Direct observational evidence for a large transient galaxy population in groups at 0.85

1.2 MB·English

by  Michael L. Balogh

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed11January2011 (MNLATEXstylefilev2.2)
Direct observational evidence for a large transient galaxy population
0.85 < z < 1
in groups at
1
Michael L. Balogh1, Sean L. McGee1,2, David J. Wilman3, Alexis Finoguenov3,4,
1
0 Laura C. Parker5, Jennifer L. Connelly3, John S. Mulchaey6, Richard G. Bower2,
2
Masayuki Tanaka7, Stefania Giodini8
n
a 1DepartmentofPhysicsandAstronomy,UniversityofWaterloo,Waterloo,Ontario,N2L3G1,Canada
J 2DepartmentofPhysics,UniversityofDurham,Durham,UK,DH13LE
0 3Max–Planck–Institutfu¨rextraterrestrischePhysik,Giessenbachstrasse85748GarchingGermany
1 4CSST,UniversityofMaryland,BaltimoreCounty,1000HilltopCircle,Baltimore,MD21250,USA
5DepartmentofPhysicsandAstronomy,McMasterUniversity,Hamilton,Ontario,L8S4M1Canada
] 6ObservatoriesoftheCarnegieInstitution,813SantaBarbaraStreet,Pasadena,California,USA
O 7InstituteforthePhysicsandMathematicsoftheUniverse,UniversityofTokyo,Kashiwa2778582,Japan
C 8LeidenObservatory,LeidenUniversity,POBox9513,2300RALeiden,theNetherlands
.
h
p
- 11January2011
o
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t
s
a ABSTRACT
[ We introduce our survey of galaxy groups at 0.85 < z < 1, as an extension of the Group

EnvironmentandEvolutionCollaboration(GEEC).Herewepresentthefirstresults,basedon
2
GeminiGMOS-Snod-and-shufflespectroscopyofsevengalaxygroupsselectedfromspectro-
v
scopicallyconfirmed,extendedXMMdetectionsinCOSMOS.Weusephotometricredshifts
9
to select potential groupmembersfor spectroscopy,and targetgalaxies with r < 24.75. In
0
5 totalwe have over100confirmedgroupmembers,andfourof the groupshave> 15 mem-
5 bers. The dynamicalmass estimates are in goodagreementwith the masses estimated from
1. theX–rayluminosity,withmostofthegroupshaving13 < logMdyn/M⊙ < 14.We com-
pute stellar masses by template-fitting the spectral energy distributions; our spectroscopic
1
0 sampleisstatisticallycompleteforallgalaxieswithMstar >∼ 1010.1M⊙,andforbluegalax-
1 ies we sample masses as low as Mstar ∼ 108.8M⊙. The fraction of total mass in galaxy
: starlight spans a range of 0.25–3per cent, for the six groupswith reliable mass determina-
v
tions.Like lower-redshiftgroups,these systemsare dominatedby redgalaxies,atallstellar
Xi massesMstar > 1010.1M⊙.Fewgroupgalaxiesinhabitthe“bluecloud”thatdominatesthe
surroundingfield;instead,wefindalargeandpossiblydistinctpopulationofgalaxieswithin-
r
a termediatecolours.The“greenvalley”thatexistsatlowredshiftisinsteadwell-populatedin
thesegroups,containing∼30percentofthegalaxies.Thesedonotappeartobeexceptionally
dusty galaxies, and about half show prominentBalmer-absorption lines. Furthermore, their
HSTmorphologiesappeartobeintermediatebetweenthoseofred-sequenceandblue-cloud
galaxiesof the same stellar mass. Unlike red-sequencegalaxies, most of the green galaxies
have a disk component, but one that is smaller and less structured than disks found in the
bluecloud.Wepostulatethattheseareatransientpopulation,migratingfromthebluecloud
to the red sequence, with a star formation rate that declines with an exponential timescale
0.6Gyr < τ < 2Gyr. Such galaxiesmay not be exclusive to the groupenvironment,as we
findexamplesalsoamongstthenon-members.However,theirprominenceamongthe group
galaxypopulation,andthemarkedlackofblue,star-forminggalaxies,providesevidencethat
thegroupenvironmenteitherdirectlyreducesstarformationinmembergalaxies,oratleast
preventsitsrejuvenationduringthenormalcycleofgalaxyevolution.
Keywords: galaxies:clusters
1 INTRODUCTION
Galaxy evolution since about z ∼ 2 is characterized gener-
ally by the cessation of star formation. On average, the star
2 Baloghet al.
formation rate density in the Universe has declined by a fac- clustersislowerthanitwasinthepast(e.g.Ellingsonetal.2001;
tor ∼ 10 since that time (e.g. Lillyetal. 1996; Hopkins 2004; McGeeetal. 2009), and recently accreted galaxies will represent
Gilbanketal. 2010). The rate of decline appears to depend on asmallfractionoftheentirepopulationwhichhasbuiltupovera
both stellar mass (e.g. Cowieetal. 1996; Juneauetal. 2005; Hubbletime. Sothere islessopportunity tocatch galaxiesinthe
Noeskeetal.2007;Belletal.2007;Gilbanketal.2010)andenvi- actoftransformation,ifsuchaprocessoccurs.
ronment(e.g.Poggiantietal.2006;Cooperetal.2008;Maieretal.
Thus,thebestplacetoobserveenvironmentally–drivenevolu-
2009; Iovinoetal. 2010; Vulcanietal. 2010; Pengetal. 2010;
tion in action is in gas-rich, low–mass galaxies inhabiting small
Cucciatietal. 2010; Sobraletal. 2010; McGeeetal. 2010). This
groups, at relatively high redshift. This was the motivation be-
leadstoafairlyconsistentempiricaldescription,atleastformas-
hindtheGroupEnvironmentandEvolutionCollaboration(GEEC),
sivegalaxies.Thegalaxypopulationiswell-modeledbyhavingstar
that began with deep Magellan LDSS3 spectroscopy of groups
formation“quenched”onrelativelyshorttimescales;thisquench-
at 0.3 < z < 0.55, selected from the CNOC2 survey fields
ing happens first for the most massive galaxies, and later for
(Wilmanetal.2005a,b).Inaseriesofpapersweshowedthatwhile
lower–mass systems (e.g. Noeskeetal. 2007). This evolution is
disk–dominated, star forminggalaxiesof agiven stellarmassare
somewhat accelerated indenser environments (Iovinoetal.2010;
lesscommoningroupsthaninthegeneralfield,thepropertiesof
Vulcanietal.2010;Lietal.2010),especiallyforthelowest-mass
the star–forming population itself seems independent of environ-
galaxiesstudiedtodate.
ment (e.g. McGeeetal. 2010). This is surprising, as it suggests
However, welackaclearunderstanding ofthephysical pro- thateitherthetransformationisveryrapid,orthenumberofgalax-
cesses driving this evolution. It appears that the dependence on iesundergoingtransformationattheepochofobservationissmall.
stellar–mass and environment are separable (Baldryetal. 2006), Atthesametime,groupgalaxieslookmuchmorelikethegeneral
and likely point to different mechanisms. Recently, Pengetal. fieldinmostrespectsthanisthecaseatlowerredshift(McGeeetal.
(2010)haveproposedaninterestingempirical descriptionofthis, 2010).Thissuggeststhatevolutioningroupsisarelativelyrecent
wherethequenching rateisdescribedbyonetermthatispropor- phenomenon(seealso Luetal.2009).
tionaltostarformationrate,andanotherthatisrelatedtothelocal
Nonetheless,groupsatz ∼ 1andbeyondstillshowalarger
density.Thisworkssurprisinglywell,butstilldoesn’tlenditselfto
population of passively–evolving galaxies than the surrounding
aclearphysicalinterpretation.
field (e.g. Cooperetal. 2010). Interestingly, however, there is in-
Galaxy formation models are too complex and under-
creasing evidence that those galaxies that are forming stars, are
constrained to provide an unequivocal answer at this time. How-
doing so at a higher rate than field galaxies of similar stellar
ever, they do indicate that parametric treatments of energy input
mass (Elbazetal. 2007; Muzzinetal. 2008; Cooperetal. 2008;
duetosupernova andsupermassiveblackholeaccretioncandoa
Ideueetal.2009;Sobraletal.2010;Lietal.2010;Kocevskietal.
reasonable jobof explaining themass–dependent quenching (e.g.
2010). It may be at thisepoch that dense environments stimulate
Crotonetal.2006;Boweretal.2008).Theenvironmental termis
starformation,leadingtoarapidconsumption ofgasandleading
accountedfortosomeextentintheso–called“halomodel”,where
to the dominant old population at z = 0. The joint density- and
the central galaxy of a given dark matter halo is treated some-
mass-dependenceofthiseffectisnicelyillustratedbySobraletal.
how differently from the satellites (e.g. Gilbank&Balogh 2008;
(2010).Thehigheraccretionrate,andyoungerageoftheUniverse,
Skibba&Sheth 2009; McGeeetal. 2009). Such a distinction is
hasledustopredictthattherewillbemorediversityamonggalaxy
expected at some level, since the hot gas that is thought to fill a
groupsatthisredshift(McGeeetal.2009).
common halo should, for the most part, cool and condense only
ontothecentralgalaxy.Muchofthehotgasassociatedwithsatel- Wehavethusembarked onanambitiousspectroscopic cam-
litegalaxieswilllikelygettidallystripped,ram-pressurestripped, paign,usingtheGMOSspectrographsonGeminiNorthandSouth
or shock heated until it becomes part of the common halo (e.g. to study the galaxy populations in ∼ 20 groups, mostly selected
McCarthyetal.2008;Kawata&Mulchaey2008;Fontetal.2008). withintheCOSMOS(Scovilleetal.2007b)surveyfields.Thedeep
Thispicturereliesonanumberofuntestedassumptions,however, spectroscopy achieves twomaingoals.First,wecanstudygalax-
and simpleimplementations of thisdo not doa verygood jobof ieswithlowstellarmass;oursampleiscompleteforredgalaxies
predictingtheobservedstarformationhistoriesofgroupedgalax- 1.0magfainterthanthezCOSMOS10ksurvey(Lillyetal.2007),
ies(Weinmannetal. 2006; Gilbank&Balogh 2008; Baloghetal. 0.65magfainterthanDEEP2(Cooperetal.2008)and∼0.25mag
2009). fainterthanEDisCSatthisredshift(Hallidayetal.2004);forblue
galaxieswearecompletetoevenfaintermagnitudes.Thisdepthis
Thestudyofgalaxyevolutionindenseenvironmentshasbeen
vitalforstudying environmental effects,whichappear tobemost
hampered by several observational difficulties. The easiest place
importantforthelowest–massgalaxies(Cucciatietal.2010).Sec-
to look is in the cores of rich clusters at relatively low redshift
(e.g.Baloghetal.1997;Poggiantietal.1999;Baloghetal.2002; ondly,thedepthallowsustoobtain∼ 20spectroscopicmembers
pergroup,whichissufficienttoestimatethedynamicalmass,and
Lewisetal. 2002; Gomezetal. 2003; Luetal. 2009, and many
dynamical state,ofeachgroup(Houetal.2009).Moreover, ital-
others). But the cores of rich clusters are extreme environments
lowsustoconsiderthegalaxypopulationofindividualgroups,or
where, for example, ram-pressure stripping can remove even the
subsetsofgroups. Thus,wecanattempttolinktransientgalaxies
coldgasfromdisks(e.g.Quilisetal.2000;Koopmann&Kenney
tothedynamicsorrecentaccretionhistoryofthelarger–scaleen-
2004),andgalaxyinteractionseffectivelydistortgalaxymorpholo-
vironment.
gies (Mooreetal. 1996). These are interesting processes, but are
unlikelytobetheexplanationfortrendsobservedinlower–density Wepresentourdata,includingafulldescriptionofthegroup
environments(e.g.Baloghetal.2004).Moreover,lowredshiftsys- andspectroscopic selection, in§2. Thebasicanalysisof redshift
tems provide additional challenges. Galaxies at low redshift are andstellarmassmeasurements,anddeterminationofgroupdynam-
generally older and more gas poor, due to normal evolution, so icalmasses,ispresentedin§3.Ourmainresults,onthestellarfrac-
there is less potential for environment to disrupt star formation. tionandthediscoveryofadominantgrouppopulationofgalaxies
Also, the accretion rate of new, relatively gas–rich galaxies, into withintermediatecolours,spectraltypeandmorphology, arepre-
Transientgalaxiesin groupsatz = 1 3
sented in § 4. Finally, we discuss the implications of our results, 2.2 Galaxygroupselection
andsummarizeourconclusions,in§5.
Foroursurvey,weareinterestedinthelowest-massgroupsthatare
Throughout the paper, we assume a cosmology with Ωm = robustlyidentified,andwithintheredshiftrange0.85<z<1.We
0.3,ΩΛ = 0.7andh = H◦/(100km/s/Mpc) = 0.7.Allmagni- consideredallgroupsintheFinoguenovetal.(inprep)catalogue,
tudesareontheABsystem.
thatliewithinthisredshiftrange,haveatleast3spectroscopically-
determined members, and were considered secure identifications.
Specifically, we consider groups that are either category 1 (good
detection,withawell-determinedX–raycentre)orcategory2(se-
curedetection,butwithanunreliableX–raycentre).Thereare21
groupssatisfyingthisselection.Ofthese,wehavegivenpreference
2 DATA tothelowest–mass,highestredshiftsystems,andhaveavoidedtar-
getsthatalreadyhave>10redshiftsusedintheidentification.The
2.1 Theparentcatalogues latterselectionismadeinparttoavoidmassiveclusters,andtoen-
sureourspectroscopy increasesthetotalnumber ofgroupsinthe
Our survey targets galaxy groups within the COSMOS field
fieldwithlarge,spectroscopically-confirmedmembership.
(Scovilleetal.2007b).Thisfieldbenefitsfromawiderangeofpub-
Ourfirstobservations,describedin§2.3,targetedsixofthese
liclyavailable,deep,multiwavelengthphotometry.Thebackboneis
groups.TheirpropertiesaretabulatedinTable1.
thewidestHubbleSpaceTelescope(HST)surveyeverundertaken,
covering ∼ 2 square degrees with the F814W filter on the Ad-
vancedCameraforSurveys(Scovilleetal.2007a).Inaddition,we
willmakeuseofthedeepX-rayobservationsobtainedwithXMM 2.3 GeminiObservations
(Hasingeretal. 2007) and Chandra (Elvisetal. 2009); 3.6µm –
2.3.1 Spectroscopictargetselection
24µmdatafromSpitzerIRACandMIPS(Sandersetal.2007),and
ground-basedopticalandnear-infraredphotometryfromavariety Acrucialpartofourstrategyistheuseofphotometricredshiftsto
ofsources(Capaketal.2007). selectpotentialgroupmembers.Wegivehighestprioritytogalax-
OursurveyisbuiltonthreeimportantcontributionstoCOS- ies that have 21.5 < r < 24.75, and a redshift within 2σzphot
MOS. First are the exquisite photometric redshifts, derived from of the estimated group redshift, where σzphot is the 68 per cent
thephotometry of 30broad, intermediate andnarrow-band filters confidence level on the photometric redshift1. Secondary prior-
(Ilbertetal.2009).Thesearederivedusingatemplate-fittingtech- ity slits are allocated to galaxies with 15 < r < 24.75 and
nique, andcalibratedbasedonlargespectroscopic samples. Even 0.7<zphot <1.5.
for the faintest galaxies in our sample, most of the objects have We designed 3–4 GMOS masks on each target, using the
photometric redshifts determined to better than 10 per cent (see GMMPSsoftware.Oneachmaskweareabletoallocate40–50slits.
§2.3.1).Secondly,weusethe10KreleaseofthezCOSMOSspec- Typically, well over half the slits on the first mask are allocated
troscopic survey (Lillyetal. 2007, 2009), from which we obtain toour toppriority objects. Thisfraction decreases on subsequent
redshiftsformoderately brightgalaxies(IAB < 22.5) over most masks,asweuseupthesetargets.Inmostcases,withthreemasks
ofthesurveyarea.Thisisasparsely–sampledredshiftsurvey,with weareabletotargetatleast40ofthesehighprioritygalaxies.All
∼40percentsamplingcompleteness. masks for a given target use the same alignment stars and are at
Thethirdpillarofoursurveycomesfromanalysisofthedeep thesamepositionangle; thusasmallfractionoftheCCDareais
X-raydata(Finoguenovetal.2007).Usinganestablishedwavelet unusableregardlessofthenumberofmasksobtained.
technique (e.g. Finoguenovetal. 2009, 2010) it has become pos-
sible to detect extended X-ray emission from groups of galaxies
out to z ∼ 1. Identification of group redshifts is done using all 2.3.2 Observations
availablespectroscopy,includingbutnotlimitedtothe10KzCOS-
MOS survey. The combination of X–ray and redshift data pro- Weobtained40hoursworthofobservations onGeminiSouth,in
videsarobust catalogueofgroupsandclusters,toenablefollow- theBand1queueduringsemester10A.Thisreturnedsciencedata
up(Finoguenovetal.,inprep).MassesareestimatedfromX–ray of2hourson-sourceexposure, for14masksin5fields,covering
scaling relations (e.g. Rykoffetal. 2008), and calibrated from a sixgroups(twoofthegroupsarelocatedwithinthesameGMOS
stackedweak-lensinganalysis(Leauthaudetal.2010).Catalogues field). All science observations were obtained in clear conditions
basedonredshiftonly(e.g.Gerkeetal.2005;Knobeletal.2009) withseeing0.8arcsecorbetterini.Thenumberofmasksactually
arepowerfultoolswithwhichtoprobetheevolutionintheaverage observedisgiveninTable1.
properties of groups. However, such catalogues suffer from con- The spectroscopy was obtained in nod & shuffle mode
tamination and projection effectswhich make it difficult tointer- (Glazebrook&Bland-Hawthorn 2001), nodding the telescope by
pretstudiesofindividualsystems.Thecross-correlationwithX-ray ±0.725arcsecfromthecentreoftheslit,every60s.Thisplacesthe
allowsonetoobtainamorerobustsampleofgroups. Thispoten- galaxyatthelowerendofthe3arcsecslithalfthetime,andatthe
tiallyintroducesascientificallyinterestingbias,ifgroupswithX– upperendtheotherhalfofthetime.Weusedmicroshufflingmode,
rayemissionhavefundamentallydifferentpopulationsfromgroups sochargewasshuffledby21pixels(3.05arcsec).Fourexposures
of similar mass without such emission. However, the main dif-
ferencebetween optically–andX–ray–selected samplesseemsto
beprimarilyjustthatthelatterpreferentiallyselectsmoremassive 1 Atourredshift ofinterest, the average uncertainty onzphot increases
systems(Jeltemaetal.2009;Finoguenovetal.2009;Baloghetal. fromσzphot ∼ 0.007forthebrightestgalaxiestoσzphoto ∼ 0.04for
2010).Wethususethiscatalogueasthebasisofourgroupselec- thoseatourlimitofr=24.75.Evenforthesefaintestobjects,90percent
tion,whichwedescribeinthefollowingsubsection. ofthegalaxieshavezphotuncertaintiesoflessthanσzphot<0.07.
4 Baloghet al.
Group RA Dec zmed Nmask Nmem σ Rrms Mdyn
deg(J2000) (km/s) Mpc (1013M⊙)
40 150.41991 1.85265 0.97225 2 16 765±124 0.32±0.03 12.84±5.50
130 150.02586 2.20367 0.93828 3 26 403±48 0.63±0.06 7.12±2.40
134 149.65073 2.20932 0.94729 3 21 384±60 0.92±0.07 9.44±3.69
150 149.97472 2.31654 0.93428 4 20 209±31 0.75±0.07 2.29±0.90
161 149.95776 2.34531 0.94330 4 9 372±56 0.45±0.07 4.36±2.01
213 150.39697 2.49136 0.87990 2 7 281±120 0.70±0.07 3.85±3.69
213a 150.42715 2.49992 0.92650 2 6 44±17 0.49±0.10 -
Table1.Properties oftheseven galaxy groupsobserved withGMOSinsemester 10A.Theposition, median redshift zmed,velocity dispersion, andthe
numberofgroupmembersaredeterminedfromourGMOSspectroscopy(combinedwithavailablezCOSMOS10kdata),asdescribedinthetext.Thenumber
ofGMOSmasksobservedineachfieldisgivenbyNmask;notethatgroups150and161liewithinthesamefield,asdogroups213and213a(aserendipitous
discoveryinthebackground).Rrmsisthermsprojecteddistanceofallgroupmembersfromthecentre.Thedynamicalmassiscomputedasdescribedinthe
text,fromσandRrms.Wedonotcomputeamassforgroup213a,sincethevelocitydistributionisunresolvedandthemembershipispoor.
of30mineachweretaken,withsmalldithersinthespectraldirec- ulatedvalues.Thisisappliedtothefinal1Dand2Dobjectspectra,
tionforthepurposeofremovingbadpixelsandinterpolatingover togiveflux-calibratedspectra.
thechipgaps. Our final step isto correct for telluricabsorption. This isof
Slits were 1 arcsec wide, and we use the R600 grism with particularimportancesincetheH&Kabsorptionlinesingalaxiesat
OG515 order blocking filter. The detector was binned 2×2, re- ourtargetredshiftz ∼ 0.95lierightontopoftheA-bandtelluric
sulting in a pixel scale of 0.146 arcsec/pix, and a dispersion of line.Weuseourstandardstarobservationtoextractspectralregions
0.93A˚/pix. The resulting spectral resolution is limited by the slit aroundA-band(7500<λ/A˚<7700)andB-band(6850<λ/A˚<
width,∼6.4A˚. 6900). A smooth continuum issubtracted, and weuse the IRAF
ScienceobservationswereinterspersedwithGCALflats.Cal- taskTELLURICtoapplythecorrectiontoeachspectrumaccording
ibrationexposuresforeachmaskconsistedofCuArarclampob- totheairmassintheimageheader.
servationsandtwilightflats.Inaddition,weobservedthestandard
starLTT6248witha1arcsec,longslit,forfluxcalibration.
2.3.4 RedshiftDetermination
Redshifts were measured by adapting the ZSPEC software,
kindly provided by R. Yan, used by the DEEP2 redshift survey
2.3.3 Datareduction
(Davisetal.2003,2007).Thisperformsacross-correlationonthe
All data were reduced in IRAF, using the GEMINI packages with 1Dextractedspectra,usinglinearcombinationsoftemplatespectra.
minormodifications.SlitswerefirstidentifiedfromtheGCALflat Thecorrespondingvariancevectorsdescribedin§2.3.3areusedto
exposures.AbadpixelmaskwascreatedusingtheroutineGBPM, weightthecross-correlation.
andasetofsixlong-andthreeshort-exposureimageflats.Pixelsin Weusetemplatesofabsorptionlineandemissionlinegalaxies
thechipgaps,andoutsidethespectroscopicfieldofview,arealso only,andforeachgalaxyZSPECreturnsashortlistofpossiblered-
markedasbadpixels. shifts,withassociatedχ2andR(Tonry&Davis1979)values.Ev-
A biasframewassubtracted fromallsciencedata. Skysub- eryspectrumisvisuallyinspected,bothinthe1Dand2Dformat.
tractionisdonesimply,usingtheGNSSKYSUBroutine,bysubtract- Particularlyvaluableisinspecting the2Dimageprior tocombin-
ingfromeachscienceframeacopyofitself,shiftedby21binned ingthepositive-andnegative-fluxobjects.Realemissionlinesare
pixels.Thisresultsinapositive-fluxobjectspectruminonehalfof clearlyidentifiedbytheirdipolesignature,andfalsepeaksdueto
theslit,andanegative-fluxspectrumintheotherhalf. cosmicraysorothereffectsareeasilyremoved.Generally,the1D
Togenerateanoisevectorweusethesametechnique,butadd spectraareboxcarsmoothedto∼10A˚ pixelsforvisualinspection.
theshiftedimages.Thisgivespixels,atthelocationofthegalaxy, Weadoptasimple,four-classmethodtoquantifyourredshift
that include object flux and twice the sky flux, which is equal to quality. Quality class 4 is assigned to galaxies with certain red-
theexpectedPoissonvariance.Weaddtothistwicethereadnoise shifts.Generallythisisreservedforgalaxieswithmultiple,robust
squared,toobtainthefinalvariancespectrum. features.Withour(unbinned)spectralresolution,weareabletojust
Eachsky-subtractedscienceimageisthencleanedofcosmic resolvethe[OII]doublet;inthiscase,acleardetectionofthedou-
rays,flat-fielded,andcutintoindividualslits.Correspondingarcs bletalonewouldwarrantaclass4redshift.Qualityclass3arealso
areextractedandwavelengthcalibrated;thiscalibrationisthenap- veryreliableredshifts,andweexpectmostofthemtobecorrect.
plied to each science frame. The four exposures are added, after Theseinclude galaxieswithagood match toCaH&K for exam-
applyingashifttoaccountforthespectralditherandignoringbad ple, but no obvious corroborating feature. Similarly, single emis-
pixelsidentifiedinthebadpixelmask,whichhasbeenpropagated sionlineswhereadoubletisnotconvincingaregenerallyassumed
through thesamereduction procedures asthescience frame.The to be [OII] and given quality class 3. We would also use this in
sameisdoneforthe“noise”framedescribedabove. cases where H&K are detected ina region of telluricabsorption,
The negative-flux spectrum is then subtracted from its posi- butthereisatleastoneotherlikelymatchtoanabsorptionfeature.
tive counterpart, for each spectrum, using by default a 3.5 pixel Wetake particular care not to assign class 3 or 4 to a galaxy for
wide aperture (this is tweaked in a few cases, where necessary). whichH&Karetheonlyidentifiablefeatures,andlieonatelluric
Awavelength-dependent sensitivityfunctionisdeterminedbyex- absorptionline.
tractingthestandardstarspectrum,andcomparingthefluxtotab- Class2objectscorrespond to“possible”redshifts. Thesein-
Transientgalaxiesin groupsatz = 1 5
tional∼0.5magdepthrelativetotheI−selectedzCOSMOSand
EDisCSsurveys.
3 ANALYSIS
3.1 SpectroscopicandRedshiftcompleteness
WefirstconsiderthesamplingcompletenessofourGMOStargets,
defined as the fraction of potential targets for which we actually
obtainedaspectrum.Here,wedefinethecompletesampletobeall
priority1galaxies(i.e.all21.5 < r < 24.75galaxiesthathavea
photometricredshiftwithin2σoftheestimatedgroupredshift)that
are withinthe GMOS fieldof view. Note that 100 per cent com-
pleteness in this sense could never have been achieved, because
someofthefieldofviewispermanentlyinaccessibleduetotheac-
quisitionstarsand,inafewcases,guiderarm.Wecharacterizeour
samplingcompletenessintermsoftheIRAC[3.6µm]magnitude,
since this is more closely related to galaxy stellar mass than the
r band magnitude on which our selection was based. Weinclude
galaxiesfromthezCOSMOS10Kspectroscopiccataloguehereas
Figure 1. The colour-magnitude distribution for all galaxies with spec- well, which mostly includes galaxies brighter than IAB = 22.5.
troscopy (from either GEEC or zCOSMOS) that lie within our GMOS Althoughourtargetselectiondoesnotexplicitlydependongalaxy
fields,andhave0.8 < zphot < 1.5,withintheir1σ uncertainties. Red colour,therelianceonphotometricredshiftuncertaintypotentially
pointsrepresentGEECtargetswithsecureredshifts(quality3or4),while introduces a colour-dependence. Thus we consider our sampling
blue points indicate secure zCOSMOS 10k redshifts (quality > 2). The completenessasafunctionofboth[3.6µm]magnitudeand(V−z)
remaining, black points, are therefore those with a spectrum and 0.8 < colour. Specifically, we divide this colour-magnitude plane into
zphot<1.5,butwithoutasecureredshift. several binsandcalculatethecompleteness ineachbin.Foreach
colour bin we define a completeness function by fitting a line to
thecompletenessasafunctionof[3.6µm]magnitude.Wefindthat,
clude some spectra that are reasonably likely to be correct (e.g.
regardlessofhowwedothis,ourcompletenessisremarkablyuni-
H&Kontopofatelluriclineandnoothercorroboratingfeatures);
form; it ranges from ∼ 0.6 to ∼ 0.75 with little dependence on
butalsosomethatarelittlemorethanguesses.Class1isreserved
colourormagnitude.Thismaybepartlybecausethesampleistoo
for“junk”,withnochanceofobtainingaredshift.
smallyettodefinetheselectionfunctionwithhighenoughpreci-
In this analysis we only consider galaxies with class 3 or 4
sion,andwewillrevisitthiswhenthesurveyiscomplete.
qualityredshifts.
Anotherpotentialsourceofincompletenessisfailuretoobtain
redshiftsfortargetedgalaxies.Wecharacteriseourredshiftsuccess
2.4 zCOSMOSObservationsandfinalcatalogue rateasafunctionofIAB magnitude, sincethiscorresponds most
closely to the wavelength of features (like Ca H&K) commonly
Wealsoincludeinour analysisredshiftsfromthe10K releaseof usedforredshiftidentificationatz ∼0.9.Thissuccessrate,shown
zCOSMOS(Lillyetal.2009).Weincludeallgalaxieswithredshift inFigure2,isdefinedasthefractionofpriority1,targetedgalax-
qualitygreaterthan2.0,whichhaveahighprobability(> 90per iesfor whichweobtained a redshift withquality 3or 4. Wefind
cent)ofbeingcorrect(notethezCOSMOSqualityflagsaredefined aremarkably highsuccessrateof > 80per cent, forIAB < 24.
differentlyfromourown).Theseprovideanicecomplementtoour Thisisatestament in part to thehigh-quality spectra obtained at
observations,bydesign,astheyarerestrictedtothebrightergalax- Gemini, and in particular the success of the nod & shuffle tech-
iesthatarenotgivenpriorityinourtargetselection. nique, which results in near-perfect sky subtraction in a spectral
Figure 1 shows our final sample in colour-magnitude space. regiondominatedbyskyemissionlines.Moreover,wearehelped
Allopticalcoloursandmagnitudesarecomputedwithina3′′diam- bytheexquisitephotometricredshifts,whichmeansourpriority1
eteraperture,onpsf–matchedimages,asdescribedinCapaketal. listcontainslittlecontaminationfromhigherredshift galaxiesfor
(2007). The galaxies plotted here are all those with an available whichredshiftsaredifficulttoobtain.
spectrum,lyingwithinourGMOSfields,andwith0.8<z <1 Weshow,asthebluepoints,theredshiftsuccessofthezCOS-
phot
withintheir1σ uncertainties.Wechoosethe(V −z)colourasit MOS10ksample,restrictedtotheregionscoveredbyourGMOS
bracketsthe4000A˚ breakatz ∼0.95,andtheIAB magnitudefor observations.Herethesuccessrateisdefinedasthefractionofse-
comparison withzCOSMOS. Red points represent GEEC targets cure(quality>2)redshiftsfromallzCOSMOStargetswithinour
with secure redshifts, while blue points indicate zCOSMOS 10k GMOSfieldsofview.Thesuccessrateisalsoveryhighhere,>70
galaxieswithsecureredshifts.Distinctsequences ofredandblue per cent for IAB < 22.5, which is their nominal completeness
galaxiesareapparent,andourredshiftsuccessrateisveryhighat limit.Wehavealsocheckedforacolourdependenceonthesesuc-
all colours (see §3.1). Our GEEC spectra probe red galaxies up cessrates,andfindnosignificanttrend,asisvisuallyapparenton
to 1.0 mag fainter than found in the 10k survey (IAB < 22.5), Figure1.Thus,bothzCOSMOSandGEECarehighlysuccessful
0.65magfainterthanDEEP2(Cooperetal.2008,r < 24.1) and at obtaining secure redshifts even for red, absorption-only galax-
∼ 0.25 mag fainter than EDisCS at this redshift (Hallidayetal. ies.Manyofthese“failures”areinfactlikelytobehigherredshift
2004,IVega <23,orIAB<∼23.3).Forbluegalaxieswegainaddi- galaxies, inwhich case they do not affect the success rate in our
6 Baloghet al.
Figure2.Top:Thehistogramshowsthenumberofpriority1,GEECspec- Figure3.Spectroscopicredshiftsarecomparedwithphotometricredshifts
tra,asafunctionofIAB magnitude.Bottom:Theredshiftsuccessrateof andtheiruncertainties.Allgalaxieswithasecureredshiftareshown.Red
GEEC,definedasthefractionofpriority1galaxies withgood(quality3 pointsarethoseGEECgalaxieswithpriority1,whicharethosethathave
or4)redshifts, isshownastheblackpoints. Theblue points represent a zphotwithin2σofthegroupredshift;blackpointsrepresentGEECgalax-
similarquantityforthezCOSMOS10ksamplewithinourfield;hereitis ieswithlowerpriority.BluepointsarezCOSMOS10kspectrawithinthe
thefractionofallgalaxiesinourGMOSfieldswithazCOSMOSspectrum samefields.Ourspectragreatlyincreasethecompletenessanddepthofthe
thathavearedshiftqualityof>2.0.Thevertical,dottedlineatIAB rep- 10ksurveyattheredshiftofthesegroups.
resents thenominal, zCOSMOScompleteness limit.WithGEEC,weare
highlycompleteforgalaxiesupto1.5magfainterthanthis.
limitedredshiftrangeofinterest.Wethereforeapplynofurthercor-
rectionsforredshift incompleteness; theonlyweight comesfrom
thesamplingfraction.
InFigure3wecompare ourspectroscopic redshiftswiththe
correspondingphotometricredshiftanditsuncertainty.Redpoints
represent secure redshifts for priority1 galaxies inGEEC.These
all,bydefinition,haveazphotwithin2σzphotofthegroupredshift
(0.88<z<0.97forthissample).Effectively,thisselectsgalaxies
thateitherhavez closetothegroupredshift,orgalaxieswith
phot
poorlydeterminedz ,whichhavelargeerrorbars.Weseethat
phot
notonlydothespectroscopicandphotometricredshiftsagreevery
well,butthatourpreselectionallowsustobeefficientattargeting
potentialgroupmembers.Thebluepointsrepresentsecureredshifts
inthezCOSMOS10ksample.
During mask design, we preferentially target galaxies with
photometricredshiftswithin2σofthemeangroupredshift.Since
we account for the uncertainty in z , we do not expect to be
phot
strongly biased against galaxies for which z is poorly deter-
phot
mined due, for example, to a lack of strong features in the spec-
trum.However,wearepotentiallybiasedagainst“catastrophicfail- Figure 4. For confirmed group members with 0.7 < zphot < 1.5 we
showthedifferencebetweenspectroscopicandphotometricredshifts,asa
ures”.Toaddressthis,inFigure4weshowthedifferencebetween
functionof(V −z)colour. Asymmetricerrorbars are68percentconfi-
thespectroscopicandphotometricredshiftforallconfirmedgroup
denceintervalsonzphot.Filledsymbolsarequalityclass4,whichmeans
members(seeSection3.3)withsecurespectroscopicredshiftsand thespectroscopicredshiftiscertain,whileopensymbolsareclass3andthus
0.7<zphot <1.5,asafunctionoftheir(V−z)colour.Redpoints generallyreliable(andusedthroughoutthisanalysis).Theredpointsiden-
identify“priority2”galaxies,whicharethoselowerprioritytargets tifythosefewgroupgalaxiesthatwereallocatedlowerpriorityduringmask
withzphotmorethan2σawayfromthegroupredshift.Thesemake designbecausetheirzphotismorethan2σawayfromthemeangroupred-
uponly∼6percentofthesample,withnomeasureablecolourde- shift.Thesemakeuponly∼6percentofthetotalgrouppopulation,with
pendence.Thereisalsonostrongdependenceonthequalityofthe noobviouscolourdependence.
spectroscopic redshift; the fraction of priority 2 galaxies that are
actuallygroupmembersissimilaramongstquality3andquality4
Transientgalaxiesin groupsatz = 1 7
cerncouldbetheomissionofthermally-pulsingAGBstars,which
becomeincreasinglyimportantathigherredshifts(Marastonetal.
2006). This will be more relevant when considering comparison
with z = 0 observations, which we defer to after completion of
thesurvey.Forourpurposeshere,weareinterestedincomparing
groupgalaxieswiththeirfieldcounterparts.Systematiceffectslike
thisonthemassestimatesarenotlikelytobeverydifferentforthe
two populations. Thus we expect our conclusions to be robust to
thisandsimilarassumptionsaboutIMFanddustgeometry.
We k-correct all colours to z = 0.9, the redshift of in-
terest for our survey, using the KCORRECT IDL software of
Blanton&Roweis(2007).Wedenote thesecolours as,for exam-
ple,(V −z)0.9.
InFigure5weshowourderivedstellarmassesasafunction
ofrmagnitude(ourselectionband),forboththephotometricfield
sample(smallpoints)andourspectroscopicsample,within0.8 <
z < 1.0. Thepointsareseparatedintothebluest galaxies,(V −
z)0.9 < 1.5, and the reddest galaxies, (V − z)0.9 > 2.7. Our
selectionlimitofr <24.75impliesweare100percentcomplete
aboveamasslimitofMstar >∼1010.5M⊙;however,wearemostly
completeforMstar >∼1010.1M⊙,missingonlysomeofthereddest
galaxies.Wewillthereforetreatthisasournominalcompleteness
Figure5.Thestellarmasses,computedforaChabrier(2003)massfunction
limitthroughoutthepaper.Notethatthebluegalaxiesarecomplete
andbasedonBruzual&Charlot(2003)models,areshownasafunctionof
r magnitude forarandom subset ofthe photometric fieldsample (small for Mstar >∼ 109.6M⊙, and weprobe massesas low asMstar <∼
points)andourspectroscopicsample(large,filledpoints),for0.8 < z < 108.8M⊙.
1.0.Thereddestgalaxies,(V−z)0.9>2.7,areshownasredpoints,while
thebluestgalaxies(V−z)0.9<1.5areshowninblue(theremainingblack
pointsarethoseofintermediatecolour).Ourselectionlimitofr < 24.75 3.3 Groupmassesandmembership
imposesacolour–dependent masslimit,ofMstar < 1010.1M⊙ forred
galaxies,andMstar<108.8M⊙forthebluest. Foreachgroup,weinspectthespatialandredshiftdistributionof
thegalaxies.Fullresultswillbepresentedforallgroups,aftercom-
pletionofthesurvey.
redshifts. We conclude therefore that our z preselection does Westartbyconsideringallgalaxieswithinr = 1Mpcofthe
phot
notintroducealargebias,thoughwewillbeabletotestthismore nominalX–raygroupcentrefromFinoguenovetal.(inprep).The
robustlyattheendofthesurvey. velocitydispersionofthesegalaxiesisdeterminedusingthegap-
Atthisearlystageofoursurvey,wehavefewduplicateobser- perestimate(Beersetal.1990).Wealsocomputethe(unweighted)
vationswithwhichtocheckourspectroscopicredshiftsanddeter- meanspatialpositionofthegalaxies,andthermsprojectedsepa-
minearobustuncertainty.Wedohaveeightgoodqualityredshifts ration from this centre, which we call Rrms. We then iteratethis
forgalaxiesthathaveanexisting,goodqualityzCOSMOSredshift. process, typically clipping galaxies with velocities > 1.5σ from
Fromthese,thereappearstobeasmallbiasinthesensethatour themedianredshift,andpositionsr>2Rrmsfromtherecomputed
redshiftsaresmallerby7.72×10−4,correspondingtoarest-frame centre2.Thisconvergesafterafewiterations,andweadoptthefinal
velocityshift of ∼ 120 km/sat theredshift of interest.Although σandRrms.Groupmembersarethendefinedasthosewithin3σof
thisshould betreatedasapreliminaryoffset,wecorrectour red- themedianredshift,andwithin2Rrmsofthecentre.Uncertainties
shifts for it here. Weuse the same eight galaxies to estimate our onσandRrmsarethencomputedusingajackknifemethod,iterat-
redshiftuncertaintyandfindσz ∼4.1×10−4,correspondingtoa ingonlyoverthislistofgroupmembers.Thus,theseuncertainties
rest-framevelocityuncertaintyof∼65km/satz=0.9. donotincludesystematicuncertaintiesduetotheclippingprocess,
which are likely to be at least ∼ 15 per cent (e.g. Wilmanetal.
2005a).
3.2 Stellarmassmeasurementsandk-corrections We show how our sample compares with other, related sur-
veys,inFigure6.Pointsshowthevelocitydispersionofeachgroup,
We fit the spectral energy distribution for each galaxy, us-
as a function of redshift, for our present survey (filled circles),
ing all available photometry, following the method described in
our previous GEEC sample at 0.3 < z < 0.6 (open triangles),
McGeeetal.(2010).Briefly,thisisdonebycomparingwithavery
and the EDisCS cluster and group sample Hallidayetal. (2004);
largegridoftemplateBruzual&Charlot(2003)models,covering
Poggiantietal.(2006,opencircles).Themedianredshifts,veloc-
awiderangeofparameters,assumingaChabrier(2003)massfunc-
itydispersions, and number of members ineach of our groups is
tion.TheprocedureisbasedonthatofSalimetal.(2007),andas-
giveninTable1.
sumesthestarformationhistoryofagalaxycanberepresentedby
anexponentialmodelwithsuperposedbursts.
Stellar mass estimates are quite robust to the details of the
2 The clipping parameters were tweaked for group 161, which is very
fits, inpart because of theavailability of Spitzer IRAC data. The
close to group 150 in redshift and position. For this group we exclude
mainassumptionsthatcouldleadtosystematicerrorsaretheini- galaxies with velocities > 1.4σ from themedian redshift, andpositions
tialmass function(IMF),the dust model (weuse Charlot&Fall r>1.6Rrmsfromtherecomputedcentre.Theproperties(dynamicaland
2000),andthechoiceofgalaxyevolutionmodel.Ofparticularcon- stellarmass)ofthisparticulargrouparequitesensitivetothischoice.
8 Baloghet al.
Figure6.Thevelocitydispersion,asafunctionofredshift,forthreesam- Figure7.Left:ThecorrelationbetweenMdynandX–rayluminosity,LX,
ples of groups and clusters with high spectroscopic completeness. The is shown for the six targeted groups in our sample (i.e. excluding 213a,
present group sample is shown as filled circles, while our lower-redshift whichisnotdetectedinX–rays).Right: Hereweshowthesamedata,butas
GEECsampleisshownwithopentriangles.Theopencirclesrepresentthe afunctionofM200,XasdetermineddirectlyfromLX assumingascaling
groupsandclustersoftheEDisCSsurvey(Hallidayetal.2004). relationandcalibratedfromstackedweaklensinganalysis(Leauthaudetal.
2010).
Weestimateadynamicalmass,fromthevelocitydispersionσ 4 RESULTS
andtheradiusRrms,
4.1 Dynamicsandstellarfractions
3
Mdyn = Rrmsσ2. (1) With at least ∼ 15 spectroscopic members for most groups, we
G
are able to make reasonable estimates of the dynamical mass, as
Theuncertaintyisdeterminedbypropagatingthejackknifeuncer- describedin§3.3.InFigure7wecomparethesemasseswiththe
taintiesonσ and Rrms.Thefactor 3inthisequation isbased on X–rayluminosity(leftpanel)andwiththevirialmassM200,X es-
theassumptionofisotropicorbitsandanisothermalpotential,but timatedfromthisluminosity(rightpanel).M200,X isestimatedas-
isonlyweaklydependentonthoseassumptions(Łokas&Mamon sumingascalingrelationandcalibratedbasedonastacked weak
2001). lensinganalysis(Leauthaudetal.2010).
Groups150and161arelocatedwithinthesameGMOSfield. Thedynamicalmassuncertaintiesaregenerallylarge,butthe
It is clear that group 161 is dynamically and spatially separated masses agree remarkably well with those estimated from the X–
from group 150, but they are very close, separated by less than rayemission.Theoneoutlier,whichhasarelativelylowdynamical
1Mpc and1000km/s(restframe).Thustheymaybepart ofthe massforitsX–rayemission,isgroup150;thisgroupisclosetoand
same,interactingsystem.Theassignmentofmemberstoonegroup likelyassociatedwith161.Itspositiononthisplotisquitesensitive
ortheotherisnotentirelyobvious,andourclippingalgorithmwas totheprecisemembershipassigned;thatis,thesystematicuncer-
tunedtoachieveareasonable–lookingseparation. taintyduetothechoiceofclippingparametersarelikelydominant
Finally, themembership of group 213 isquite poor, and our overthestatisticaluncertainties.
completeness in thisfield isworse than elsewhere. Wedid, how- Total stellar masses can often be determined with much
ever,detectaserendipitousgroup(named213a)athigherredshift, greaterprecision,withthedominant statisticalnoisetermcoming
z = 0.9254.Thisgrouphassixmembers,andanominalvelocity from the uncertainty in the radius Rrms within which the stellar
dispersionofonly44±17km/s,restframe—smallerthanwould massissummed.Thetotalstellarmassiscomputedoverallgalax-
beexpectedfromredshiftuncertaintiesalone,andlikelydominated ies in the sample; since we are incomplete for red galaxies with
bysystematicuncertainty.Thegroup’spositionissignificantlyoff- M <1010.1M⊙,thisisactuallyalowerlimit.Forthebluegalax-
set from the X–ray detection, which is likely still correctly iden- ies, about 75 per cent of the stellar mass is at M > 1010.1M⊙.
tified with the lower redshift system. We include both groups in Assumingthistobetruefortheredpopulationaswell,weestimate
ouranalysis;however,wedonotattempttodetermineadynamical thatourstellarmassesunderestimatethetruetotalbylessthan10
mass of group 213a since the velocity dispersion is clearly unre- percent.Thestatisticaluncertaintyiscalculatedfromthejackknife
solved. uncertaintiesinRrms andσ.Inseveralcases,thegroupmembers
Ourobservationsconfirmthatthesearealllow-masssystems, withr < Rrms and|∆z| < 3σ donot change withintheuncer-
asexpectedfromtheirX–rayfluxes.Thedynamicalmassesofthe taintiesonthesequantities,whichleadstoastellarmassestimate
targetedgroupsrangefrom2.3±0.9×1013M⊙ to1.3±0.5× with,formally,zerouncertainty.Wethereforeimposeanarbitrary,
1014M⊙. minimumuncertaintyof10percentontotalstellarmasses.
Transientgalaxiesin groupsatz = 1 9
Figure9.Left:Thecolours(k-correctedtoz =0.9)andstellarmassesofgalaxiesareshownfortwosamples.Thesmall,blackpointsaregalaxiesfromthe
fullCOSMOSphotometriccatalogue,withr < 24.75and0.8 < zphot < 1.0.Weonlyshowarandom10%ofthepoints,forlegibility.Thisrepresents
theparentdistributionfromwhichourspectroscopicsurveys(overamuchsmallerfield)aredrawn.Thelarge,filledpointsrepresentgalaxieswithsecure,
spectroscopicredshiftsat0.8 < z < 1,thatliewithinourGMOSfieldsbutarenotassignedtoagroupinoursurvey.Werefertotheseas“non-Xgroup”
environmentgalaxies,todistinguishthemfromthegeneralfield(representedbythesmallpoints)whichcontainsgalaxiesinallenvironments.Thepoints
circledinredindicategalaxieswith24µmdetections.Notetheabsenceoflow-mass,red-sequencegalaxiesinthenon-Xgroupgalaxies,andthedominanceof
thebluesequenceforMstar<1010.6M⊙.Forillustrationpurposesonly,thelargeredandblueellipsesapproximatelyidentifythe“redsequence”and“blue
cloud”populations.Thesolidlineindicatesourapproximatecompletenesslimitimposedbyther > 24.75selection.Right:Thesame,butnowthefilled
pointscorrespondtogroupmembers.Here,thefaintendoftheredsequence,visibleintheparentphotometricredshiftcatalogue,isfilledinwithconfirmed
groupmembers.Moreover,thereappearstobeaprominentthirdpopulation,thatliesbetweentheblueandredsequencesat(V −z)0.9∼2.
InFigure8weshowtherelationshipbetweendynamicaland
stellarmass,forallsixtargetedgroups.Weexcludetheserendipi-
tousgroup213a,sincethevelocitydispersionisunresolved; with
onlysixmemberswecannotdetermineareliabledynamicalmass.
Fortheremainder,thestellarfractionsrangefrom∼0.25percent
to ∼ 3 per cent, similar to what is observed both locally (e.g.
Baloghetal.2010)andathigherredshift(Giodinietal.2009).The
groupwiththehigheststellarfractionisgroup150which,asmen-
tionedpreviously,hasalargersystematicuncertaintyonitsdynam-
ical mass due to its close proximity to 161. The membership of
group161itselfisverysensitivetothechoiceofRrms,andtheun-
certaintyinthisquantityleadstoalargeuncertaintyinstellarmass.
Thus,althoughtherearehintsofrealdiversityinthestellarfraction
amongstourgroups,thesampleistoosmallatthistimetodrawany
strongconclusions.
4.2 Galaxypopulations
Ourmainresultfromthesefirstdataisshown inFigure9,asthe
correlationbetween(V−z)0.9colourandstellarmassforthegroup
andnon-Xgroupsamples.Thesmall,blackpointsrepresentgalax-
Figure8.Weshowthemeasureddynamicalmassasafunctionoftotalstel-
iesintheentireCOSMOSphotometric redshift catalogue, within
larmass,foreachofourgroupsexcepttheserendipitousgroup213a.The
0.8 < z < 1.0andrestrictedtor < 24.75,theGEECselec-
phot stellarmassisthesumofallspectroscopically confirmedgroupmembers
tionlimit.Intheright panel, thelargepointscorrespond tospec-
withinaradiusRrms.Thedottedlineindicatesaconstantstellarfractionof
troscopically confirmed group members, as defined in § 3.3. On 1percent;thesolidlinesoneithersiderepresentfractionsof0.25percent
theotherhand,largepointsintheleftpanelrepresentallgalaxies and3percent,asindicated.
withsecureredshifts0.8<zspec <1thatareunassociatedwitha
groupinourcatalogue.Wewillrefertothisspectroscopicsample
as“non-Xgroup”galaxies,sincetheyareconfirmednon-members
10 Baloghet al.
Figure10.Left:Thedistributionof(V −z)0.9coloursareshown,forallgalaxieswithMstar>1010.1M⊙.Thesolid,blacklinerepresentsthedistribution
ofourspectroscopic,non-groupgalaxysample,whichwesuggestrepresentsthe“non-Xgroup”environment.Itshowsthewell-knownbimodaldistribution,
consistingofredandbluegalaxypopulations.Thered,dashedhistogramrepresentsourspectroscopicgroupsample;therearefewbluegalaxiesand,instead
thepopulationisdominatedbyred-sequencegalaxies,andanapparentlydistinctpopulationofintermediate-colour(“green”)galaxies.Finally,thethin,dotted
blackhistogramisthedistributionofallgalaxiesinCOSMOS,with0.8<zphot<1.0,r<24.75andMstar >1010.1M⊙;thishasbeenrenormalizedto
matchtheareaoftheredhistogram.Thisrepresentstheglobal,“field”populationofgalaxies.Right:Thesame,butwherethegroupsampleisnowweighted
forsamplingincompleteness,andtheCOSMOScomparisonsampleisappropriatelyrescaled.
ofourX–raygroupsample.Wecannotexcludethepossibilitythat tobeconsiderablyredder,populatingtheredenvelopeoftheblue
thissamplestillcontainsgalaxiesthataremembersofothergroups, cloud,andthegreenvalleyitself.
notidentifiedinX-rayemission.ThesolidlineinFigure9indicates The population of galaxies between the red and blue galax-
ourcolour-dependent completenesslimit,asaconsequenceofthe ies is of particular interest, as it may represent a transient phase.
r > 24.75magselection.Thisiscomputedbysimplylookingat To explore this further, we show the distribution of colours, for
theaveragestellarmassofgalaxiesnearthemagnitudelimit,asa all galaxies with Mstar > 1010.1M⊙, in Figure 10. The black,
functionofcolour. solid histogram represents the distribution for our “non-Xgroup”
spectroscopic sample; while the sample size is small, it shows
Wefirstnotetheusualstructureinthiscolour-magnitudedia-
thewell-known bimodal distributionof colours, witha minimum
gram,witha“redsequence”and“bluecloud”population.Weshow
around (V − z)0.9 ∼ 2.3. This is statistically consistent3 with
the approximate locations of these populations with red and blue
the colour distribution of the full COSMOS “field” sample (for
ellipses,respectively.Theredsequenceinthegroupsiswellpopu-
0.8 < z < 1.0)inthesamestellarmassrange,shownasthe
latedatallmassesaboveourcompletenesslimit,M >1010.1M⊙. thin,dottpehdotline.Thered,dottedhistogramrepresentsourspectro-
In fact, there is some indication that the lowest-mass (M <
scopicgroupsample.Theleftpanelshowsthesedataunweighted
1010.5M⊙),redgalaxiesappearonlyingroups,astheyareabsent forcompleteness,whiletherightpanelincludesthecompleteness
inthe“non-Xgroup” sample. However thesampleistoosmallto
corrections. In both cases, the groups show a marked deficit of
bedefinitiveaboutthis,andwewilldeferafullanalysisofthelu-
bluegalaxies,whiletheintermediate-colour population isatleast
minosityfunctionshapestotheendofthesurvey.
asabundantasinthegeneralfield.
The second interesing observation from Figure 9 is that the Remarkably, the distribution suggests that this intermediate
blue sequence appears to be very underpopulated amongst group populationmaybedistinctfromboththeredpeakandbluecloud.
galaxies,compared withthefield,forM >∼ 1010.2M⊙.Theblue Unfortunately the sample is not yet quite large enough to unam-
galaxiesthatdoexistheretendtolieattherededgeofthecloud; biguouslyidentifythese“green”galaxieswiththegroupenviron-
moreover, the “green valley” between the two sequences is well- ment; a KS test on the unweighted distributions inFigure 9con-
populated,apointwewillreturntobelow. firmsthatthegroupdistributionhasa∼ 1percentprobabilityof
beingdrawnfromthesamedistributionastheparent,fieldpopula-
We have matched our redshift catalogue with the deep
tion;ora∼ 2.5percentchanceofbeingdrawnfromthespectro-
Spitzer MIPS catalogue, which has a 5σ detection limit of 0.071
scopic,“non-Xgroup”sample.Itisalsointerestingthatthe“green”
mJy (Sandersetal. 2007). Using the calibration of Riekeetal.
population(with1.9<(V −z)0.9 <2.4)makesupasimilarfrac-
(2009), this limit corresponds to a star formation rate of SFR∼
tionofthetotalinthegeneralfield(∼25percent)asinthegroups
11.8M⊙yr−1 atz ∼ 1.These24µmdetectionsarecircledinred
inFigure9.Thenon-Xgroupsamplehasalargepopulationofmas-
sive,bluegalaxieswith24µmdetections,indicatingsubstantialstar 3 AKStestshowsthereisa38percentprobabilitythatthetwodistribu-
formationrates. Inthegroups, themassive24µm detections tend tionsaredrawnfromthesameparentpopulation.