A journey from the outskirts to the cores of groups I: Color- and mass-segregation in 20K-zCOSMOS groups

5.1 MB·English

by  V. Presotto

Astronomy&Astrophysicsmanuscriptno.Presotto˙2011 (cid:13)c ESO2012
10-1-2012
A journey from the outskirts to the cores of groups I:
⋆
Color- and mass-segregation in 20K-zCOSMOS groups
V.Presotto1,2,A.Iovino2,M.Scodeggio3,O.Cucciati4,C.Knobel5,M.Bolzonella6,P.Oesch5,7,A.Finoguenov8,
M.Tanaka9,K.Kovacˇ5,10,Y.Peng5,G.Zamorani6,S.Bardelli6,L.Pozzetti6,P.Kampczyk5,C.Lo´pez-Sanjuan13,
D.Vergani6,24,E.Zucca6,L.A.M.Tasca13,C.M.Carollo5,T.Contini11,12,J.-P.Kneib13,O.LeFe`vre13,S.Lilly5,
V.Mainieri14,A.Renzini15,A.Bongiorno8,K.Caputi16,S.delaTorre16,L.deRavel16,P.Franzetti3,B.Garilli3,13,
F.Lamareille11,12,J.-F.LeBorgne11,12,V.LeBrun13,C.Maier5,17,M.Mignoli6,R.Pello`11,E.Perez-Montero11,12,18,
E.Ricciardelli19,J.D.Silverman9,L.Tresse13,L.Barnes5,R.Bordoloi5,A.Cappi6,A.Cimatti20,G.Coppa8,
2 A.M.Koekemoer21,H.J.McCracken22,M.Moresco20,P.Nair6,andN.Welikala23
1
0 (Affiliationscanbefoundafterthereferences)
2
ReceivedMonthDay,Year;acceptedMonthDay,Year

n
a
J ABSTRACT

9
Context.Studyingtheevolutionofgalaxieslocatedwithingroupsmayhaveimportantimplicationsforourunderstandingoftheglobalevolution

] ofthegalaxypopulationasawhole.Thefractionofgalaxiesboundingroupsatz∼0isashighas60%andthereforeanymechanism(amongthe
O manysuggested)thatcouldquenchstarformationwhenagalaxyentersgroupenvironmentwouldbeanimportantdriverforgalaxyevolution.
C Aims.UsingthegroupcatalogobtainedfromzCOSMOSspectroscopicdataandthecomplementaryphotometricdatafromtheCOSMOSsurvey,
weexploresegregationeffectsoccurringingroupsofgalaxiesatintermediate/highredshifts.Ouraimistorevealif,andhowsignificantly,group
.
h environmentaffectstheevolutionofinfallinggalaxies.
p Methods.Webuilttwocompositegroupsatintermediate(0.2≤z≤0.45)andhigh(0.45<z≤0.8)redshifts,andwedividedthecorresponding
- compositegroupgalaxiesintothreesamplesaccordingtotheirdistancefromthegroupcenter.Thesamplesroughlycorrespondtogalaxieslocated
o
in agroup’s inner core, intermediate, and infall region. Weexplored how galaxy stellar masses and colors - working in narrow bins of stellar
r
masses-varyasafunctionofthegalaxydistancefromthegroupcenter.
t
s Results.Wefoundthatthemostmassivegalaxiesinoursample(log(M /M ) ≥10.6)donotdisplayanystronggroup-centricdependenceof
gal ⊙
a thefractionsofred/blueobjects.Forgalaxiesoflowermasses(9.8≤log(M /M )≤10.6)thereisaradialdependenceinthechangingmixof
[ gal ⊙
redandbluegalaxies.Thisdependenceismostevidentinpoorgroups,whereasrichergroupsdonotdisplayanyobvioustrendofthebluefraction.
1 Interestingly,masssegregationshowstheoppositebehavior:itisvisibleonlyinrichgroups,whilepoorergroupshaveaaconstantmixofgalaxy
v stellarmassesasafunctionofradius.
3 Conclusions.These findings can be explained in a simple scenario where color- and mass-segregation originate from different physical pro-
7 cesses.Whiledynamicalfrictionistheobviouscauseforestablishingmasssegregation,bothstarvationandgalaxy-galaxycollisionsareplausible
6 mechanismstoquenchstarformationingroupsatafasterratethaninthefield.Inpoorergroupstheenvironmentaleffectsarecaughtinaction
1 superimposedtoseculargalaxyevolution.Theirmembergalaxiesdisplayincreasingbluefractionswhenmovingfromthegroupcentertomore
. externalregions,presumablyreflectingtherecentaccretionhistoryofthesegroups.
1
0 Keywords.cosmology:observations-galaxies:groups:general-galaxies:evolution
2
1
v: 1. Introduction low-densityregions,whereasred,inactive,ellipticalgalaxiesfa-
i vorhigh-densityregions.
X Thestrikingbi-modalityofthecolor-magnitudeandofthecolor-
Thesetwodistinctgalaxyevolutionaryfamiliescanoriginate
mass diagrams raises important questions about galaxy forma-
r either from a priori differences set at galaxy formation epoch,
a tion and evolution. What are the physical processes responsi-
the so-called nature scenario, or from environmentally driven
ble for the sharp partition into blue cloud and red sequence
processes taking place during the galaxy evolutionary history,
galaxies? Does the environment play a key role in this pro-
theso-callednurturescenario.
cess by boosting the transition into the red sequence region?
What are the timescales for this transition? There is much ev- The currently accepted ΛCDM model predicts the hierar-
idence of correlations between galaxy properties and their en- chical growth of structures: as time proceeds, smaller struc-
vironment, the oldest and best known being the morphology- turesmergetoformprogressivelylargerones.Thisprocessim-
densityrelation(seeOemler1974;Dressler1980,althoughthe plies that the fraction of galaxies located in groups progres-
firstmentionofit datesbackto Hubble).Ingeneral,blue,star- sivelyincreasessincez ∼ 1.5,uptotheLocalUniversevalues,
forming, disk-dominated galaxies are located preferentially in wheremostgalaxiesarefoundingroups(Huchra&Geller1982;
Ekeetal. 2004; Berlindetal. 2006; Knobeletal. 2009). As a
Send offprint requests to: Valentina Presotto consequence,atleastpartofthe observeddeclineofthe global
([email protected]) star-formation rate (SFR) from z ∼ 1.5 until today (Lillyetal.
⋆ BasedonobservationsmadeattheEuropeanSouthernObservatory 1996; Madauetal. 1998; Hopkins 2004) could be accelerated
(ESO)VeryLargeTelescope(VLT)underLargeProgram175.A-0839 byenvironmentallydrivenphenomena.
1
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
In this context group environment plays a dominant role ies with environment. To isolate the true environmentaleffect,
(Baloghetal. 2004; Wilmanetal. 2005; Iovinoetal. 2010; the analysis must be performed in narrow galaxy stellar mass
Pengetal.2010),becauseonlyasmallfractionofgalaxieslive bins.Muchoftheearlierworkperformedatintermediate/highz
indenserenvironmentssuchasclustercores.Thereareobserva- wasbasedonincompleteand/orscarcesamplesofgroups,where
tionalindicationsthatthecolortransitionfrombluetoredgalax- oftenthestatistics wasnothighenoughto performsuch anac-
iesproceedsfasterinagroupthaninthecoevalfieldpopulation, curateanalysisinsmallmassbins.
aneffectthatbecomesevidentatredshiftslowerthanz ∼ 1and In this paperwe will study mass - and color segregationin
forgalaxiesofmasseslog(Mgal/M⊙)≤10.6(Iovinoetal.2010; groupsoverwide redshiftandgalaxystellar mass rangesusing
Kovacˇetal.2010;Bolzonellaetal.2010;Pengetal.2010). thespectroscopicdatafromtherecentlycompletedzCOSMOS-
However, the physical phenomena responsible for accel- Bright, a large survey reaching out to z ∼ 1 with a fairly
erating the color transition from blue to red galaxies within high and uniformsampling rate (Lillyetal. 2007), and its new
groups are yet to be described. While the extreme local den- groupcatalog(Knobeletal.2011).Wewillbenefitalsofromthe
sities reached within cluster cores enable efficient ram pres- widerangeofphotometricdataavailablefortheCOSMOSsur-
sure stripping of the galaxy cold gas on timescales of a few vey (Scovilleetal. 2007; Ilbertetal. 2009; Oeschetal. 2010).
Myr (Gunn&Gott 1972; Abadietal. 1999), within the groups Galaxycolorsaretheeasiestparametertomeasureamongthose
different physical processes have been proposed. On one hand thatexhibitadistinctivebi-modality,andweselectedrest-frame
galaxy-groupinteractionslike’strangulation’,starvationorhalo (U −B)color,bracketingthe4000Åbreak,asagoodindicator
gasstrippingcanremovewarmandhotgasfromagalaxyhalo, ofthegalaxyaveragestar-formationhistoriesoverlongertime-
efficientlycuttingoffthestarformationgassupply(Larsonetal. scalesthanemissionlineindicatorssuchase.g.,[OII].
1980;Coleetal.2000;Baloghetal.2000;Kawata&Mulchaey Toshedlightonhowrapidlyandsignificantlystarformation
2008).Alternatively,mergers/collisionsandclosetidalencoun- issuppressedingroupsandtoovercomethelownumberstatis-
tersamonggroupmembergalaxiestogetherwithgalaxy-galaxy tics for individual systems (typically 7-8 members per group),
harassment at the typical velocity dispersion of bound groups we built stacked groupsby co-addingspatial informationfrom
mayalsoresultinstar-formationquenching(Mooreetal.1996). groupmembergalaxies.This strategyenabledus to establish a
Thesephysicalprocessesdonotrequireextremelocaldensities, statisticallyreliablesampleandtorevealtrendsofgalaxyprop-
andquenchstarformationinamoregradualandgentlewayon ertiesasafunctionofthegroup-centricdistanceandofvarying
timescalesofseveralGyr. grouprichnesses.
Among the observable effects of these processes are seg- Thepaperisorganizedasfollows:inSect.2wedescribethe
regation phenomena, that is, not only differences between data of our analysis, includingthe algorithmchosen to add the
group - and field galaxy properties, but also radial trends of groupmembergalaxieswithphotometricredshift.InSect.3we
galaxy properties (e.g., colors, morphologies ...) as a func- illustrate the construction of the realistic mock catalogs with
tion of distance from the group/cluster center. These phenom- which we tested our algorithms. In Sect.4 we explain how we
ena have already been extensively studied in galaxy clusters, stacked group member galaxiesto build a composite group.In
where e.g., a strong radial dependence in the star-formation Sects.5and6 wepresentouranalysisanditsresults, whichwe
rateisobserved(Hashimoto&Oemler1999;Baloghetal.1999; discussinSect.7.OurconclusionsaresummarizedinSect.8.A
Lewisetal. 2002; Baloghetal. 2004; Tanakaetal. 2004). The concordance cosmology is adopted throughoutour paper, with
observed quenching of star-formation activity starts at large h = H /70 km s−1 Mpc−1, Ω = 0.25 and Ω = 0.75. All
70 0 m Λ
cluster-centric distances and low projected densities, in the magnitudesarequotedintheABsystemthroughout.
so-called infalling regions, and even at large radii field star-
formation values are not yet reached. This result suggests that
galaxy transformationstarts to occur in the infalling filaments, 2. Data
which consist of chains of groups in which field galaxies are
affected by the group environment which changes their star- It is widely accepted in the literature that classical galaxy
forming blue field-like properties into passive, red cluster-like color/morphologytrendsindifferentenvironmentsarebetterin-
galaxies. But even if groups seem to be the key environment vestigatedinbinsofmass-volume-limitedsamples(Tascaetal.
to search for the nurture scenario, still the observational ev- 2009;Iovinoetal.2010;Cucciatietal.2010;Kovacˇetal.2010;
idence for related segregation phenomena is quite scarce and Xueetal. 2010; Cooperetal. 2010; Gru¨tzbauchetal. 2011).
holds mainly for the local Universe (Postman&Geller 1984; This strategy enables one to break the degeneracy caused by
Mahdavietal. 1999; Tranetal. 2001; Carlbergetal. 2001a,b; therelationshipsbetweengalaxystellarmassesandenvironment
Girardietal.2003;Dom´ınguezetal.2002;Wilmanetal.2009; andbetweengalaxystellarmassesandcolors/morphologies.
Baietal.2010;Ribeiroetal.2010). The recently completed zCOSMOS-bright survey with its
A complication to consider is the strong correlation be- highanduniformsamplingrateoffersuniqueopportunitytoex-
tween galaxy properties such as colors, morphologies and plorethepresence/evolutionofthesetrendsoverawiderangeof
star formation, with galaxy stellar mass (Cowieetal. 1996; cosmictime.
Gavazzietal.1996;Blantonetal.2003;Kauffmannetal.2003;
Brinchmannetal. 2004; Baldryetal. 2004) and the additional
2.1.COSMOSandzCOSMOSsurveys
correlation between the galaxy stellar mass itself and environ-
ment: galaxies in less dense environment tend to be less mas- The COSMOS survey is a large HST-ACS survey,with I-band
sivethanthoselocatedindenserenvironment(Hoggetal.2003; exposuresdowntoI = 28onafieldof2deg2 (Scovilleetal.
AB
Kauffmannetal. 2004; Blantonetal. 2005; Scodeggioetal. 2007). The COSMOS field has been the object of extensive
2009; Bolzonellaetal. 2010). Thus any study performed on multiwavelength ground- and space-based observations span-
samples of galaxies containing a wide range of stellar masses ningtheentirespectrum:X-ray,UV,optical/NIR,mid-infrared,
cannot distinguish between true environmental effects and ef- mm/submillimeterandradio,providingfluxesmeasuredover30
fectssimplyinducedbythedifferingmassdistributionsofgalax- bands (Hasingeretal. 2007; Taniguchietal. 2007; Capaketal.
2
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
ricredshifts(Ilbertetal.2009).Basedonacomparisonwiththe
zCOSMOS spectroscopicredshifts,Ilbertetal. (2009) estimate
anaccuracyofσ =0.007×(1+z)forgalaxiesbrighterthan
zphot s
I = 22.5.Applyingthe ZEBRA code(Feldmannetal. 2006)
AB
to 30 bands, Oesch (2011) obtains a similar accuracy (private
communication).
Inouranalysisweusedphotometricredshiftvaluesobtained
bytheZEBRAcode.
ForallgalaxiesbrighterthanI =22.5,absoluterest-frame
AB
magnitudes and stellar masses were obtained using standard
multi-colorspectralenergydistribution(SED)fittingtechniques,
usingthesecurespectroscopicredshift,ifavailable,orthepho-
tometricone.Rest-frameabsolutemagnitudeswereobtainedus-
ingtheZEBRAcode(seeFeldmannetal.2006forthedetailsof
thecode),whilestellarmasseswereobtainedusingthehyperz-
masscode(Pozzettietal.2010;Bolzonellaetal.2010).Fromthe
availablestellarpopulationsynthesislibrariesweadoptedthose
of Bruzual&Charlot (2003), assuming a Chabrier initial mass
function(Chabrier2003).
Fig.1. Ra-Dec distribution of the 16623 zCOSMOS-bright
galaxies with secure redshift z ≤ 2 (the so-called 20K sam-
ple). The area within the red box (149.55 ≤ ra ≤ 150.67 and
1.75 ≤ dec ≤ 2.70) has a nearly uniform sampling rate of 2.2.Thespectroscopicgroupcatalog
∼62%.
The group catalog used in this paper is a subset of the 20K
group catalog described in Knobeletal. (2011), Knobeletal.
2007;Lillyetal.2007;Sandersetal.2007;Bertoldietal.2007;
(see also 2009 for an earlier version of the catalog). The 20K
Schinnereretal.2007;Koekemoeretal.2007;McCrackenetal.
group catalog consists of 1496 groups with at least two spec-
2010).
troscopicmembergalaxies(188withatleastfivespectroscopic
The zCOSMOS survey was planned to provide the cru- members).Knobeletal.(2011)usesa“multi-passprocedure”to
cial high-quality redshift information to the COSMOS field achieveanimpressivequalityingroupreconstruction,astested
(Lillyetal. 2007). It benefitted of ∼ 600 hr of observations at usingrealisticmockcatalogs.Thismethod,whencombinedwith
VLTusingtheVIMOSspectrographanditconsistsoftwoparts: the standard friends-of-friends(FOF) algorithm, yields for the
zCOSMOS-bright,and zCOSMOS-deep.The zCOSMOS-deep resultinggroupcatalogvaluesofcompleteness(i.e.,fractionof
targets ∼ 10000 galaxies within the central 1 deg2 of the real detected groups) and purity (i.e., fraction of non-spurious
COSMOSfield,selectedthroughcolorcriteriatohave1.4<∼z<∼ groups)thatareextremelygoodandstableasafunctionofboth
3.0.ThezCOSMOS-brightispurelymagnitude-limitedandcov- redshiftand numberof membersobservedin the reconstructed
ersthewholeareaof1.7deg2oftheCOSMOSfield.Itprovides groups. Typical values of these two quantities, for groups re-
redshiftsfor∼ 20000galaxiesdowntoIAB ≤ 22.5asmeasured constructedwithfiveormorespectroscopicobservedmembers,
from the HST-ACS imaging. The success rate in redshift mea- are around∼ 80%atall redshiftsanddo notdecrease substan-
surementsisveryhigh,95%intheredshiftrange0.5<z< 0.8, tiallyforgroupswithlowernumberofobservedmembers.The
and the velocity accuracy is ∼ 100 km s−1(Lillyetal. 2009). interloperfraction,i.e.,thefractionoffieldgalaxieserroneously
Eachobservedobjecthasbeenassigneda flagaccordingtothe classed asgroupmembers,alwaysremainsbelow∼ 20%atall
reliability of its measured redshift. Classes 3.x, 4.x redshifts, redshifts for groupsreconstructed with more than five spectro-
plus Classes 1.5, 2.4, 2.5, 9.3, and 9.5 are considered a secure scopicobservedmembers,withonlyaslightincreaseforgroups
set, with an overallreliability of 99% (see Lillyetal. 2009 for with lower numberof observedmembers.Anotherpointworth
details). noticingisthatthealgorithmtodetectgroupstreatseachgalaxy
OurworkisbasedonthethezCOSMOS-brightsurveyfinal as a point in Ra-Dec-redshiftspace, therefore avoiding any in-
release:thesocalled20K sample(simply20K hereafter),total- terloper/completenessdependenceongalaxypropertiessuchas
ing16623galaxieswithz ≤ 2andsecureredshiftsaccordingto colorsormasses(seeKnobeletal.2011formoredetails).
theaboveflagclassification(18206objectsintotal,irrespective
ofredshiftandincludingstars). Inthispapertheanalysisisrestrictedtogroupswithatleast
Fig.1showsthespatialdistributionofthe20Kgalaxies.The fivespectroscopicallyobservedmembersthatarelocatedwithin
redsquare correspondsto the regionwith the highestsampling the high sampling rate box introduced in Fig.1. From now on
rate, approximately ∼ 62% of the parent galaxy catalog. Its we will call this sample the spectroscopic group sample: it to-
boundariesare 149.55 ≤ ra ≤ 150.67and 1.75 ≤ dec ≤ 2.70. tals178groupsand1437groupmembergalaxiesatz ≤ 1.Our
Within this region are 13619 galaxies with secure redshift and choiceenablesustoworkwithgroupsthathavebestvaluesfor
z≤1(15730objectsintotal,irrespectiveofredshiftandinclud- purityandinterloperfraction,andtosecureareliabledefinition
ingstars)andtheirskydistributionisremarkablyuniform. ofgroupcenterandradius.Thesetwoparametersarecrucialto
ForobjectsbrighterthanI =22.5andwithoutsecurespec- buildthecompositegroupandforouralgorithmwhichretrieves
AB
troscopicredshift,thewealthofancillaryphotometricdatapro- groupmemberswithoutspectroscopicredshiftinformationd(see
videdbytheCOSMOSsurveyprovidesgoodqualityphotomet- laterSect.2.4and4).
3
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
2.3.Thespectroscopicfieldsample Toovercomethisdrawback,we developeda slightlydiffer-
entstrategy,whosemainadvantageisthatitassignseachgalaxy
To define the field galaxysample, we started by selecting 20K only one spectroscopic group, thus avoiding multiple assign-
galaxies located within the high sampling rate box and out- mentsof a galaxytodifferentgroups,andthe needtoadoptan
sideanyofthereconstructedgroupsofKnobeletal.(2011).We arbitraryprobabilitycut-offtobypassthisproblem.
therefore discarded from this sample galaxies located in pairs, For a detailed description of our algorithm we refer the
tripletsandquadruplets,i.e.,membersofthegroupswithfewer readertoAppendixA,whileinSect.3wewillpresentextensive
observedmembersarenotconsideredinourscienceanalysis. teststhatweperformedonmockcatalogstocheckthereliability
To performthe fairest comparisonbetween groupand field ofthefinalspec+photo-zgroupcatalog.
samples, we took into accountthe possibility of spurioustrend Sufficeistosaythatwechosetheselectionfunctiontoiden-
introduced by residual group contamination or by the differ- tifyputativephotometric-redshiftmembersinawaytonotonly
ent redshift ranges covered by the group/field galaxy samples. keep the fraction of interlopers as low as possible, but also to
Galaxieslyingintheclosestproximityofgroupscouldbecon- avoid introducinganyradialdependencyof the interloperfrac-
taminatedbyspectroscopicgroupmembersmissedbythegroup- tion.Thelastpointisimportantbecausewewillbelookingfor
finding algorithm. In addition, the redshift distribution of the radialdependenciesofgalaxyproperties.
spectroscopicgroupcatalogisfarfrombeinguniform,display- As already mentioned in Sect.2.2, we chose a conservative
ing prominentpeaks, especially at low redshift where the 20K definition of the spectroscopic group sample, restricting our-
fieldofviewlimitsthecosmicvolumeexploredandweneedto selves to only 178 groups detected with at least five spectro-
considertheappropriatecoevalfieldpopulation. scopic members within the the high sampling rate box intro-
To take into accountthese two factors, we furthermore re- ducedin Fig.1box(149.55 ≤ ra ≤ 150.666and1.75 ≤ dec ≤
strictedthefieldsampletogalaxieslocatedwithinvelocitydis- 2.7).Withinthisareaanduptoz=1.0thereare13619galaxies
tances2000≤|∆v|≤5000kms−1 fromthespectroscopicgroup withreliablespectroscopicredshiftand11994withanestimated
sample, - therefore following the same redshift ditribution of photometricredshift.
group member galaxies - and with radial projected distances Ouralgorithmaddsanother684membergalaxieswithpho-
R > 4 × R from any group of 20K group catalog, R tometric redshifts to the already existing 1437 spectroscopic
fudge fudge
beingan estimate of the virialradiusprovidedby Knobeletal. group member galaxies, and from now on this is the group
(2011)(seeSect.4.2fordetails).Fromnowonwewillcallthis sample we will use. The final number of groups with more
setofgalaxiesthefieldsample,totalling6556galaxiesatz≤1. than10(15)membersafterapplyingouralgorithmistwice(three
times) that in the spectroscopic group catalog, i.e., there are
We also introduced a complementary set of field galaxies
78(41)groupsinsteadoftheoriginal39(14)groups.Thenumber
that we call near-field galaxies. These are 20K galaxieswithin
of groupswith more than 20 membersis six times the original
the high sampling rate box that do not belong to any of the
one:25groupsinsteadoftheoriginalfourgroups.
reconstructed groups of Knobeletal. (2011), but with velocity
distances |∆v| ≤ 2000 km s−1 and radial projected distances Asafinalpointwenoticethatwerepeatedallanalysespre-
sented in this paper considering only galaxies from the spec-
R≤4×R fromatleastonegroupofthespectroscopicgroup
fudge troscopic group catalog and our results remained entirely un-
sample.Thenear-fieldsamplesodefinedtotals1694galaxiesat
changed,albeitatalowersignificance.
z ≤ 1 and contains, by definition, galaxies located in the close
proximityofthespectroscopicgroupsample.InSect.6.3wewill
use thissample to check forpossible environmentaleffects ex- 3. ThezCOSMOSmockcatalogs
tendingoutside groupradii,e.g.,colordifferencesofnear-field
galaxypopulationwithrespecttothegeneralfieldsample. The use of realistic mock galaxy catalogs is important for as-
sessing the reliability of the algorithm we adopted to produce
the spec+photo-z group catalog and to validate the procedures
2.4.Addingphoto-zs:thespec+photo-zgroupcatalog we chose to define group centers and richnesses (see Sect.2.4
andSect.4).
Foreachgroupthenumberofavailablemembergalaxiesdown
We took advantage of the 24 COSMOS mock light-cones
toI =22.5islimitedbytheincompletesamplingrateof20K.
AB provided by Kitzbichler&White (2007). These mock light-
To increase this number, we took advantage of the exquisite
cones are based on the Millennium DM N-body simulations
quality of the photometric redshifts available in the COSMOS
ofSpringel(2005)andusesemianalyticrecipesofCrotonetal.
field,see Sect.2toincorporatein ouranalysisphotometricred-
(2006)asupdatedbyDeLucia&Blaizot(2007)forpopulating
shiftsforgalaxiesbrighterthanI = 22.5andwithoutreliable
AB thesimulationsvolumewithgalaxies.
spectroscopic data. A higher number of group member galax-
Fromeachofthese24light-conesweextractedthreediffer-
iesenablesonetoimprovecenteringandrichnessestimatesfor
enttypesofmockcatalogs:
eachgroup,quantitiescrucialtoproperlycenterandrescaledis-
tinct groupsto build a composite one (see e.g., Carlbergetal. 1. The 40K mock catalogs: 100%complete to I = 22.5. In
AB
1997). thesecatalogsallgalaxiesbrighterthanI =22.5arespec-
AB
In Knobeletal. (2011) a probabilityapproachwas adopted troscopicallyobservedwitha100%successrate.Weadded
to retrieve member galaxies brighter than I = 22.5 and that toeachgalaxyredshiftanerrorof100kms−1toaccountfor
AB
have no spectroscopic information.To each galaxy a probabil- the typical zCOSMOS spectroscopic redshift error as esti-
ity, p ,ofbeingpartofagroupwasassigned,dependingonits matedfromobservations(seeLillyetal.2009).
in
projectedradialandvelocitydistancefromthegroupcenter(we 2. The 20K mock catalogs: mimicking the 20K zCOSMOS
referthe readertothepaperbyKnobeletal.(2011) fora more spectroscopic sample. We applied the same observational
detailed description of the adopted method). The drawback of strategyadoptedto selectthe spectroscopiczCOSMOS tar-
thisapproachisthateachgalaxymayhavemultipleassociations gets:usingthe slitpositioningalgorithmSPOC on the40K
todifferentgroups. catalogs, see Bottinietal. (2005), and accounting for the
4
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
spectroscopicredshiftfailuresbyincludingthesameredshift externalrings,asinSect.4.2,andadoptedexactlythesamemass
successrateastherealdata. and redshift limits adopted subsequently in our analysis, see
3. The 20K+photo-zs mock catalogs: mimicking the data set Sect.5.
weusedinouranalysis.Thespectroscopicgalaxiesarethose In the top right panel of Fig.2 we show the fraction of in-
listedinthe20Kmockcatalogs,whileaphotometricredshift terlopersineachofthesethreeregionsforthe20K/20K+photo-
isprovidedfortheremaininggalaxiesofthe40Kmockcat- zs mock groups(black stars and red triangles, respectively),as
alogs.Forthephotometricredshiftgalaxysamplewerepro- obtained using the low-z, mass-limited, mock group samples.
ducedthephotometricredshifterrorσ =0.007×(1+z). Noticethatthetrendintroducedbythegroup-findingalgorithm
zphot s
We alsotookintoaccountthepresenceofcatastrophicfail- inthe20Kmockreconstructedgroupsisnotmodifiedbyadding
uresinestimatingphoto-z,thatis,theexcessofgalaxieswith photo-zs members. In the same panel the orange squares dis-
errorslargerthan∆phot-z= 3σ withrespecttothesim- playthefractionofinterlopersinthe20K samplespectroscopic
zphot
pleGaussiandistribution.Forthesetofphotometricredshift groupcatalog,estimatedusingtheprobabilities,p ,associated
in,i
adoptedinouranalysiswewereabletoestimate apercent- toeachobservedgroupspectroscopicmember:
ageof∼8%(byusingthe20Ksubsetflagged4.xor3.xand
comparing their spectroscopic redshift to their photometric Ntot,obs
redshift). Group members with such high values of phot-z PI =1− X pin,i/Ntot,obs, (1)
error cannot be retrieved by our algorithm and are a con-
i=1
siderablesourceofincompletenessingroupreconstruction.
Wechoseafairlyconservativeapproachandalsoconsidered asprovidedbyKnobeletal.(2011).Thesevalueshavebeen
catastrophic errors of 10% in the 20K+photo-zsmock cat- calibrated in Knobeletal. (2011) using simulations, and there-
alogs,byrandomlypermutingthephotometricredshiftsfor fore are by constructionagree well with the interloper fraction
10% of the galaxies while keeping the galaxyra-dec fixed. estimatedfrom20Kmockgroups.Inturn,becauseaddingphot-
coordinates. z members does not alter the trends significantly, these values
agree well with the interloper fraction for the 20K+photo-zs
Weappliedtothe20Kmockcatalogsthesamegroupfinding mock groups. The picture does not change when plotting the
algorithmusedforthe20Ksample(seeKnobeletal.2009).We same quantitiesfor the high-z, mass-limited,mock groupsam-
then selected groups with at least five spectroscopic members ples,orwhenselectingsubsetsofgroupsaccordinge.g.,totheir
located within the high sampling rate box introduced in Fig.1 richness.ThereforewealwaysusedthePIvaluesobtainedfrom
and applied to the 20K+photo-zsmock catalogs the algorithm spectroscopic group members to estimate the interlopers’ con-
describedinSect.2.4. tamination as a function of the distance from the group center
The COSMOS mock light-conesprovidedark matter halos forourspec+photo-zgroupcatalog,seeSect.6.2.
IDsthatcaneasilybeusedtoidentifyrealgroupsandrealgroup
members, i.e., the set of galaxies located within the same dark
matterhaloineachmockcatalog(seealsoKnobeletal.2009). 4. Buildingthestackedgroup
Ifwedefinecompletenessastheratioofthereconstructedgroup
members in the 20K/20K+photo-zsmock catalogs to the total To exploregalaxypropertiesas afunctionofthe distancefrom
number of real group members in the 40K mock catalogs, the the center of the group, we needed to build ensemble systems,
improvement introduced by our algorithm is shown in the top becausethescarcityofindividualgroupmemberspreventsade-
leftpanelofFig.2.Thispanelshowsthedistributionofthecom- tailedanalysisofeachgroup.Inthissectionweillustrateindetail
pletenessforallgroupsofthe20Kmocksandforthoseobtained the steps of building the so-called stacked-group: a composite
afterapplyingouralgorithmtothe20K+photo-zsmocks(black groupobtainedbyspatiallyco-addingallgroupmembergalax-
dot-dashedlineandred-solidline,respectively).Wewereableto ies(simplySG fromnowon).
improvethe median completeness from 67% of the 20K mock The two main ingredients to build a SG are precise re-
catalog up to 90% in the 20K+photo-zsmocks: the number of centeringand scalingof allavailable groups.Itis thereforeex-
groupsthatare 100%completeis threetimes largerthanusing tremelyimportantthateachgroupcenterandrichnessisdefined
onlythe20Kmocks.Asaconsequence,thegrouprichness,de- as reliably as possible, so that the trends we are searching for
fined as the numberof membersbrighterthan an adopted rest- arenotsmoothedout.Wewilldiscussprecisedefinitionofboth
frame absolute magnitude cut-off, is also easier to recover in quantitiesinthissection.Weremindthereaderthatfromnowon
a reliable way. For more than half of the cases the richness as anynumberquoted,unlessexplicitlystated,includesbothspec-
measuredforreconstructedgroupsinthe20K+photo-zsmocks troscopic and photometric redshift group member galaxies, as
equalsthe same quantityas obtainedfromthe 40K mockcata- obtainedfromthealgorithmdiscussedinSect.2.4andwhichwe
logs. describeinmoredetailinAppendixA.
Our algorithmachievesthis remarkableresultwhileadding
anegligiblefractionofinterlopermembers,i.e.,galaxiesthatdo
4.1.Groupcentering
notsharethe same darkmatterhalo in the 40Kmock catalogs.
Forhalfofthegroupsweaddedlessthan3%ofnewinterlopers A goodgroupcenter definitionis essentialto our scienceanal-
withrespecttothetotalmembersinthe20K+photo-zscatalog, ysis, because we will be searchingforradialtrendsthatcan be
thereforeattaining the same interlopersfraction as in the spec- easily erased by errorsin groupcentering.After addingphoto-
troscopicgroupcatalog. zs, as discussed in the previous section, 50% of the groups in
We also checkedforanydependenceof the interloperfrac- oursamplepossessmorethanninememberswhichmakesgroup
tiononthenormalizedgroup-centricdistanceR /R ,R center definition more robust. However the simple methods of
gal fudge fudge
beingan estimate of the virialradiusprovidedby Knobeletal. estimating group centers, such as the median of members co-
(2011 see Sect. 4.2 for details). For this test we divided each ordinates,provideonlyroughestimatesofthegroupcenter,es-
group into a central part and two concentric intermediate and pecially for the numerically poorer groups. We therefore tried
5
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
Fig.2.Summaryoftheresultsobtainedwithouralgorithm.Topleft:completenessdistribution(seetextfordefinition)forthe20Kmocks(black
dot-dashedline)andforthe20K+photo-zsmockgroups(redsolidline).Topright:fractionofinterlopersasafunctionofnormalizedgroup-centric
distanceforthe20K/20K+photo-zsmockgroups(blackstarsandredtriangles,respectively).Orangesquaresrefertothefractionofinterlopers,
PI,forrealdata,ascalibratedonthemocks,seetextfordetails.Bottomleft:fordifferentmocksasindicatedonx-axis,themediandistancetothe
centralgalaxyoftheVWcenter(violettriangles)andthemediancenter(cyanstars).Bottomright:DistributionofthedistanceoftheVWcenter
tothecentralgalaxypositionforthe20Kmocks(blackdot-dashedline)andforthe20K+photo-zsmocks(redsolidline).
an alternativestrategy,taking into accountsky-projectedgroup We used our set of mock catalogs to test the advantagesof
galaxydensities. this center definitionwith respectto simpler ones,like the me-
Using the 2D-Voronoiareas as proxy for density measure- dian of the member galaxies coordinates (median center from
ment,wedefinedtheVoronoi-weightedcenter(VWcenterfrom nowon).We assumedthepositionofitscentralgalaxyasfidu-
nowon)as cialcenterforeachgroup,asprovidedbythemocks.
ra = PiN=1rai/AV,i, dec = PiN=1deci/AV,i. (2) In the bottom left panel of Fig.2 we show the median dis-
VW N 1/A VW N 1/A tanceoftheVWcenter(violettriangles)andthatofthemedian
Pi=1 V,i Pi=1 V,i center (cyan stars) for each of the three mock catalogs defined
where AV,i is the 2D-Voronoi area associated to the i-th in Sect.2.4. Error bars show the rms among mock catalogs ex-
galaxymember,thatis, the projectedareacontainingallpoints tracted from the different24 light-cones.We note that the VW
closer to the i-th galaxy than to any other member galaxy. We centerprovidesabetterestimateofthecenterwithrespecttothe
usedgalaxieslocatedoutside3×Rgr (whereRgr istheradiusof mediancenter onaverage.Furthermore,the VW centers, when
the minumumcircle containingall groupmembers)andwithin appliedtothegroupswhithphoto-zsaddedusingourouralgo-
1×σzphot toavoiddivergenceof2D-Voronoiareasforgalaxies rithm,arenearlyindistinguishablefromthoseobtainedwhenall
locatedattheperipheryofgroups. membersdowntoI =22.5possessspectroscopicredshift.The
AB
This way galaxies that are located in group denser regions medianvalueofthedistanceoftheVWcentersfromthegroup
willhavea smallerAV,i andtheywill weighmore,while those centralgalaxyis40h−1 Kpc for20K+photo-zsmocks,withan
thatareinlessdenseregionswillaffectthecenterdetermination improvementofnearly7040%incenteringwithrespecttothe20K
less.Thismethodthusprovidesacenterforthegroup,whichis mocks.
locatedbydefinitionintheareaofgreatestgalaxyover-density,
and is not affected by the details of the spatial distribution of InthebottomrightpanelofFig.2weshowthedistancehis-
galaxies at the outskirt. For a similar approach see Diazetal. togram of the VW center to the central galaxy position for the
(2005). 20Kmocksasablackdot-dashedline andforthe 20K+photoz
6
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
mocksasa redsolid line.Theimprovementin groupcentering Vice versa in the external regions of groups the contamination
obtainedwhenaddingphoto-zmembersisquiteobvious. byfieldgalaxies,onaveragebluerandlessmassivethangroup
WethereforeadoptedtheVWmethodtodefinethecenterof galaxies,willtendtointroducespurioussegregationtrends,and
eachgroup. weneedtoaccountforthemcarefully.
4.2.Grouprescaling
5. Analysis
Theprocedureofstackinggroupsofdifferentsizesandmasses
intoanensemblesystemrequiresrescalingofindividualgalaxy 5.1.Selectingmassvolume-limitedsamples
group-centric-distances.Instudiesofgalaxyclusters,projected
cluster-centric-distances R are generally rescaled with R or Wefocusedouranalysisontworedshiftbins:0.2≤z≤0.45and
vir
R , whose estimate is proportionalto cluster velocity disper- 0.45 < z ≤ 0.8,where we definedtheclassical volume-limited
200
sion σ , that is, a proxy of cluster mass (Carlbergetal. 1997; samplestakingintoaccounttheluminosityevolutionofindivid-
v
Bivianoetal. 2002;Katgertetal. 2004).However,the problem ualgalaxies.FollowingZuccaetal.(2009),weadoptedalinear
isnottrivialwhendealingwithgalaxygroups,wheretheuncer- evolution with redshift: MB∗ev = −20.3− 5 log h70 − 1.1 z to
taintiesintheestimateofvelocitydispersions,masses,size,and parametrizetheevolutionofMB∗ oftheluminosityfunction.The
the group dynamical state in general are larger, because of the correspondingevolvingcut-offmagnitudesareMcut−off = MB∗ev+
smallnumberofgroupmembers. 2.1(+0.8)forthelow(high)redshiftbin.For0.2 ≤ z ≤ 0.45the
IntheliteraturetherearedifferentapproachesinrescalingR volume-limitedsample consistsof829outof 1128totalgalax-
forgroups,using1)thevirialradiusR orR ,2)anestimateof ies,belongingto79groups.For0.45<z≤0.8itconsistsof510
vir 200
thermsofthepositionofmembergalaxiesR ,and3)sometimes out of 660 total galaxies, belonging to 64 groups (see Tab.1).
H
radialdistancesarenotrescaledatall(seeGirardietal.2003for The total volume-limited field sample consists of 1869(2893)
adetailedreview). galaxies,whilethenear-fieldvolumelimitedsampleconsistsof
Ourgroupsspanawiderangeofsizes,andthereforearescal- 683(612)galaxiesforthelow(high)redshiftbin.Intheleftpanel
ing of physical distances seemed unavoidable. We decided to of Fig.3 we show the MB∗ versusredshiftdistributionof the to-
useR asthescalingfactor,providedbyKnobeletal.(2011). tal galaxysample (blackpoints) and thatof both spectroscopic
fudge
Thisfudgequantity,asmanyotheronescorrelatingwiththeob- and photometric redshift group member galaxies (red points).
served group richness, was estimated and calibrated using our Thecyansolidlineandthevioletdashedlinecorrespondtothe
realistic mock catalogs. In brief, given an observed group at magnitudecut-offsdefiningthelow-andhigh-redshiftvolume-
redshift z with richness N, defined as the number of members limitedsamples.Inthefollowing,thegrouprichnessN foreach
brighterthananadoptedrest-frameabsolutemagnitudecut-off, groupisdefinedasthenumberof(phot+spec-z)membergalax-
its R correspondsto the mean R amongallreconstructed iessurvivingtothemoreconservativeabsoluterest-framemag-
mockfudggreoups wjth the same N andvriredshift (see Knobeletal. nitudecut-off:MB∗ev+0.8,unlessexplicitelystated.Thisquantity
2011 for more details on how this quantity is calculated). The correlates,albeitwith a largescatter, with the mass of the halo
quantityR correlateswithM ,anestimateofthemass wherethegroupresidesandthereforeisagoodproxyforit(see
fudge halofudge
ofthegroupwell,whichadditionallyshowsitsrelevanceforour Knobeletal.2009).
analysis (see Knobeletal. 2009, 2011for more detailson how The flux-limited target definition of zCOSMOS-bright,
boththesequantitiesareestimated). IAB ≤ 22.5,translatesintoaB-bandrest-frameselectionatz ∼
Because our goal is to distinguish property of galaxies lo- 0.8.Thereforethe20K galaxysample,whenrest-frameB-band
cated in regions with different physical properties rescaling by selectionisadopted,isfreefromsignificantcolor-dependentin-
R ,aquantityrelatedtoR ,suitsourneedswell.Indeed,the completenessin(U−B)rest-framecolorsuptoz∼1.However
fudge vir
virialradiusisa scalingfactorformanytimescalesofdifferent the (U − B) rest-frame color completeness in the B-band rest
processes such as the crossing time, the relaxation time or the frameselectiondoesnotimplycompletenessinmassselection:
mergingtime(Boselli&Gavazzi2006;Weinmannetal.2006). theB-bandrest-frameselectionisbiasedtowardblue,low-mass
Allgalaxiesthatareinsidethevirialradiusareexperiencingthe galaxies,whilemissingthecorrespondingred,equallylow-mass
group potential effects either for the first time or many times. ones. Environmentaltrendsobservedin samplesselected using
In contrast, those galaxies that are outside the virial radius are rest-frameB-bandmagnitudescouldthereforebesimplythere-
amixedpopulationofbothin-fallinggalaxiesandgalaxiesthat sultsofthisincompletenesscoupledwithdifferentgalaxymass
oncepassedthroughthevirialradiusbutnowareintheoutskirts, distributionsindifferentenvironments(Bolzonellaetal.2010).
theso-calledback-splashpopulation(Gilletal.2005). To separate true environmental effects from mass-driven
Beforestackinggroups,wethereforerescaledeachmember ones,weusedinouranalysismassvolume-limitedsamples,that
galaxydistance to the VWcenter,R , with the corresponding is, samples complete down to a fixed galaxy mass cut-off. To
gal
R ofitsgroup.Belowwewilluseonlyscaleddistances,R, obtainthem, we followedthe same approachasin Iovinoetal.
fudge
unlessotherwisespecified. (2010).Inbrief,wefirstcalculatedthelimitingstellarmassfor
Weaddafinalcaveat:whendiscussingourresults,weshould each galaxy in the 20K sample, i.e., the stellar mass it would
take into account projection effects. We observed the 2-D pro- haveatitsspectroscopicredshift,ifitsapparentmagnitudewere
jection of a 3-D distribution of member galaxies. Assuming a equaltothelimitingmagnitudeofoursurvey:log(M (z ))=
lim gal
spherical distribution, this implies that e.g., the inner observed log(M )+0.4(I −22.5).Wethenusedtheseestimatedlim-
gal AB
regionincludesgalaxieslocatedintheoutergroupshellsthatare iting masses to define, in bins of (U − B) rest-framecolors for
locatedalongthelineofsightofthegroupcentralpart.Hencea eachredshiftbin,themassM belowwhich85%ofgalaxies
cut−off
fractionof galaxiesobserved,in projection,in the innerregion of that colorlie. We fitted M to obtain a color-dependent
cut−off
actually belongs to the outskirts. These projection effects will masslimitcut-off.ThevalueofM forthereddestgalaxies
cut−off
tendto smooththeradialtrendswearelookingfor,so thatany ineachredshiftbinistheonethatweusedasthelimitingmass
observedtrendisalowerlimitfortherealtrendpresentin3-D. forthatbin.
7
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
Fig.3.Leftpanel:redshiftdistributionofthezCOSMOS-brightgalaxies(black).Redpointsrepresentgroupmembergalaxieswith
spectroscopicandphotometricredshiftsasobtainedfromouralgorithm.Thecyansolidlineandthevioletdashedlinecorrespond
tothetwodifferentmagnitudecut-offsadoptedtodefinethevolume-limitedsamplesofthelow-andhigh-redshiftbinrespectively,
seetextfordetails.Centralandrightpanel:(U −B)rest-framecolorversusmassforthelowestandthehighestredshiftbin.The
bluedashedlinecorrespondstothecolor-dependentM ,whiletheredsolidlinecorrespondstothefixedM forourmass
cut−off cut−off
volume-limitedsample.Thereddot-dashedlineshighlightthemassrangesadoptedinthemass-segregationanalysis,seeSect.6.5
fordetails.Thecyandottedlinecorrespondstotheseparationbetweenredandbluegalaxies(seetextforitsprecisedefinition).
InthecentralandrightpanelofFig.3weshowthe(U −B) Table 1. Number of volume-limited and mass-volume-limited
rest-framecolorversusthestellarmassforthelowestandhigh- (spec+phot-z)groupmembergalaxies.Inbracketswereportthe
est redshift bin respectively. The blue dashed line shows the number of spectroscopic-only group members. The number of
color-dependent M , while the red solid line shows the groupscontainingthesegalaxiesislistedinthelastcolumn.
cut−off
valuechosentodefinemass-limitedsamples:log(M /M ) ≥
gal ⊙
M = 9.8 and log(M /M ) ≥ M = 10.56 for the Redshift Vol-lim Vol-Mass-lim
cut−off gal ⊙ cut−off
lowestandhighestredshiftbins. Ngals Ngals Ngr
To define the mass-dependentcolor cut separating the blue 0.2≤z≤0.45 829(570) 571(410) 79
0.45<z≤0.8 510(391) 265(200) 64
andredgalaxies,weperformeda robustfitoftheredsequence
asafunctionofthegalaxystellarmassinthehigh-zbin,wherea
largenumberofobservedgalaxiesdisplaysaprominentandwell
definedredsequence.Thecolorcutwasthenobtainbyshifting Before moving to a detailed study of the group member
thefittinglineby2·rms ,whererms ∼ 0.08isthedisper- galaxiesproperties,itisinterestingtocomparethegeneralprop-
red red
sionoftheredgalaxiesalongtheredsequence.Weadoptedthe ertiesofgroupsinlow-andhigh-redshiftbins,tohighlightany
same color cut for the low-z bin. Numerically,the stellar mass redshift-dependenttrendinthegroupsampleweusedinoursci-
dependentcolorcutis enceanalysis.
In Fig.4 we compare from left to right R - the virial
(U−B)=0.094·log(M /M )+0.05, (3) fudge
gal ⊙
radius estimate (Knobeletal. 2011), M - the mass of the
fudge
anditisshownbythecyandottedlinesinFig.3. groupcalibratedwiththemocksasinKnobeletal.(2009),and
We tested thatourresultsdo notchangeif we applya con- thegrouprichness,N,asdefinedinsection5.1,forlow-(black
stantcolorcut,(U −B)=1,toseparateredandbluegalaxies,a dot-dashedline)andhigh-(redsolidline)redshiftgalaxygroups.
simplerdefinitionthatcorrespondsequallywelltothedipofthe The KS test always rejects with more than 99.99% confidence
bimodaldistribution. thehypothesisthatpropertiesoflowandhighredshiftgroupsare
For the lowest redshift bin the final group mass-complete drawnfromthesamedistribution.Inthelow-redshiftbinonthe
samplecontains571galaxies,whileforthehighestredshiftbinit meanwedealwithsmaller,lessmassiveandpoorergroupsthan
contains265galaxies.Themass-completefieldsamplesconsist thoseinthehighest-redshiftbin.Thisisnotanunexpectedresult
of 743(728)galaxiesforthe lowest(highest)redshiftbin, while giventhatzCOSMOSisaflux-limitedsurveyandthereforethe
thenear-fieldsamplesconsistof293(211)galaxiesforthelow- observedpopulationofbothgalaxiesandgroupsvarieswithin-
est(highest)redshiftbin. creasingredshift.Asaconsequence,thegroupdetectionworks
onlyonprogressivelybrighter/moremassivegalaxiesmovingto
higherredshifts.Weshallneedtotakeintoaccountthesediffer-
5.2.Low-zandhigh-zstacked-groups
enceswhendiscussingourresults. We definea subsetofricher
Foreachofthetworedshiftbinsdefinedintheprevioussection, groupsforthelow-redshiftbinusingrichnessN,definedforthis
0.2 ≤ z ≤ 0.45 and 0.45 < z ≤ 0.8, we proceeded to build bin as the number of member galaxies surviving the evolving
the correspondingSG. Notice thatwhile forcenteringpurposes magnitude cut-off: M = M∗ + 2.1 (see Sect. left panel
cut−off Bev
we used all spec-z and phot-z galaxies available in our group of Fig.3). We adopted a separation of N ≤ (>)12 to distin-
catalog,irrespectiveof their mass andB-band rest-framelumi- guishbetweenpoor(rich)groups,avalueroughlycorresponding
nosity,forouranalysiswewilluseonlygalaxieswithinthemass toM ≤(>)13.3,sothatrichlow-zgroupsarevirtuallyindis-
fudge
volume-limitedsamplesasdefinedinTable1. tinguishableinmassdistributionfromthehigh-redshiftsample.
8
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
Fig.4.Comparisonofthegeneralpropertiesofgroupsinthe0.2 ≤ z ≤ 0.45(blackdot-dashedline)and0.45 < z ≤ 0.8(redsolid
line)redshiftbin.Fromlefttorightwe compareR (theestimateofthevirialradius),logM (theestimateofthemassof
fudge fudge
thegroup),bothfudgequantitiesarecalibratedwiththemocksasdefinedinKnobeletal.(2009)),andN,asdefinedinSect.5.1.
Indeed, while a KS test comparing the distributions of M 6. Results
fudge
of poor and rich groups defined this way rejects the hypothe-
sis that they are drawn from the same distribution with more We will start our analysis by exploring how galaxy colors are
than99.99%confidence,theKStestcomparingdistributionsof affected by group environment, irrespective of galaxy position
M ofrichlow-zgroupsandofhigh-zgroupsdoesnotreveal withinthegroup(seeSect.6.1).Wewillthenmovetoinvestigate
fudge
anysignificantdifferencebetweenthetwo. thepresenceofcolorsegregationwithinthegroupenvironment
(Sect.6.2),andiftheeffectofthegroupenvironmentextendsto
scales somewhat larger than those of the group size itself (see
To explore how galaxy population properties change as a
function of group-centric distance, we first sorted SG galaxies Sect.6.3).Thankstothehighstatisticofthe20Kwewillalsobe
intoincreasingscaleddistancesfromSG centerandthendivided abletoinvestigateifandhowobservedtrendsdependongroup
richness and on galaxy stellar mass (see Sect.6.4). Finally we
theirdistributionintothreeequipopulatedbinscorrespondingto
inner,intermediate,andperipheralSG regions. will search for evidenceof mass segregationinside groupsand
howitmightdependongrouprichnessandaffectobservedmass
trends(seeSect.6.5).
In Table 2 we list the exact radial ranges of each of these
three regions, all values are normalized to R . The corre-
fudge
sponding three median distances are R ∼ 0.15, R ∼ 0.4 and
6.1.F andgalaxystellarmassesingroupsvsfield
R∼0.85,respectively,thereforetheseregionscanbeconsidered blue
as the groupinner core,intermediate,and moreexternal/in-fall The cumulative galaxy stellar mass distributions of the mass-
region. complete group and field samples are shown in the top pan-
els of Fig.6, red solid and cyan dot-dashed lines, respectively,
Given that the median R is ∼ 500 h−1 Kpc in both the left(right) panels refer to the low(high) redshift bin. The
fudge 70
redshift bins, the inner region extends typically up to ∼ 150 KS test rejectsthe hypothesisthat groupand field galaxymass
h−1 Kpc. Because the VW center is on the average only ∼ 40 distributions are drawn from the same population with more
70
h−1 Kpc awayfromthegroupfiducialcenter(seeSec.4.1),our than 99.99% confidence for both redshift bins. Group envi-
70
errorincenteringisnegligiblewithrespecttothemedianinner ronment hosts preferentially more massive galaxies than the
region size, and should not have a significant impact when ex- fieldone,confirmingwell-knownliteratureresults(Iovinoetal.
ploringthegroup-centricdependenceofgalaxyproperties. 2010;Kovacˇetal.2010;Bolzonellaetal.2010).
Asaconsequence,toexplorethepresenceofcolortrendsas
Theskydistributionofgalaxiesbelongingtothelow-z(left) a function of environment,we need to separate the joint effect
andhigh-z(right)compositegroupisshowninFig.5.Pointsare ofmassandenvironmentandtoperformtheanalysisinnarrow
coded according to the (U − B) colors of the galaxies, while mass bins of galaxy stellar mass. We adopted a galaxy stellar
point dimensions are scaled according to galaxy masses. As a mass bin of 0.4 dex,which is approximatelytwice our errorin
referencewedrawdashedcirclescorrespondingtothe division estimating galaxy stellar masses (Pozzettietal. 2010). Bottom
betweenthedifferentregionsineachcompositegroup.Wenote panelsofFig.6showF atfixedgalaxystellarmassingroups
blue
thattheoverallshapeofthecompositegrouphasawell-defined (redcircles)andfield(cyanstars),whileTable3listsindetailthe
peakcorrespondingtothecenter,whiletheprojecteddensityde- F valuesand theirerrors.Both at highandlow redshift,the
blue
creases as we move from the center to the outskirts. A visual bluefractionincreaseswhenmovingtowardlessmassivegalax-
inspection of the galaxy sky distribution already shows rough ies.F isalwayshigherinthefieldthaninthegroup,adiffer-
blue
differencesin masses and colors dependingon the area we ex- encethatdecreasesmovingtomoremassivegalaxies.Themost
plore.Inthenextsectionweproceedtoextensivelyanalyzethese massivegalaxies(log(M /M ) > 11.0)donotshowanysig-
gal ⊙
trendsandtheirdependenceonintrinsicgalaxy/groupproperties, nificant F evolution with redshift within the error bars. For
blue
properlyaccountingforpossiblefieldcontaminationeffects. thegalaxieswith10.6≤ log(M /M ) ≤ 11.0,F decreases
gal ⊙ blue
9
Presotto,V.etal:SegregationeffectsinthezCOSMOS-20Kgroupsample
Fig.5. Sky distribution of the galaxies belonging to the low-z (left) and high-z (right) composite group. Ra-dec positions are
expressed in terms of the rescaled distances R. Points are colored according to the (U − B) colors of the galaxies, while point
dimensionsarescaledaccordingtothemassesofthegalaxies.Asareferencewedrawdashedcirclescorrespondingtothedifferent
central/intermediate/externalregionlimitsineachcompositegroup.
Table2.RadialrangeexploredinthethreeSG regions.AlldistancesRarenormalizedtoR .
fudge
Redshift 1st region 2ndregion 3rd region
range R range R range R
median median median
0.2≤z≤0.45 R≤0.30 0.15 0.30<R≤0.68 0.47 R>0.68 0.94
0.45<z≤0.8 R≤0.23 0.13 0.23<R≤0.51 0.37 R>0.51 0.74
forbothgroupandfieldenvironmentwhenmovingfromhighto Table3.Observedbluefractionsingroupsandfieldfordifferent
lowredshift. galaxystellarmassbins.
Notice that on average, groups in the low-z bin are poorer
thanthoseinthehigh-zbin.Aswewillshowbelow,seeSect.6.4, Sample0.2≤z≤0.45 group field
F dependsonthegrouprichness,F beinglowerinricher
blue blue 9.8≤log(M /M )≤10.2 0.33+0.03 0.70+0.03
groups. As a consequence,the decrease of Fblue across the ex- gal ⊙ −0.03 −0.03
ploredredshiftrangeforthesampleofgroupgalaxiesshouldbe
10.2≤log(M /M )≤10.6 0.18+0.03 0.49+0.03
evenmorepronounced. gal ⊙ −0.03 −0.03
At fixed galaxy stellar mass, the migration to the red se- 10.6≤log(M /M )≤11.0 0.22+0.03 0.32+0.03
gal ⊙ −0.03 −0.03
quencehappensearlierinthegroupsandlaterinthefield,sug-
gestingthepresenceofphysicalmechanismsabletoremovegas 11.0≤log(M /M )≤11.4 0.11+0.04 0.25+0.07
gal ⊙ −0.03 −0.06
that causes the earlier quenchingof galaxiesin groups. We re-
mindthereaderthatanycontaminationofthegroupsampleby Sample0.45<z≤0.8 group field
fieldgalaxiesandviceversa,forwhichwearenotapplyingany
correction,willonlyrendertheobservedtrendslessprominent. 10.6≤log(Mgal/M⊙)≤11.0 0.28−+00..0034 0.41−+00..0022
The real, corrected, trends therefore would be even more pro-
11.0≤log(M /M )≤11.4 0.11+0.04 0.23+0.03
nounced.ThisresultexcellentlyagreeswithourpreviouszCOS- gal ⊙ −0.03 −0.03
MOS results on groups (Iovinoetal. 2010; Bolzonellaetal.
2010;Pengetal.2010).
Thequestionswewilladdressinthefollowingsectionsare:
dothegroupmembergalaxiesallsharethesame F valueir-
blue
respectiveoftheirpositionwithinthegroup?Dothegalaxieslo-
catedinthecentralregionofgroupssharethesamemassdistri-
butionasthegalaxieslocatedinthegroupoutskirts?Ideally,the
firstquestionisbetteraddressedinnarrowbinsofgalaxystellar statistics,whensplittingoursampleaccordingtodistancesfrom
mass to avoid the mass-color degeneracy. However, even with groupcenterandthereforewewillstartouranalysisworkingin
suchadatasetasthe20K,we arestilllimitedbysmallnumber cumulativemassrangesinthenextsection.
10