconstraintheoriesofgalaxyevolution. thanclusters,witharelativepaucityofmembersandsignif- Section 5 presentsour finalconclusions. icantly lower density hot plasma. Therefore, in most cases Throughout this paper we assume a ΛCDM cosmology the group selection criteria is either not well understood, of ΩM =0.3, ΩΛ =0.7 and H0=75kms−1Mpc−1. orbiased in some way.In particular, onesuccessful method has been to search for the most overdense, compact groups (Hickson 1982; Severgnini & Saracco 2001; Coziol et al. 2004);however,suchsystemsmaybeatypicaloftheaverage 2 DATA groupenvironment.Todaynewopportunitiesareaffordedby 2.1 CNOC2: The Intermediate Redshift Sample large, complete catalogues of nearby groups compiled from redshiftsurveyssuchasSDSSandthe2dFGalaxyRedshift A complete description of the data and reduction methods Survey(e.g. Ekeet al. 2004). canbefound inPaperI.Insummary,theintermediatered- One way to directly observe the influence of galaxy shiftgroupsampleisselectedfromtheCNOC2redshiftsur- groups is to trace their redshift evolution. In rich clusters, vey in the range 0.1 ≤ z ≤ 0.55 (Carlberg et al. 2001b; astrongevolutioninthefraction ofbluegalaxies, f ,was Yeeetal. 2000). Weobtained deeper,morecompletemulti- blue detected by Butcher & Oemler (1984) and later by others object-spectroscopy in the regions of 26 of these groups (in (e.g. Margoniner et al. 2001; De Propris et al. 2003b), al- 20fields)at0.3≤z ≤0.55usingLDSS2onthe6.5mBaade thougheventhisresultisstillamatterofsomedebate(e.g. telescopeatLasCompanasObservatoryinChile.Thefields Andreon&Ettori1999; Andreonet al.2004). InAllington- werechosen tomaximize thenumberof groupsin theCarl- Smith et al. (1993), a sample of groups were photometri- bergetal.(2001b)samplealongthelineofsight,withinthis cally selected in the vicinity of bright radio galaxies at low redshift range.Redshiftsweremeasured for 74% ofgalaxies (z≤0.25)andintermediate(0.25≤z≤0.50)redshift.They targetted with Rc≤ 22 (with ∼ 60% success rate in the tentativelyconfirmananalogousevolutioninthefractionof faintest bin 21.5 <Rc≤ 22) and galaxies have been reas- (cid:13)c 0000RAS,MNRAS000,000–000 Galaxy Groups at 0.3 ≤ z ≤ 0.55. II. Evolution to z ∼ 0 3 signed togroupswith anewdeterminationof thegroupve- locity dispersion. The galaxies have each been weighted by afactorWC toaccountforradialandmagnitude-dependent selection functions (see Paper I). The CNOC2 field galaxy sample is defined to include ′′ all galaxies within 240 of a targetted group centre, lying withintheredshiftrange0.3≤z≤0.55butexcludingthose galaxiesassignedtothetargettedgrouptoavoidbiasingthe field towards the group environment. The final magnitude– limited field sample is therefore representative of the Uni- verse in the0.3≤z≤0.55 redshift range. TheCNOC2groupsamplecontains240galaxieswithin 1h−1Mpc of the group centre and the field sample contains 75 334 galaxies. Figure1.Luminosityfunctionsofthegroupgalaxies(a)andfield galaxies(b)for2dFandCNOC2.FieldandGroupCNOC2data are each normalised to match the total number in the CNOC2 sampleforMbJ<−20.25 2.2 2dFGRS: The Local Sample 2.3 The galaxy luminosity function in groups and Thelocal redshiftgalaxy samplecomes from thelarge 2dF- the field GRS with over 220 000 galaxy spectra selected in the bJ- 2.3.1 K-corrected rest frame magnitudes band.Thegalaxysampleiseffectivelyvolumelimited(with lowincompletenessthesampleisrepresentativeofthewhole Galaxies in both catalogues are k-corrected to give rest- population)intheredshiftrange0.05≤z ≤0.1forgalaxies frame absolute magnitudes in the bJ-band. The 2dF k- with M ≤−18.85. Althoughtherewereproblemswith the corrections are taken from Norberg et al. (2002), and are bJ atmosphericdispersioncorrectorpriortoAugust1999which generally small. For the CNOC2 survey, k-corrections have affect the instrument throughput (Lewis et al. 2002b), we been calculated using no-evolution models; we will explore find our results are unchanged if we exclude data obtained thesensitivityofourresultstotheassumedmodelofgalaxy in thisperiod. evolution in the discussion (Section 4). We note that k+e The 2dFGRS Percolation-Inferred Galaxy Group cata- corrections are also available for galaxies in the original logue(2PIGG Ekeet al.2004) is alsobased on afriends-of- CNOC2sample (Shepherdet al. 2001). Foreach galaxy, we friendspercolation algorithm.Anaxialratio(definedasthe first choose a mixture of observed local SEDs (King & El- line-of-sight length relative to the projected spatial length) lis 1985) for which the model B-I colour matches the ob- of ∼ 11 is used to link 2dFGRS galaxies together, form- servedcolour at thegiven redshift.Then therest-frame ab- ing a large catalogue of local groups. Velocity dispersions solute bJ magnitude can be determined from the models, ofthe2PIGG(andCNOC2) groupsarecalculated with the given the observed Rc magnitude, SED mixture and red- gapper algorithm. Full details of the 2PIGG group-finding shift. The transformation from observed Rc-band magni- algorithm and description of the catalogue can be found in tudetorest-framebJ ischosenbecauseitiscloselymatched Eke et al. (2004). We only investigate groups with number at CNOC2 redshifts and because it directly transforms the of known members Nm ≥ 10 because the contamination of CNOC2 spectroscopic selection band to the 2dF selection thatgroupcataloguewithunphysicalsystemsbecomeslarge band. Luminosities are then corrected for galactic extinc- insmallergroups.WenotethattheCNOC2groupdetection tion on a patch-to-patch basis, computed by extrapolating algorithm(Carlbergetal.2001b)requiresmorebrightmem- from B and V band extinction values obtained from NED bers in close proximity to each other, and therefore likely (Schlegel et al. 1998, variation within each patch is negligi- suffers from less contamination. Even with the Nm ≥ 10 ble). We make no correction for internal extinction, also to requirement for 2dFGRS groups, we find that the range of allow direct comparison with local galaxies in 2dF. group velocity dispersion matches that seen in the CNOC2 group sample. From now on we will refer to the 2dFGRS 2.3.2 Luminosity limits sample simply as the 2dF sample. The2dFfieldisdefinedasallgalaxiesinthe2dFgalaxy Figure 1 shows the superimposed luminosity functions of catalogueandrepresentstheglobalgalaxypopulationinthe the2dFandCNOC2groupandfieldsamples. Thevolume– 0.05≤z≤0.1redshiftrange,withinmagnitudelimits.Since limited 2dF sample is > 90% complete for M ≤ −19 and bJ thesegroups were untargetted,this definition is compatible soweapplynocompletenesscorrection.TheSchechterfunc- with our CNOC2 field definition. tion computed for the 2dF survey (Norberg et al. 2002) is The 2dF group sample contains 5490 galaxies within shown for comparison. The CNOC2 galaxies are weighted 1h−751Mpc of the group centre and the field sample contains byWC tocorrectfortheselection function.Forcomparison 50981 galaxies. between2dFandCNOC2groupandfieldsamples,thedata (cid:13)c 0000RAS,MNRAS000,000–000 4 D. J. Wilman et al. arenormalisedsothatthereisthesamenumberofweighted galaxies brighter than M = −20.25. At these magnitudes, bJ neithersample suffersanyincompleteness duetofalling be- lowtheapparentmagnitudelimit intheredshiftrangecon- sidered. We note that the enhanced bright to faint galaxy ratio seen in CNOC2 groups relative to the field (Paper I) is also seen in thelocal 2dF groups. AlsoshowninFigure1aresomecriticalvaluesoflumi- nosity.ThevalueofM∗inNorbergetal.(2002),appropriate to our cosmological model, is ∼ −20.28, and the 2dF data are complete down to −18.85, or equivalently ∼ M∗+1.5. OurCNOC2dataspanawiderangeinredshiftandthusthe luminosity limit corresponding to our apparent magnitude limit of Rc=22 is redshift dependent. At the upper limit of our redshift range, z = 0.55, a Rc= 22 galaxy with a mean k-correctionwill transform toarest-frameluminosity M =−19.75 andat thelower redshift limit ofz=0.3, the bJ same galaxy would transform to M =−17.93. In the case Figure 2.NormaliseddistributionsofEW[OII]ofthegroup(a) bJ of thereddest galaxies with larger K-corrections, theselim- andfield(b)galaxiesin2dF(0.05≤z≤0.1)andCNOC2(0.3≤ its would lie at MbJ=−20.07 (z =0.55) and MbJ=−18.06 z≤0.55)sampleswhereMbJ≤−18.5.Thedataisnormalisedto matchthetotal numberofgalaxiesintheCNOC2analaysis. (z=0.3), so we are incomplete below thesemagnitudes. MostgalaxiesinourCNOC2and2dFcataloguesliebe- lowthebrightestCNOC2luminositylimitofM =−20.07. bJ inggalaxypropertiesatdifferentredshifts.Inparticular,we To enable us to compare the 2dF and CNOC2 galaxy sam- showinSection2.4.3thatouranalysisisinsensitivetoaper- ples independently of differences in the luminosity function turebias in EW[OII]. (which may be partly intrinsic but is mostly due to selec- tion effects), we choose to apply an additional luminosity weighting to the CNOC2 galaxies. This weighting is calcu- 2.4.1 Fair comparison of EW[OII] in 2dF and CNOC2 lated within each bin in luminosity using the formula: Details of the CNOC2 equivalent width measurement pro- NCNOC2 cessaregiveninPaperI.In2dFGRS,theequivalentwidths Wlum=N2dF/ X WC (1) of [OII] are measured in a similar way to Hα using a com- i=1 pletelyautomatedfittingprocedure,(seeLewisetal.2002a, which corresponds to the difference between the field lumi- for details). In the fitting of the [OII] emission line, many nosity functions. It is applied in the range −20.25 ≤M ≤ 2dF measurements are classified as no line present. In our bJ −18.5, where the CNOC2 data become incomplete at the analysis,thesearesetto0˚Aandthenall2dFmeasurements highredshiftend.Thechoiceofafaintfinalluminositylimit are smoothed with a gaussian kernel of width 2˚A to match ofMbJ=−18.5(∼M∗+1.75)makesmaximaluseofthedata the mean error on CNOC2 EW[OII] measurements (much and allows the properties of faint galaxies to be compared greater than the 2dF line measurement error of ≪ 1˚A). with those of brighter galaxies. We emphasize that whilst We note that the fraction of galaxies with EW[OII]>5˚A we are incomplete at MbJ>∼ −19.75 at z = 0.55 in CNOC2 is unchanged by this smoothing to within < 1%. Figure 2 and MbJ>∼ −18.85 in 2dFGRS, this has no impact on any shows the distribution of EW[OII] in our 2dF and CNOC2 analysis of galaxy properties as a function of luminosity or group and field galaxy catalogues. We limit the group data oncomparisons between thegroupandfield galaxy popula- to within 1h−1Mpc (projected) of the group centre in all 75 tions. Also, when studying galaxy properties as a function cases. The CNOC2 galaxies are weighted by a combined of luminosity, the analysis is independent of the CNOC2 completenessandluminosityweightingtomatchthe2dFlu- galaxy weighting, including little effect from weighting by minosity function, Wtot=WC×Wlum.The CNOC2 galax- theselection function. ies are limited to Rc≤ 22 and all galaxies are limited to M ≤ −18.5. Finally, the distribution of 2dF EW[OII] is bJ normalised to provide an equal number of galaxies to that 2.4 Measurement of star formation using EW[OII] found in CNOC2, for presentation only. This is done inde- pendentlyfor thegroup and field populations. The Hα emission line disappears entirely from the LDSS2 spectrographwindowatz>0.21,limitedbytheinstrument sensitivity of the current optics and detector. Therefore,we 2.4.2 Diagnostics of Star Formation for a Galaxy usethe[OII]λ3727emissionlineequivalentwidth(EW[OII]) Population to study the relative levels of star formation in our galaxy samples. In Paper I we outlined the reasons why EW[OII] We are motivated by the findings of Strateva et al. (2001); is sufficient to reveal trends of star formation with galaxy Blantonetal.(2003);Baldryetal.(2004) andBalogh etal. environment.Incontrast tousingthelineflux,theeffect of (2004a) who show that galaxy populations have a bimodal normalising by the continuum when computing the equiv- distribution in colour and EW[Hα]. Balogh et al. (2004a,b) alent width reduces uncertainties related to absorption by show that the fraction of red, passive galaxies is strongly dust and aperture bias, which are relevant when compar- dependent upon local galaxy density. The division between (cid:13)c 0000RAS,MNRAS000,000–000 Galaxy Groups at 0.3 ≤ z ≤ 0.55. II. Evolution to z ∼ 0 5 passive and star forming galaxies in the EW[Hα] distribu- tionoccursat∼4˚A(Baloghetal.2004a).Wedonotexpect to see such a clear bimodality in EW[OII] since EW[Hα] = 4˚A typically corresponds to EW[OII] < 2˚A, below the measurement error in EW[OII] for CNOC2. Greater intrin- sic scatter in the SFR-[OII] relation than in the SFR-Hα relation also works to mask the division between the two populations.Thus,althoughwecannotcleanlyseparatethe two populations, we impose an arbitrary division at 5˚A in the CNOC2 and smoothed 2dF data. We expect the pop- ulation with EW[OII]<5˚A to be dominated by the passive population1 and the population with EW[OII]≥5˚A to be dominatedbythestar-formingpopulation,andthisdivision is sufficient to reveal trends in the data (see e.g. Hammer et al. 1997; Zabludoff & Mulchaey 2000). To assess therelative normalisation of the two popula- tions, we define fp as the fraction of passive galaxies. The level of [OII] emission in the star forming galaxies is char- acterised by <EW[OII]|SF>, which represents the median Figure3.ThefractionofgalaxieswithEW[OII]<5˚A(thepassive EW[OII] restricted to star–forming galaxies. population), fp, in 2dF and CNOC2 groups (within 1 projected Wenotethatitwillalsobeinterestingtoderivethefrac- Mpc of group centre) and field, as a function of MbJ. The 2dF tionofbluegalaxiesusingCNOC2colours,foramoredirect pointsareslightlyoffsetinMbJforclarity. comparison with classical studies of theButcher-Oemleref- fect. However, this analysis is not straightforward, because 3 RESULTS of the complex dependence of CNOC2 colour apertures on galaxy size,galaxy typeandredshift;themeagerness ofthe 3.1 Evolutionary and Environmental group red sequence and the difficulties in making a direct Dependencies of EW[OII] comparison with 2dFGRS. Many of these problems can be Acomparisonofthe2dFandCNOC2EW[OII]distributions overcomebycomputingthefractionofgalaxiesineachpeak inFigure2showsthatthefractionofgalaxiesinthe0˚Apeak of a bimodal colour distribution. This analysis will be pre- dependson both epoch and environment.Inparticular, the sented in a futurepaper, currently in preparation. 0˚Apeakinthe2dFdataismuchmoreprominentthaninthe CNOC2 survey, for both field and groups. At both epochs, however,thegroupgalaxypopulationismorebiasedtowards the 0˚A peak than the corresponding field population. We now explore thesetrends in more detail. 2.4.3 Aperture bias Systematic effects on the measurements of EW[OII] can be 3.1.1 The dependence of f on redshift, environment and induced by the relative aperture sizes used in the 2dFGRS p luminosity andCNOC2spectroscopy.Inparticular,the2dFfibresgen- erally sample light from a smaller physical radius than the In Figure 3, the fraction of passive galaxies, fp is plotted CNOC2 slits, and this might lead to an overestimate of fp against rest-frame MbJluminosity. Statistical errors are es- byexcludingthestar forming regions in face-on disk galax- timatedusingaJackkniferesamplingmethod(Efron1982). ies. In Appendix A, we use SDSS resolved photometry to Wecan see that: estimate the effects of aperture bias across our magnitude • In all samples, fp is a strong function of luminosity range.Wefindthatthefractionofgalaxies foundinthered withfaintergalaxiesfarmorelikelytobestarforming than peak of the bimodal colour distribution is no greater when brighter galaxies at equivalent redshifts. This is consistent considering colours measured inside 3′′ SDSS fibres, rather with many previousresults, e.g. Kauffmann et al. (2003). than the total colour. This is because both red and blue • fp is significantly greater in the galaxy groups than in galaxies have similar colour gradients, likely due to metal- thefieldatbothlowandintermediateredshiftandalsoright licityratherthanstarformation.Thusweconcludethatthe across theluminosity range investigated. effects of aperture bias do not strongly affect our measure- • fp is strongly redshift dependent, both in the field and mentsof fp. ingalaxygroups.AtbrightermagnitudesthanM =−18.5 bJ (∼ M∗+1.75 in 2dF), fp in groups evolves from ∼ 42% at 0.3 ≤ z ≤ 0.55 in CNOC2 to ∼ 71% at 0.05 ≤ z ≤ 0.1 in 2dF. In our field samples (defined to represent the global population),fp evolvesfrom∼25%to∼47%overthesame redshift interval. The observed field evolution is consistent 1 We note that the shape of the negative side of the 0˚A peak with the equivalent strong evolution in the fraction of pas- inthe EW[OII] distributionfromthe fullCNOC2surveyiscon- sistent with a gaussian function, supporting the hypothesis that sive galaxies in the Canada-France Redshift Survey (Ham- this peak is dominated by galaxies with no [OII] emission and meretal.1997)andtheglobaldeclineinstarformationrate normallydistributederrors(Whitaker etal.2004). since z ∼ 1 (e.g. Madau et al. 1998). We refer to Whitaker (cid:13)c 0000RAS,MNRAS000,000–000 6 D. J. Wilman et al. Figure 4. Cumulative distributions of EW[OII] in star forming Figure6.ThefractionofgalaxieswithEW[OII]<5˚A(thepassive galaxies in the field and group galaxies of 2dF (0.05 ≤ z ≤ 0.1) population) (fp) in 2dF and CNOC2 groups, as a function of and CNOC2 (0.3 ≤ z ≤ 0.55) samples. The arrows indicate the physical radius. The field level is overplotted and only galaxies meanvaluesofEW[OII]forstarforminggalaxiesineachsample. whereMbJ≤−18.5areconsidered. analysis of the EW[Hα] distribution (Balogh et al. 2004a). In the CNOC2 sample, the distribution shows a small en- hancementinhighlystarforminggalaxies(EW[OII]>∼30˚A) relative to the 2dF galaxies in both groups and the field. The mean values of EW[OII] for each sample are indicated by the arrows in Figure 4. These values of 26.4˚A (groups) and31.3˚A (field)inCNOC2arewithin∼20percentofthe mean values in the 2dF (20.9˚A in groups and 25.8˚A in the field) in 2dF. This difference is much smaller than the evo- lution in fp, which is almost a factor of two. Therefore, the evolution of the total star formation rate is driven more by theevolutionoffp thanbyevolutionofthemeanproperties of star forming galaxies. In Figure 5 we show the median EW[OII] among star– forminggalaxies,<EW[OII]|SF>,asafunctionofluminosity intheCNOC2and 2dFgroup andfield samples. Errors are again computed using a Jackknife resampling method. The Figure 5. The median EW[OII] (˚A) of star-forming galaxies, median EW[OII] is significantly larger for fainter galaxies <EW[OII]|SF>, in the 2dF and CNOC2 galaxies in the groups than for bright galaxies. Furthermore, the evolution in the (within1projectedMpcofgroupcentre)andfield,asafunction EW[OII]distributionislargely manifested asanincreasein ofMbJ.The2dFpointsareslightlyoffsetinMbJforclarity. <EW[OII]|SF> in galaxies with MbJ>∼−21.5. et al., 2004, in preparation, for a more detailed and thor- oughdiscussionoftheevolutionofstarformationrateinthe 3.1.3 Dependence of fp in groups upon group-centric global CNOC2population.Thisprovidesaclearanalogy at radius and velocity dispersion lower densities to theobserved evolution of theblue galaxy InFigure6weplotfp asafunctionoftheprojectedphysical fraction inclusters(Butcher&Oemler1984) andtosimilar distance, dr, from the group centre. It is clear that even in evolutioninrichgroups,estimatedbyAllington-Smithetal. thebettersampled2dFgroups,thetotalfraction ofpassive (1993) using photometric data. galaxies, fp merely declines from ∼ 0.76 in the innermost 0.125 h−1 Mpc bin to ∼ 0.65 in the 0.5 h−1 Mpc< δ(r) ≤ 75 75 1 h−1 Mpc bin. This is a weak, but statistically significant 3.1.2 The properties of the star forming population 75 trend. The value of fp for the total 2dF field population is Figure 4showsthecumulativedistributionofEW[OII],for ∼ 0.47, much lower than in the groups. Similarly, in the star forming galaxies only, in the 2dF and CNOC2 group CNOC2 field, fp = 0.25 is much smaller than that in the and field samples. Interestingly, the shape of the distribu- combined group population where fp = 0.42. We only see tion at a particular epoch (i.e. 2dF or CNOC2) is approxi- a trend with dr in the inner regions of the CNOC2 group mately independent of environment, consistent with earlier population, as shown in Paper I. However, a trend as weak (cid:13)c 0000RAS,MNRAS000,000–000 Galaxy Groups at 0.3 ≤ z ≤ 0.55. II. Evolution to z ∼ 0 7 butis seen both in thegeneral field and in small groups, as previouslyfoundbyAllington-Smithetal.(1993).Thesere- sults represent clear evidence that a significant proportion of galaxies in both environments have ceased forming stars sincez ∼0.5.Asimpleargument,neglectingdensityandlu- minosity evolution, sees 50% of star forming group galaxies and 35% of star forming field galaxies at z ∼0.4 (CNOC2) becoming passive galaxies by z ∼0.1 (2dF). This evolution is modelled in more detail in Section 4.2. In contrast, while we find significant redshift evolution in the shape of the EW[OII] distribution for star-forming galaxies, there is little or no dependence on environment. Thissuggeststhatthisevolutionresultsfromverylocal(i.e. internal to the galaxy) processes that drive an evolution in SFR or metallicity, rather than external, environmental influences. We caution however that amongst star-forming galaxies, it is not possible with current data to rule out an aperturebias in themeasurement ofEW[OII], which might Figure7.FractionofgalaxieswithEW[OII]<5˚A(passivegalax- leadtounderestimationofEW[OII]in2dFGRSstarforming ies) (fp) in 2dF and CNOC2 groups, as a function of group line galaxies. ofsightvelocitydispersion.Thepointsrepresent2dFgroupsand In the following sections, we make a first attempt to thetrianglesrepresentCNOC2groups. quantitatively decouple the environmental dependence of galaxy evolution from the global SFR evolution. To fully understand this, we require a homogeneous sample over a as that seen in 2dF group galaxies would bemasked by the large range of environments. We are currently working on statistical errors. providing a fair comparison study in cluster cores (Nakata Wehavealsoinvestigatedhowthestarformationprop- ertiesofgalaxygroups2 dependuponthegroupvelocitydis- etal.2004) usingspectroscopicdataoveranequivalentred- shift range. Understanding the importance of galaxy evo- persion,σ(v)intr.InPaperIwefoundthatthereislittlede- lution in groups with respect to cluster cores will help to pendenceoffponσ(v)intr whencomputedoverawiderange isolate the environments in which galaxy evolution is most of galaxy luminosity.Figure 7shows fp in CNOC2and 2dF active. Complementary studies of the evolution in isolated groups as a function of σ(v)intr. There are no clear trends galaxies would be of especially great benefit to understand visibleinFigure7otherthantheevolutionfrom lowerfp in the role of environment in driving galaxy evolution, and in CNOC2groupstothatseenin2dFgroups.Itisalsonotice- able that there are very few 2dF groups with high velocity particular, theevolution of fp. dispersion (> 400km s−1) and low fp (< 0.4), which is a common regime for CNOC2 groups. We note that the en- 4.2 Modelling and Interpretation hancement of fp in CNOC2 groups over the CNOC2 field holds when we exclude the 2 groups with velocity disper- InthisSection,weshowhowthestrongevolutioninfp seen sion > 600km s−1 (for more details see Paper I). The only inourresultscanbeinterpretedinthecontextofgalaxyevo- CNOC2groupwithfp >0.7(group138)ischaracterizedby lution models. By combining theBruzual & Charlot (2003) ahighnumberofconfirmedmembers(35comparedto19in models of luminosity evolution with a range of star forma- the group with thenext highest σ(v)intr) and might better tion histories, we attempt to recreate a realistic evolution beconsideredapoorcluster(seee.g.Nakataetal.2004,for scenario which can reproduce theobserved evolution. thebehaviour of fp in clusters). We assume that CNOC2 galaxies represent a popula- tion equivalent to the progenitors of the 2dF population; therefore, using the stellar population models of Bruzual & Charlot(2003)itispossibletocreatemock-2dFpopulations 4 DISCUSSION byevolvingtheCNOC2galaxiestothemeanredshiftof2dF 4.1 Implications galaxies (z2dF = 0.08) in accordance with a chosen set of modelparameters.Wepresenttwomodelmethodswithdif- We have detected a strong evolution in the fraction of pas- ferentevolutionaryscenariosbutsimilarbasicmethodology. sive galaxies, fp (defined as the fraction of galaxies with Both models allow us toestimate the evolution of EW[OII] EW[OII]< 5˚A), in both groups and the field since the Uni- andM withinagivensetofparameters.Thequiescentevo- verse was ∼1/3 its current age. Thus, the Butcher–Oemler bJ lution scenarioischaracterizedbythelackofenvironmental effect(e.g. Butcher&Oemler1984; Margoniner etal. 2001; evolution. There are no sudden events which drastically al- DeProprisetal.2003b)isnotstrictlyaclusterphenomenon, ter a galaxy’s star formation. Bright star-forming galaxies simply decline exponentially in their star formation with a constant e-folding timescale and thusfade in luminosity. In 2 WenotethatwhilstfpinCNOC2groupsiscomputedwitheach the truncation scenario, we incorporate into our evolution galaxy weighted by its combined completeness and luminosity weighting Wtot (strictly only applicable when the full stacked modelaprobability of each galaxy undergoingatruncation groupisconsidered)wefindthatfpisinsensitivetothisweighting event, in which it suddenly ceases star formation. In this andveryclosetothevalueobtainedwithnoweightingapplied. model, there is also a probability that a high redshift field (cid:13)c 0000RAS,MNRAS000,000–000 8 D. J. Wilman et al. Figure 8. Thefraction of passivegalaxies, fp, inthe 2dF(solid Figure 9. Similar to Figure 8, but assuming an extreme quies- line) and CNOC2 group (within 1 projected h−1Mpc of group centevolutionmodelfortheCNOC2population,withaSalpeter 75 centre) and field galaxy populations, as a function of MbJ. The IMF,zform=3,solarmetallicityandnodustextinction(dashed- CNOC2galaxiesareshownasobserved(dotted-line),andevolved line).Thisrepresentsthemaximumevolutionexpectedfromany to z2dF using a quiescent model with a Kennicutt IMF, zform= realisticquiescent model. The 2dF points areslightly negatively 10, solar metallicity and dust extinction (dashed-line). The 2dF offset, and the CNOC2 data points slightly positively offset in pointsareslightlynegativelyoffset,andtheCNOC2datapoints MbJforclarity. slightlypositivelyoffsetinMbJforclarity. 4.2.2 The Truncation Scenario galaxycaninfallontoagalaxygrouptobecomealocalgroup galaxy. Next we consider a model in which galaxies undergo trans- formationsthatcausethemtoceaseformingstars.Wehave shown in Section 4.2.1 that some form of galaxy transfor- mation appears to be required to reproduce our observed 4.2.1 The Quiescent Evolution Scenario evolution in fp. Here, we constrain the probability of these Quiescent evolution describes the evolution of galaxies in transformationsrequiredtomatchtheobservedevolutionin whicheverygalaxy’sstarformationdeclinesoveritslifetime fp and its dependenceupon galaxy luminosity and environ- with a single e-folding timescale. The modelling procedure ment.ThemodellingprocedureisdescribedinAppendixB2. for quiescent evolution is described in Appendix B1, which In this scenario, a CNOC2 galaxy either continues with its alsodescribestheeffectsofallowingeachparametertovary. e-foldingdeclinein starformation, orhasitsstarformation To investigate the effects of quiescent evolution on fp, we truncatedinstantaneouslywithaprobabilityPtruncperGyr, chooseacontrolmodelandanextrememodel.Forourcon- at a random point during its evolution to z =0.08. The 2dF trol model we choose a Kennicutt IMF, a galaxy formation timescale of the transformation is likely to havelittle effect redshiftzform=10andsolarmetallicity.Wealsoincorporate on theevolution of fp. Weneglect thepossibility that some atwo-componentdustprescriptionintothemodel(Granato transformationsareaccompaniedwithastrongstarburstor et al. 2000). Figure 8 shows fp as a function of luminosity involvethemergingofgalaxiesaseitherofthesepossibilities intheobservedCNOC2and2dFsamples,togetherwiththe cannot be constrained by simply considering the evolution equivalenttrendintheevolvedCNOC2populationobtained in fp. In a later paper we will consider our data in thecon- using this model. The evolution of fp in the model signifi- text of a more complete galaxy formation model (e.g. Cole cantly underestimates the trend seen in the real data. This et al. 2000). is partly because galaxies which become passive also tend We adopt a realistic set of parameters governing to fade into a fainter bin of luminosity, leaving the trend the spectrophotometric evolution, with a Kennicutt IMF, offp withluminosityapproximatelyunchanged.Forourex- zform= 10, solar metallicity and a basic dust prescription trememodel,wedeliberatelychooseparameterswhichmax- (Granato et al. 2000). The progenitors of 2dF group galax- imize the evolution in fp, as discussed in Appendix B1. We ies are chosen by selecting all CNOC2 group galaxies plus choose a Salpeter IMF, z = 3, solar metallicity and we a fraction of the CNOC2 field galaxies such that the com- form ignoretheeffectsofdust.Figure9showsthesame2dFand bined set is made up of ξ % CNOC2 group members and gr CNOC2 data as in Figure 8 but this time overplotted with (1−ξ )%CNOC2fieldgalaxies.Asourfieldrepresentsthe gr theevolvedCNOC2populationobtainedusingthisextreme global population, the progenitors of 2dF field galaxies are model. In this case, galaxies fainter than ∼ M∗ −0.5 still simply the CNOC2 field galaxies. Following the model pre- show a significant deficit of passive galaxies (low fp) in the scription, we obtain best fit values for Ptrunc as a function evolved CNOC2 galaxies when compared to the data. This oflocalluminosityandenvironmenttofittheobservedevo- provides strong evidence that transformations are required lution in fp. Figures 10 and 11 show Ptrunc as a function toreproducetheobservedevolutioninfpbothingroupsand of luminosity in groups and the field with ξgr = 100% (lo- theglobal (field) population. calgroupmemberswereallgroupmembersatzCNOC2)and (cid:13)c 0000RAS,MNRAS000,000–000 Galaxy Groups at 0.3 ≤ z ≤ 0.55. II. Evolution to z ∼ 0 9 (ii) Assuming no density evolution since z ∼ 0.45, ξ = gr 100%, (Figure 10), we see no evidencethat Ptrunc is larger in groups than in the field. This means that there must be some global mechanism in which star formation can be ef- fectively reduced to zero over a short period of time rather than simply declining in a quiescent manner as assumed in Section4.2.1.Howevertheexistenceofamoreevolvedpopu- lation(higherfp)ingroupssuggeststhatthestarformation history prior to z ∼ 0.45 must depend upon environment in some way. This could be either a nurturing environmen- tal process at z > 0.45, or an earlier formation time for galaxies in groups (nature). We emphasize that our model is designed to simply match the observed evolution of fp. It cannot simultaneously match evolution of the luminos- ity function, which requires a better understanding of the volume-averagedgalaxydensity.Wearealsoconstrainedby our definition of “field” which spans the full range of envi- ronment. Figure 10. The probability (per Gyr) that star formation has (iii) If we assume a strong density evolution with only been truncated (Ptrunc) as a function of MbJ as determined 50%oflocalgroupgalaxiesingroupsatz∼0.45,ξ =50%, by modelling the evolution of fp between the CNOC2 (0.3 ≤ gr zCNOC2 ≤ 0.55) and 2dF (0.05 ≤ z2dF ≤ 0.1) samples. The (Figure 11), then a marginally larger Ptrunc is invoked in crosses represent the group galaxies and the diamonds repre- groupsthanin thenodensity evolution (ξgr =100%) case, sent the field population which has been artificially offset by although not significantly so. Evenat faint luminosities the 0.05magforclarity.Here,thereisassumedtobenodensityevo- differences between Ptrunc in groups and the field is still of lution,i.e.allgroupgalaxies werealreadyingroups byzCNOC2 low significance (< 2σ in the −19.5 ≤MbJ≤ −18.5 bin). (ξgr = 100%). The vertical line represents the location of a M∗ Physically, an enhanced Ptrunc with greater density evolu- galaxyin2dFasdeterminedfromtheluminosityfunctionofNor- tion is consistent with a second transformation process oc- bergetal.(2002). curingduringclusteringasagalaxyisinfallingintoalarger dark matter halo. A strong density evolution with a Ptrunc whichremainsconstantwithredshiftcouldtheoreticallyex- plainthelargervalueoffp ingroupsthaninthefield.How- ever, realisations of dark matter halo merger trees suggest that the actual fraction of 2dF group galaxies in groups by z=0.45 was ∼80% (ξ =80%) (Lacey & Cole 1993). gr (iv) There are no clear trends of Ptrunc with galaxy lu- minosity in groups. In the field there is a suggestion (∼ 2σ significance) that Ptrunc decreases in the faintest bin (−19.5≤M ≤−18.5). bJ Weacknowledgethatourmodelissimple,andneglects themass and luminosity enhancing effects of galaxy-galaxy mergers. 2dFGRS studies of the local luminosity function, and its dependence upon environment and galaxy spectral type,suggestthatgalaxieswithearlyspectral-typesbecome more important in higher density regimes, particularly at lowluminosities(DeProprisetal.2003a;Crotonetal.2004). DeProprisetal.(2003a) showthatasimplemodel(similar Figure 11.As Figure 10, but assuming local 2dF groups com- toours),inwhich starformation canbesuppressed inclus- prise50%CNOC2groupgalaxiesand50%CNOC2fieldgalaxies ters,canexplainmostofthedifferencesbetweenthecluster (ξ =50%). gr andfieldluminosityfunctions. Theyclaim thatmergers are onlyrequiredtoexplainthesmallpopulation ofverybright early typecluster galaxies. ξ =50% (local group members were 50% group members gr The precise importance of galaxy mergers remains to and50%fieldgalaxiesatzCNOC2)respectively.Wenotethat beseen. There is significant evolution since z∼1 of the lu- adopting a Salpeter IMF does not significantly alter these minosityfunction ofred galaxies in theCOMBO-17survey, results. and Bell et al. (2004) conclude that mergers are required Figures 10 and 11 show thefollowing: to explain at least some of this evolution. In clusters, com- (i) Ptrunc issignificantlygreaterthanzero,implyingthat parisons of the K-band luminosity function over a similar galaxytransformationsarerequiredoverourredshiftrange, redshift range suggest there is little evolution in the stellar both in groups and the field. This is independent of the mass of cluster galaxies (De Propris et al. 1999; Kodama assumed evolution of clustering power and agrees with our & Bower 2003). However, mergers are expected to be more conclusions in Section 4.2.1. common in groups than in clusters (e.g. Barnes 1985). In a (cid:13)c 0000RAS,MNRAS000,000–000 10 D. J. Wilman et al. future paper, we will investigate the importance of mergers they fall into groups (nurture). However, it is also possi- ingroups,usingexistingdataonCNOC2and2dFgroupsto ble to imagine a nature scenario in which more strongly maptheevolutionofthegroupK-bandluminosityfunction. clustered galaxies form first and all galaxies undergo trans- In a separate paper we will also make comparisons of forming events, independently of their environment. A bet- the data with results from semi-analytic models of galaxy ter understandingof the environmental influence on galaxy formation. Observational constraints on the bimodality of propertieswill bemadepossible bycomparisons with semi- galaxyproperties,andthedependenceongalaxyluminosity, analyticmodels,galaxiesinotherenvironments(e.g.Nakata environmentandredshiftwillplacestronglimitationsonthe et al. 2004) and higher redshift galaxy systems. physicalprocessesregulatingstarformationinthesemodels. 6 ACKNOWLEDGEMENTS: 5 CONCLUSIONS Wewould liketothanktheMagellan staff fortheirtremen- dous support. RGB is supported by a PPARC Senior Re- In this paper, we have examined the evolution of galaxies searchFellowship.DJW,MLBandRJWalsothankPPARC and the effects of the group environment in kinematically for their support. VRE is a Royal Society University Re- selected groups from the CNOC2 (Carlberg et al. 2001b, search Fellow. We are grateful to Dan Kelson for the use supplementedwith newand deeperMagellan spectroscopy) of his spectral reduction software and to David Gilbank for and 2dFGRS (Eke et al. 2004) surveys. The data span the his help when learning to use it. We would like to thank redshift range 0.05 ≤ z ≤ 0.55 and luminosities down to IanLewisforhismeasurementsofEW[OII]in2dFGRS.We MbJ≤ −18.5 (locally ∼ M∗ +1.75). Motivated by the ap- alsoacknowledgeTomShanksandPhilOutramfortheirob- parently fundamental differences between the blue, star- servations at Magellan and the full CNOC2, 2dFGRS and formingandthered,passivepopulationsofgalaxies(Balogh SDSSteamsforoutstandingdatasets.ThanksalsogotoBob etal.2004a;Blantonetal.2003)wehavearbitrarilydivided NicholandChrisMillerfortheirhelpinproducingtheSDSS our galaxies into passive (EW[OII]< 5˚A) and star-forming catalogues.WethankGustavoBruzualandSt´ephaneChar- (EW[OII]≥5˚A) populations. We have then shown that the lot for their publically available spectro-photometric evolu- fraction of passive galaxies fp is a strong function of: tionary modelling software GALAXEV and Carlton Baugh and Cedric Lacey for helping to develop software used to • redshift: fp declines strongly with redshift, both in model the galaxy properties we required. Finally, we thank groupsandthefieldandoverthefullluminosityrangetoat leastz∼0.45.ThisisequivalenttoaButcher-Oemlertrend the anonymous referee for some useful feedback which has helped to improvethis paper. in the emission line properties of group galaxies and in the global population. • environment:fp issignificantly higheringroupsthan the field across the full luminosity range, both locally and REFERENCES at z∼0.45. • luminosity:fp increasessteeplywithluminosityacross Allington-Smith, J. R., Ellis, R., Zirbel, E. L., & Oemler, ourrange(MbJ<∼−18.5)ingroupsandthefielduptoatleast A.J. 1993, ApJ, 404, 521 z∼0.45. Andreon,S. & Ettori, S.1999, ApJ, 516, 647 Andreon,S.,Willis,J.,Quintana,H.,Valtchanov,I.,Pierre, UsingthestellarpopulationmodelsofBruzual&Char- M., & Pacaud, F. 2004, MNRAS,353, 353 lot (2003), we have shown that the rate of evolution in fp Baldry, I. K., Glazebrook, K., Baugh, C. 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