Where do Wet, Dry, and Mixed Galaxy Mergers Occur? A Study of the Environments of Close Galaxy Pairs in the DEEP2 Galaxy Redshift Survey

0.57 MB·English

by  Lihwai Lin

DRAFTVERSIONJANUARY25,2010
PreprinttypesetusingLATEXstyleemulateapjv.11/26/03
WHEREDOWET,DRY,ANDMIXEDGALAXYMERGERSOCCUR?ASTUDYOFTHEENVIRONMENTSOFCLOSE
GALAXYPAIRSINTHEDEEP2GALAXYREDSHIFTSURVEY
LIHWAILIN1,MICHAELC.COOPER2,3,HUNG-YUJIAN4,DAVIDC.KOO5,DAVIDR.PATTON6,RENBINYAN7,CHRISTOPHERN.
A.WILLMER2,ALISONL.COIL8,TZIHONGCHIUEH4,DARRENJ.CROTON9,BRIANF.GERKE10,JENNIFERLOTZ11,12,PURAGRA
GUHATHAKURTA5,ANDJEFFREYA.NEWMAN13
DraftversionJanuary25,2010
0 ABSTRACT
1 We study the environmentof wet, dry, and mixed galaxy mergers at 0.75<z<1.2 using close pairs in
0
theDEEP2GalaxyRedshiftSurvey,aimingtoestablishaclearpictureofhowthecosmicevolutionofvarious
2
merger types relate to the observed large-scale extra-galactic environmentand its role in the growth of red-

n sequence galaxies. We find that the typical environment of mixed and dry mergers is denser than that of
a wet mergers,mostly due to the color-densityrelation. While the galaxycompanionrate (N ) is observedto
c
J increasewithoverdensity,usingN-bodysimulationswefindthatthefractionofpairsthatwilleventuallymerge

5 decreaseswiththelocaldensity,predominantlybecauseinterlopersaremorecommonindenseenvironments.
2 Aftertakingintoaccountthemergerprobabilityofpairsasafunctionoflocaldensity,wefindonlymarginal
environmentdependenceofthefractionalmergerrateforwetmergersovertheredshiftrangewehaveprobed.

] On the other hand, the fractional dry merger rate increases rapidly with local density due to the increased
O
populationofredgalaxiesindenseenvironments. Inotherwords,whilewetmergerstransformgalaxiesfrom
C thebluecloudintotheredsequenceatasimilarfractionalrateacrossdifferentenvironments(assumingthatthe
. successrateofwetmergerstoyieldredgalaxiesdoesnotdependonenvironment),thedryandmixedmergers
h
are most effective in overdense regions. We also find that the environmentdistribution of K+A galaxies is
p
similartothatofwetmergersaloneandofwet+mixedmergers,suggestingapossibleconnectionbetweenK+A
-
o galaxies and wet and/or wet+mixed mergers. Based on our results, we therefore expect that the properties,
r including structures and masses, of red-sequence galaxies should be different between those in underdense
t
s regionsandinoverdenseregionssincethedrymergersaresignificantlymoreimportantindenseenvironments.
a
Weconcludethat,asearlyasz∼1,high-densityregionsarethepreferredenvironmentinwhichdrymergers
[
occur, and that present-day red-sequence galaxies in overdense environments have, on average, undergone

1 1.2±0.3 dry mergers since this time, accounting for (38±10)% of their mass accretion in the last 8 billion
v years.Ourfindingssuggestthatdrymergersarecrucialinthemass-assemblyofmassiveredgalaxiesindense
0 environments,suchasBrightestClusterGalaxies(BCGs)ingalaxygroupsandclusters.
6
Subjectheadings:galaxies:interactions-galaxies:evolution-large-scalestructureofuniverse
5
4
1. 1. INTRODUCTION Whether this bottom-up scenario also holds for galaxies
0 Within the framework of hierarchical structure formation, whichresideindarkmatterhalosremainsachallengingques-
0 tion in the theories of galaxy formation and evolution. The
dark-matterhalosgrowthroughsuccessivemergerswithother
1 keysto pindowntheimportanceofmergersin theassembly
halos and through accretion of the surrounding mass (Blu-
: history of galaxies are the study of galaxy mergerrates as a
v menthal et al. 1984; Davis et al. 1985; Stewart et al. 2009).
function of cosmic time (Carlberg et al. 2000; Patton et al.
i
X 1 Institute ofAstronomy&Astrophysics, Academia Sinica, Taipei106, 2002; Conselice et al. 2003; Lin et al. 2004, 2008; Lotz et
r Taiwan;Email:[email protected] al.2008a;deRaveletal.2009;Blucketal.2009)andtoun-
a 2StewardObservatory,UniversityofArizona,933N.CherryAvenue,Tuc- derstand the level of triggered star formation during galaxy
son,AZ85721USA interactions(Lambasetal. 2003;Nikolicetal.2004;Woods
3SpitzerFellow
etal. 2006;Lin etal. 2007;Bartonet al. 2007;Ellison etal.
4DepartmentofPhysics,NationalTaiwanUniversity,Taipei,Taiwan
5 UCO/Lick Observatory, Department of Astronomy and Astrophysics, 2008).
UniversityofCalifornia,SantaCruz,CA95064 In addition to assembling galaxy masses, galaxy mergers
6 Department of Physics and Astronomy, Trent University, 1600 West have also been suggested to be responsible for the change
BankDrive,Peterborough,ONK9J7B8Canada of galaxy properties (Hopkins et al. 2006). Galaxy popula-
7DepartmentofAstronomyandAstrophysics,UniversityofToronto,50
tionshavebeenshowntoevolvedifferentlysinceredshift1.5:
St.GeorgeStreet,Toronto,ONM5S3H4,Canada
8DepartmentofPhysicsandCenterforAstrophysicsandSpaceSciences, whilethecharacteristicnumberdensitiesofbluegalaxiesre-
UniversityofCalifornia,SanDiego,9500GilmanDr.,LaJolla,CA92093 mainfairlyconstant,thenumberandstellarmassdensitiesof
9 Centre for Astrophysics & Supercomputing, Swinburne University of redgalaxieshaveatleastdoubledoverthisperiod(Belletal.
Technology,P.O.Box218,Hawthorn,VIC3122,Australia 2004;Willmeretal.2006;Faberetal.2007).Thegrowthrate
10KavliInstituteforParticleAstrophysicsandCosmology,StanfordLin-
of the red galaxies is much faster than the predictions from
earAcceleratorCenter,2575SandHillRd.,M/S29,MenloPark,CA94025,
USA purely passive evolution, suggesting that additional physical
11NationalOpticalAstronomyObservatory,950N.CherryAve.,Tucson, mechanisms are required to truncate the star formation in
AZ85719 someofthebluegalaxiesandturnthemintothered-sequence
12LeoGoldbergFellow
(Bell et al. 2007). More recently, there have been studies
13PhysicsandAstronomyDept.,UniversityofPittsburgh,Pittsburgh,PA,
15620 examining the connection between galaxy mergers and the
2 Linetal.
establishment of red-sequence galaxies (van Dokkum 2005; Inthiswork,weadoptedtheformerapproachasaprimaryen-
Bell et al. 2006; Lin et al. 2008; Skelton et al. 2009). Us- vironmentmeasurementtoinvestigate(1)whichenvironment
ingclosepairsfoundintheDEEP2Survey,Linetal.(2008) hosts most wet/dry/mixed mergers and (2) the pair fraction
found that the present red galaxies might have experienced and the fractional merger rate as a function of environment
onaverage0.7,0.2,and0.4wet, dry,andmixedmergersre- at0.75<z<1.2, using the blue-blue,red-red, andblue-red
spectivelysincez∼1,suggestingakeyroleofgalaxymerg- pairs selected from the DEEP2 sample (Lin et al. 2008). It
ers in the evolution history of red galaxies. In addition, the is worth noting that there are potential caveats in our analy-
relative role of different types of mergers also evolve with sisusinggalaxycolorstoclassifywet/dry/mixedmergers.At
redshift, indicating that the effect of quenching star forma- z∼1, it is found that about 20% of red galaxies appear to
tionandmassbuild-upthroughmergersalsoevolvewithtime. beeitheredge-ondisksordustygalaxiesandhencearelikely
Meanwhile, it becomesincreasinglyclear thatdense regions to be gas-rich (Weiner et al. 2005) whereas there also exist
suchasgalaxygroupsandclustersmightbetheplaceswhere blue spheroidals that could be gas-poor, although these are
the transformation of blue galaxies into red galaxies occurs relativelyrare objects(Cassata etal. 2007). As notedin Lin
mosteffectively. Ifgalaxymergersarethedominantquench- et al. (2008)that both cases of contamination make up only
ing mechanism, one should also expect a clear environment a minority of the red sequence and blue clouds respectively,
dependenceofgalaxymergerrates.However,therehavebeen classifying different types of mergers based on their colors
very few attempts to probe the connection between mergers shouldbeafairapproximation.
andenvironmentobservationally(McIntoshetal.2008;Darg Majoruncertaintiesinconvertingthepairfractionintothe
etal.2009). mergerfractionand mergerratescome fromthe handlingof
Anotherwaytoprobetheconnectionbetweenmergersand thefractionofpairsthatwillmergeC andmergertime-scale
mg
the formationof red galaxiesis to comparethe environment T (Kitzbichler&White2008;Lotzetal.2008b). Themost
mg
distribution of mergers to the poststarburst galaxies, the so- commonwayof assessing mergertime-scale is computedas
called ’K+A’ or ’E+A’ galaxies (Dressler & Gunn 1983). the"dynamicalfrictiontime"(Binney&Tremaine1987;Wet-
Such galaxiesare identified throughtheir strong Balmer ab- zeletal.2009)orisestimatedfromtheN-body/hydrodynamic
sorptionandlittleHαor[OII]emissions,indicatingthatthey simulationsof galaxy mergers(Conselice 2006;Jiang et al.
had recent star formation with the last ∼ 1 Gyr or so, but 2008;Lotz et al. 2008b). On the other hand, the fraction of
no on-going star formation. These K+A galaxies are sug- pairsselectedwithprojectedseparationwith/withouttheline-
gestedtobethetransitionphasebetweenstarforminggalax- of-sightvelocitiesthatarephysically-associatedpairsandwill
iesandthedeadred-sequencegalaxies,andhencetheycould mergewithinashorttimeisoftenobtainedbycorrectingfor
be the direct progenitors of early-type red galaxies. There chanceprojection(Patton&Atfield2008;Bundyetal.2009).
have been many studies looking at the environmentof post- Both T andC are usually assumed to be a constant at a
mg mg
starburst galaxies (Goto 2005; Hogg et al. 2006; Yan et al. givenredshiftandmassbin,independentoftheenvironment.
2009;Poggiantiet al. 2008), with the aim of identifyingthe However, such approachesmay not be adequate when com-
process that truncates the star formation activity (mergers, paringthemergerrateacrossdifferentenvironmentsbecause
ram-pressurestripping, AGN feedback, etc.). By comparing closepairsindenseenvironmentsarenotsimplyisolatedtwo-
theenvironmentdistributionofK+Agalaxiesintheliterature bodysystems,butarealsoinfluencedbynearbygalaxies,sur-
to thatofgalaxymergers,we can gaininsighton the impor- rounding material, and the gravitational potential from the
tanceofgalaxymergersintheformationofK+Agalaxiesand group/clusterhosthalos. Inorderto makea faircomparison
hencethebuilt-upofred-sequencegalaxies. ofmergerfrequenciesacrossvariousenvironments,weadopt
Sincegalaxyinteractionsrequiremorethanonegalaxyby animprovedestimateofT andC asafunctionofenviron-
mg mg
definition, it is expected that galaxy mergers tend to reside mentobtainedfromcosmologicalsimulationswhenconvert-
in dense regions. Galaxy groupsare thoughtto be preferred ingthepairfractionintothefractionalmergerrate.
placesforgalaxymergersbecauseoftheirlowervelocitydis- Thepaperis organizedas follows. In §2, we describeour
persionsasopposedtothegalaxyclusters. Bystudyingfour sampleselectionandtheapproachofmeasuringenvironment.
X-ray luminous groups at intermediate redshifts (z ∼ 0.4), In§3,wepresentourresultsonthepairfractionsforblueand
Tranetal.(2008)suggestedthatdrymergersareanimportant redgalaxies,thecomputationofbothT andC ,aswellas
mg mg
process to build up massive galaxies in the cores of galaxy the derivedfractionalmergerratesfor differentmergercate-
groups/clusters;McIntoshetal.(2008),usinggroupandclus- gories. A discussionis givenin §4, followedbyourconclu-
ter samples in SDSS also foundthat the frequencyof merg- sions in §5. Throughout this paper we adopt the following
ers between luminous red galaxies (LRGs) is significantly cosmology:H0=100hkms- 1Mpc- 1,Ωm=0.3andΩΛ=0.7.
higheringroupsandclusterscomparedtooverallpopulation TheHubbleconstanth=0.7isadoptedwhencalculatingrest-
ofLRGs. While mostofpreviousstudiesexaminingthe en- frame magnitudes. Unless indicated otherwise, magnitudes
vironment of mergers focused on the dry mergers in dense aregivenintheABsystem.
environments, to date no quantitative measurement of wet,
2. DATA,SAMPLESELECTIONS,ANDMETHODS
dry,andmixedmergerratesasafunctionofenvironmenthas
beenobtained. Thispaperaimstoaddresstheissueofwhere 2.1. TheDEEP2RedshiftSurvey
galaxiesbuild up their masses and where the transformation The DEEP2 Redshift Survey (DEEP2 for short) has mea-
ofgalaxieshappensbyexploringtheenvironmentofvarious sured redshifts for ∼50,000 galaxies at z∼1 (Davis et al.
typesofinteractinggalaxies.Therearetwomethodsthathave 2003, 2007) using the DEIMOS spectrograph (Faber et al.
been used in DEEP2 to classify environments: one is to use 2003)onthe10-mKeckIItelescope. Thesurveycoversfour
theprojectednth-nearestneighborsurfacedensityΣ (Cooper fieldswithField1(EGS:ExtendedGrothStrip)beingastrip
n
etal.2006),whichgivestheestimatesoflocaldensityofin- of0.25× 2squaredegreesandFields2, 3and4 eachbeing
dividualgalaxies;the otheristo classifythegalaxyenviron- 0.5×2squaredegrees. ThephotometryisbasedonBRI im-
ments into "field" and "groups/clusters"(Gerke et al. 2005). agestakenwith the12K×8KcameraontheCanada-France-
EnvironmentofWET,DRY,ANDMIXEDGALAXYMERGERS 3
Hawaii Telescope (Coil et al. 2004). Galaxies are selected 2.4. GalaxyGroupCatalogs
forspectroscopyusinga limitofR =24.1mag. Exceptin
AB Although the overdensity δ is a good representation of
Field 1, a two-color cut was also applied to exclude galax- 3
local environment, it does not carry specific information re-
ies with redshifts z<0.75. A 1200 line/mm grating (R ∼
gardingwhatkindofphysicalenvironmentlikefield,groups,
5000)isusedwith a spectralrangeof6400- 9000Å, where andclustersitcorrespondsto. Acomplementarywaytoclas-
the [OII] 3727 Å doublet would be visible at z∼0.7- 1.4. sify the environment is to cross reference the galaxy sam-
Thedata used herecontains∼20,000galaxieswith reliable ple to the group/cluster catalogs. However, performing the
redshiftmeasurementsfromFields1,3and4. Therest-frame group/cluster finding is not always possible, depending on
B-bandmagnitudes(MB)andU- BcolorsforDEEP2galaxies the availability of redshifts, sampling rate, and other wave-
at0.75<z<0.9arederivedinasimilarwayasWillmeretal. lengthdata(e.g. X-ray). DEEP2targeted∼65%ofallofthe
(2006). Forgalaxieswith0.9<z<1.2,therest-frameU- B galaxiesdowntoR=24.1,and∼70%ofthesegalaxiesyield
coloriscomputedusingtheobservedR- zmegacolorwhenever successfulredshifts. Hencetheoverallredshiftsamplingrate
it is available, where zmega is the z-band magnitudeobtained of DEEP2 is about 50%, which allows identifying potential
fromCFHT/MegacamobservationsforDEEP2Fieldsin2004 group candidates. The group catalog used in this paper is
and2005(Lin,L.etal.,inpreparation). based on the version generated by Gerke et al. (2007), who
applied the Voronoi-Delaunay Method (VDM) group finder
2.2. ClosePairSampleinDEEP2
(Marinoni et al. 2002) on the DEEP2 redshift sample. For
The DEEP2 close pairs used in this work are identical to detailed discussion on the DEEP2 group catalog, see Gerke
thosedescribedinLinetal.(2008). Webeginwithasample et al. (2005, 2007). It was shown that this group catalog
ofgalaxiescovering- 21<Me <- 19(ABmag),whereMe is is most sensitive to groups with modest virial masses in the
B B
the evolution-correctedabsolute magnitude, defined as M + range5×1012<M <5×1013M (200<σ <400 kms- 1
B vir v
Qz. The value of Q is chosen to be 1.3 in order to select ) (Coil et al. 2006). In §3.2, we present the distributions of
galaxieswiththesamerangerelativetotheL∗oftheevolving varioustypesofpairsagainstgroupproperties.However,due
luminosityfunction(Faberetal.2007).Kinematicclosepairs to the incompletenss of group member identifications in the
are then identified such that their projected separations(r ) DEEP2sample,wethereforefocusourfinaldiscussiononthe
p
satisfy10h- 1kpc≤r ≤r (physicallength)andrest-frame resultsobtainedusingthelocaldensitymeasurements.
p max
relative velocities (|∆v|) are less than 500 kms- 1 (Patton et
2.5. Theselectionfunctionandthespectroscopicweight
al.2000;Linetal.2004). Inthiswork,weidentifythepairs
usingr =50h- 1kpcinordertohavesufficientsamplewhen Asmentionedin§2.4,theoverallredshiftsamplingrateof
max
dividingpairsintoseveraldifferentenvironmentbins. DEEP2isabout50%,whichhasanimpactonmeasuringthe
Galaxies are further divided into the blue cloud and red true pair fraction and hence the merger rate. In order to re-
sequence using the rest-frame magnitude dependent cut for cover the intrinsic number of pairs, one must consider the
DEEP2(inABmagnitudes): completenesscorrectionsaccountingforthespectroscopicse-
lectioneffects.DetailedcalculationsandresultsoftheDEEP2
U- B=- 0.032(M +21.62)+1.035. (1)
B selectionfunctionswerepresentedinourpreviouswork(Lin
Blue-blue pairs, red-redpairs, and blue-redpairs are clas- etal.2008);herewesummarizemainstepsofthesecalcula-
sifiedaccordingtotherest-frameU- Bcolorcombinationof tions.Tomeasurethespectroscopicweightwforeachgalaxy
the galaxiescomprisingthe pair, representingthe candidates intheDEEP2survey,wecomparedthesamplewithsuccess-
of‘wet’,‘dry’,and‘mixed’mergers(Linetal.2008).Intotal, fulredshiftstoallobjectsinthephotometriccatalogthatsat-
wehave101blue-bluepairs,26red-redpairs,and52blue-red isfythesurvey’slimitingmagnitudeandanyphotometricred-
pairsovertheredshiftrange0.75<z<1.2. shiftcut. Weparameterizetheselectionfunctiontobe(Linet
al.2008;Yeeetal.1996;Pattonetal.2002):
2.3. LocalEnvironmentIndicator
S (B- R,R- I,R)S (µ ,R) S
For each galaxy in the DEEP2 redshift sample, the local S=SmScSSBSxy=Sm(R) c SB R xy ,
S (R) S (R) S (R)
density environment is measured using the projected third- m m m
(3)
nearest-neighborsurfacedensity(Σ ).Thedetailedprocedure
3 whereS isthemagnitudeselectionfunction,S istheappar-
m c
is described in Cooper et al. (2005, 2006). Here we briefly
entcolorselectionfunction,S isthe surfacebrightnessse-
SB
summarize the steps of computing the overdensity δ used
3 lection functionand S representsthe geometric(localden-
in this work. Σ is first calculated as Σ = 3/(πD2 ), where xy
3 3 p,3 sity) selection function. S , S , and S are all normalized
D2p,3correspondstotheprojecteddistanceofthethird-nearest to the magnitude selectioncfunScBtion, Smx.y The spectroscopic
neighbor that is within the line-of-sight velocity interval of weightw foreachgalaxyisthus1/S,which isderivedfrom
±1000 kms- 1 . For each galaxy, we then derive the over- its apparent R mag, B- R and R- I colors, R band surface
density δ3 as the local-sky completeness-corrected density brightness,andlocalgalaxydensity.
Σ /w ,denotedasΣ ′,dividedbythemediandensityΣ ′(z) ThemagnitudeselectionfunctionS (R)foreachgalaxyis
3 p 3 3 m
atthatredshiftcomputedinbinsof△z=0.04: computed as the ratio between the number of galaxies with
1+δ =Σ′/median(Σ′(z)), (2) goodredshiftqualities to the total numberof galaxiesin the
3 3 3 target catalog in both cases, considering a magnitude bin
wherew isthelocal-skycompleteness.(1+δ ) isthusamea- of ±0.25 mag centered on the magnitude and colors of the
p 3
sureof theoverdensityrelativeto the mediandensity,which galaxy.ThecolorselectionfunctionS (B- R,R- I,R)iscom-
c
takesintoaccountthevariationintheredshiftdependenceof puted by countinggalaxieswithin ± 0.25 R magnitudeover
the sampling rate. As discussed in Cooper et al. (2005), δ a B- R and R- I color range of ± 0.25 mag. Similarly, the
3
isshowntobearobustenvironmentmeasurefortheDEEP2 surfacebrightnessselectionfunctionisdefinedwithin±0.25
sample. mag in µ and ± 0.25 mag in R. The geometric selection
R
4 Linetal.
functionS (xy,R)issimilartothemagnitudeselectionfunc- latedasafunctionofthevelocitydispersionandthenumber
xy
tionbutcomputedonaspatially-defined(i.e.,localized)scale. of members of galaxy groups for a subset of DEEP2 sam-
We take theratio betweenthe numberofgalaxieswith good ples that have group identifications as presented in Gerke et
qualityredshiftsandthe totalnumberin thetargetedcatalog al. (2007). The two upper panels of Fig. 2 plot the over-
inanareaofradius120"withina±0.25R-magnituderange. density against velocity dispersion (left) and the number of
Besides the selection function for each individual galaxy, groupmembers(right)forpairedgalaxies.Asexpected,there
we also investigatethe selection dependenceon pair separa- isacleartrendthatthelocaldensityofgalaxiesbelongingto
tion. We measure the angular separation of all pairs in the groupswith greatervelocitydispersionor groupmembersis
redshiftcatalog(z-zpairs)andinthetargetcatalog(p-ppairs) onaveragelarger. Thisillustratesthatthe localenvironment
respectivelyandthencountthenumberofpairs(N andN ) measureδ usedinthisworkingeneralcorrelateswellwith
zz pp 3
withineachangularseparationbin. Whilecountingthepairs physicalenvironments(field, groups,clusters). As shownin
in the redshiftcatalog, each componentof the pair countsis the two bottom panels of Fig. 2, the fraction of red-redand
weighted by the geometric selection function S (xy) to ex- blue-red pairs in groups with greater velocity dispersion or
xy
cludeanyeffectduetothevarianceinthelocalsamplingrate. moregroupmembersissignificantlyhigherthanthatofblue-
TheangularselectionfunctionSθ iscomputedastheratiobe- bluepairs. Morespecifically,themajorityofblue-bluepairs
tweentheweightedNzz andNpp. Theangularweight,wθ,for are foundin field-like environmentswhile red-red and blue-
eachgalaxyishence1/Sθ. red pairs tend to be found in group and/or cluster-like envi-
ronments.
It is worth pointing out that there are ∼ 11% of the pair
3. RESULTS
samplewhosetwomembersdonotbelongtothesamegroup.
3.1. EnvironmentDistributionofBlue-Blue/Red-Red/Mixed Thiscanhappenwhenasinglegroupissplitintotwoormore
Pairs
smallergroupsbythegroupfinder,oragroupisnotproperly
Fig. 1(a) shows the projected positions of wet, dry, and identified owing to the incompleteness of the spectroscopic
mixedpairsfoundinoneoftheDEEP2fields(Field4),over- sample.Asaresult,therearesomepairsthatareidentifiedas
laid with contours tracing the mean density along the line- ’fieldgalaxy’basedonthegroupfinder. Thesearethepaired
of-sight. Visuallyitrevealsthatblue-bluepairsappearinall galaxiesassignedtohaveonegroupmemberasshowninthe
kindsofenvironments,fromlowtohighdensityregions. On lower-rightpanelof Fig. 2. Such an effectmakesthe group
theotherhand,red-redpairsandblue-redpairstendtoliein results rather hard to interpretcomparedto the local density
denserenvironments. Quantitativecomparisonsbetween the results, which are relatively insensitive to the spectroscopic
localenvironmentofthethreetypesofpairs,aftercorrecting incompleteness. We thereforefocuson thediscussionofthe
forthespectroscopicincompleteness,areshowninFig. 1(b). environmenteffectsbasedontheresultsusingthelocalden-
We first note that all blue-blue, red-red, and blue-red pairs sityintherestpartofthispaper.
have median local density greater than the average environ-
ment(log (1+δ ) ∼0). This is expected: paired galaxies, 3.2. TheEnvironmentdependenceofthePairFraction
10 3
by definition, have a close companion nearby and thus their The above analysis on the environment distributions of
separationfromthe3rd-nearestneighborwillbesmaller(and wet/dry/mixed merging galaxy pairs provides insight into
hencedenser)onaveragebyconstruction. Themostinterest- whichenvironmentsplayhosttomostgalaxymergers. How-
ingresultofFig. 1(b)liesinthedifferenceinthedensitydis- ever, this is different from asking in which environments
tributionamongblue-blue,red-red,andblue-redpairs. While galaxymergersare more likely to occur. In this section, we
blue-blue pairs favor median-density environments, red-red investigatethelatterissue bystudyingtherelativefrequency
andblue-redpairsare preferentiallylocatedin overdensere- of galaxy interactions across different environments, i.e., to
gions. count the paired galaxies relative to the parent sample as a
To better understand whether such density distribution of functionofenvironment. We notethatthepairfractiondoes
pairs is related to the color-density relation, i.e., the change notnecessarilycorrespondtothemergerfractionbecausenot
ofthefractionofredgalaxiesacrossdifferentlocaldensities every kinematic pair defined observationally will eventually
(Hogget al. 2004;Cooperet al. 2006;Cucciati et al. 2006), merge into one system. Such phenomenon is in particular
weperformthefollowinganalysis: fora givenlocaldensity, more frequent in dense environments due to chance projec-
wecomputethered-galaxyfractioncorrectedbythespectro- tionaswellasmany-bodyinteractions. We willaddressthis
scopicincompleteness,andthenderivethepredictedrelative issuein§3.3and§3.4.
fraction among wet, dry, and mixed pairs assuming that the Themethodwe adoptto computethe pair fraction,N , is
c
redandbluegalaxiesarerandomlydistributed. Asillustrated thesameasdescribedinourpreviouswork(see§3.1of Lin
in Fig. 1(c), the increased fractions of dry and mixed pairs etal. 2008),exceptthatwefurtherbinthesamplebythe lo-
withrespecttothelocaldensityfollowasimilartrendasex- cal densities. The pair fraction N is defined as the average
c
pected fromthe color-densityrelation. However, we find an numberofcompanionspergalaxy:
excess of dry and mixed pairs toward overdense regions at Ntot w w(θ)
a ∼ 2-σ levelcomparedto the aboveexpectation,indicating N = i=1 j j ij, (4)
that the red and blue galaxies are not uniformly distributed c P PNtot
andthatthereexistsaclusteringeffectatverysmallscalesin where N is the total numberof galaxieswithin the chosen
tot
thoseoverdenseenvironments. absolutemagnituderange,w isthespectroscopicweightfor
j
. the jth companionbelongingto the ith galaxy, and w(θ) is
ij
Thedifferenceinthedensitydistributionofwet/dry/mixed theangularselectionweightforeachpairasdescribedin§2.5.
mergerssuggeststhatthephysicalenvironmentwherevarious Theaveragedvalueof the overallspectroscopicweightw of
typesofmergersoccurisessentiallydifferent.Tointerpretour ourpairedgalaxiesisabout2.1andthatoftheangularselec-
results, we also investigate how different mergers are popu- tionweightisabout1.2intheredshiftrangeof0.75<z<1.2.
EnvironmentofWET,DRY,ANDMIXEDGALAXYMERGERS 5
FIG. 1.—(a)Thelocationsofblue-blue(bluedots),red-red(reddots),andblue-red(greendots)pairsintheDEEP2Field4,representingcandidatesofwet,
6drlye,vaenlsd:m<ix0e,d0m-e0rg.3e,rs0r.3es-pe0c.6ti,v0el.y6i-n0p.9a,rt0o.9ft-h1e.D2,EaEnPd2>sp1e.c2tr(ofsrocompilcigshatmtopldeaartk0)..7(5b)<Tzh<ed1i.s2tr.iTbuhteiognreoyfclooncatoludresnrseiptyr,es(e1n+tδth3e)o,vfoerrdpeanisrietdygloagla1x0(ie1s+wδ3e)igwhtietdh
bytheirspectroscopicincompletenessandangularselectionfunctions. Thepairedsampleisagaindividedintob-b(bluehistogram),r-r(redhistogram),and
b-r(greenhistogram)pairs. The(1+δ3) distributionsoftheblue/redgalaxiesofthefullsamplearealsoshownasblacksolid/dash-dottedlinesforcomparison
(thenumbersofblueandredgalaxieshavebeenreducedbyafactorof12and4respectively). Itisclearlyseenthatmixedanddrymergersoccurindenser
environmentsthanwetmergersdo.(c)Therelativefractionofb-b(bluesymbols),r-r(redsymbols),andb-r(greensymbols)pairsasafunctionof(1+δ3).The
errorbarsrepresentPoissonerrors.The1-σpredictionsbasedontheobservedcolor-densityrelationareshownasgreyareasforcomparison(lighttodark:wet,
dry,andmixedpairs).Ourcomparisonshowsthattheobservedrelativefractionofthethreetypesofpairscanbeexplainedbythechangeofred-galaxyfraction
acrossdifferentlocaldensities.Inoverdenseregions,however,thereexistsa2-σdifferencebetweentheobservedpairfractionsandthepredictionsfollowingthe
color-densityrelation,whichindicatesthattheclusteringofblueandredgalaxiesatsmallscalesmaybedifferentinthoseenvironments.
A correction factor of 2.4 in addition to the usual spectro- ThereforehowtheNb,Nr ,andNmvaryagainstenvironment
c c c
scopic and angularseparation correctionsis also applied for dependsontherelativeredandbluefractionatagivenenvi-
eachredcompanionatz>1toaccountforthemissingfaint ronment,asdiscussedin§3.1
red galaxies in the high-redshift DEEP2 sample (Lin et al. Fig. 3 displaysthe pair fraction as a functionof overden-
2008). Four types of pair fraction are measured here: a) N sity(1+δ ) inlogspacefortworedshiftbins0.75<z<1.0
c 3
fromallpairsregardlessof colors; b)theaveragenumberof and1.0<z<1.2. TherapidriseofN withincreasingden-
c
bluecompanionsperbluegalaxyNb ;c)theaveragenumber sity is similar for all four types of pairs for the two red-
c
ofredcompanionsperredgalaxyNr ;d)theaveragenumber shift bins considered. However, the relative companionrate
c
of companionsof galaxieswith oppositecolor to that of the amongwet/dry/mixedpairschangesacrossdifferentenviron-
primarygalaxiesNm.Notethatb)andc)areequivalenttothe ments. In underdense regions, the blue companion rate for
c
pairfractionwithinthebluecloudandredsequence,respec- blue galaxies (blue points) is in general higher than the red
tively.Herewedividetheenvironmentintothreeregimes:un- companion rate for red galaxies (red points). On the other
derdense environment, intermediate environment, and over- hand, the opposite holds in overdense environments. This
dense environment. Naively one would think that the pair suggeststhatwhiletheoverallcompanionrateisenhancedin
fraction should increase with the local density. In fact, this denseenvironments,thelevelofenhancementdependsonthe
is not necessarily true for the blue pair fraction (Nb ), red typesofgalaxies.Inordertovisualizetheenhancementofthe
c
pairfraction(Nr ),ormixedpairfraction(Nm )individually. companionrateasafunctionofenvironment,theglobalval-
c c
Onemustkeepinmindthattheoverdensityiscomputedusing uesofN atagivenredshiftobtainedbyLinetal.(2008)are
c
allgalaxiesofallcolorsandnotsegregatedbygalaxycolors. alsoshownashorizontalarrowsineachpanelofFig.3.Quan-
6 Linetal.
FIG.2.—Thetwoupperpanelsplottheoverdensityagainstvelocitydispersion(left)andthenumberofgroupmembers(right)forpairedgalaxies. Theblack
symbolsandassociatederrorbarsindicatethemedianvaluewiththeroot-mean-squaredofthescatterineachbin.Thetwolowerpanelsdisplaythehistograms
ofvelocitydispersionofgroupsandthenumberofgroupmembersforgroupsthathostpairedgalaxies.Blue,green,andredcolorsdenoteforblue-blue,red-red,
andblue-redpairsrespectively.
titatively speaking, the companion rate is about 1.3-5 times in a 100 h- 1 Mpc box on a side. The resulting mass of a
greater in high-density regions compared to the global rate, dark matter particle is mdm = 6.188 × 108h- 1M⊙. The dis-
dependingonthetypeofpair. tincthalosandsubstructures(subhalos)areidentifiedusinga
variantversionofHierarchicalFriends-of-FriendsAlgorithm
3.3. EstimatesofCmgandTmgasaFunctionoftheLocal (Klypinetal.1999,HFOF),withtheminimumparticlenum-
Environment berof30.Theclosehalo-halopairsareselectedinawaythat
As mentionedin §3.2, the pair fractionis notequal to the they satisfy the observational criteria in both projected sep-
galaxymergerfraction unless all pairs are mergingsystems. aration and in line-of-sight velocity difference to mimic the
Itispossiblethatthepairsampleissubjecttocontamination observedpairs. Thehalosinpairscanbeeitherdistincthalos
from interlopers owing to the difficulty of disentangling the (nosmallersubstructurescontainedornothostedbyalarger
Hubbleexpansionand the galaxypeculiarvelocity. In order halo) or subhalos. For each halo, we compute its local den-
to take into account any possible environment effect on the sity usingtheseparationfromitsnearestnth neighborthatis
merger time-scale (Tmg) and the fraction of merger in pairs abovecertainmasscutMmin. BothnandMminaredetermined
(C ),weconstructamockgalaxycatalogbasedonthedark soastomatchthemediandistancetothe3rd-nearestneighbor
mg
matterhalosandsubhalostakenfromacosmologicalN-body intheDEEP2sample. Empirically,wefoundthatn=6inthe
simulation, and trace merger histories of halo-halo pairs to simulationstraces the same comovingscale as n=3 doesin
examinethedependenceofC andT onthelocaldensity. the observed data set. We note that the overallDEEP2 red-
mg mg
A full description will be presented in a forthcoming paper shiftcompletenessis∼50%,whichmeansthattheobserved
(Jian, H.-Y. et al., in preparation). Here we briefly describe 3rd-nearestneighborroughlycorrespondstothe‘true’6th-or
thetechniquesthatareusedtostudythisproblemandpresent 7th-nearest neighbor. Therefore, our choice of n= 6 in the
the most relevantresults. We make use of the cosmological simulations should be a reasonable approach. In the rest of
N-bodysimulationsandthedarkmatterhalosaspresentedin thispaper,wereferδn totheoverdensitymeasuredusingthe
Jianetal.(2008). Thesimulationusedherehasbeenevolved 6th nearest neighbor for halos. The resulting (1+δn) distri-
in the concordanceflat ΛCDM model: Ωm = 0.3, ΩΛ = 0.7, bution of halosis very similar to the (1+δ3) distribution of
and Ω = 0.05. It contains 5123 pure dark matter particles
b
EnvironmentofWET,DRY,ANDMIXEDGALAXYMERGERS 7
0000000.......5555555
lllllllllllllloooooooooooooowwwwwwwwwwwwww mmmmmmmmmmmmmmeeeeeeeeeeeeeedddddddddddddd hhhhhhhhhhhhhhiiiiiiiiiiiiiigggggggggggggghhhhhhhhhhhhhh lllllllllllllloooooooooooooowwwwwwwwwwwwww mmmmmmmmmmmmmmeeeeeeeeeeeeeedddddddddddddd hhhhhhhhhhhhhhiiiiiiiiiiiiiigggggggggggggghhhhhhhhhhhhhh
0000000.......4444444
00000000000000..............7777777777777755555555555555 <<<<<<<<<<<<<< zzzzzzzzzzzzzz <<<<<<<<<<<<<< 11111111111111..............00000000000000 ((((((((((((((aaaaaaaaaaaaaa)))))))))))))) 11111111111111..............00000000000000 <<<<<<<<<<<<<< zzzzzzzzzzzzzz <<<<<<<<<<<<<< 11111111111111..............22222222222222 ((((((((((((((bbbbbbbbbbbbbb))))))))))))))
all
0000000.......3333333
wet
ccccccc
NNNNNNN
0000000.......2222222 dry
mix
0000000.......1111111
0000000
-------1111111.......5555555 -------1111111 -------0000000.......5555555 0000000 0000000.......5555555 1111111 1111111.......5555555 2222222 -------1111111.......5555555 -------1111111 -------0000000.......5555555 0000000 0000000.......5555555 1111111 1111111.......5555555 2222222 2222222.......5555555
llllllloooooooggggggg (((((((1111111 +++++++ ddddddd ))))))) llllllloooooooggggggg (((((((1111111 +++++++ ddddddd )))))))
11111110000000 3333333 11111110000000 3333333
FIG.3.—Thepairfractionasafunctionofoverdensity(1+δ3).Hereweshowtheresultsforfourtypesofpairs:blue-bluepairs(bluetriangles),red-redpairs
(redsolidcircles),blue-redpairs(greensolidsquares),andallpairsregardlesstheircolors(blackopensquares).Thedenominatorsusedforcomputingtheabove
fourquantitiesarethenumbersofblue,red,blue+red,andblue+redgalaxiesrespectively. Theerrorbarsshownintheplotarecalculatedbybootstrapping. We
setNc=0andputtheerrorstobethePoissonerrorsfor5objectswhennopairisfoundatagivenenvironment.Thehorizontalarrowsappearedintheright-lower
cornerofeachpanelindicatetheglobalpairfractioninthesameredshiftrangetakenfromLinetal. (2008)computedwithoutseparatingdataintodifferent
environmentbins.Thereexistsclearenvironmentdependenceofthepairfractionintheconsideredtworedshiftbins,beinghigheratdenserenvironments.
sity, C is a strong function of (1+δ ) , being smaller in
mg n
higher density regions. This suggests that differentenviron-
0.9
Simulation (n = 6) at z = 1.0 mentshaveastrongimpactondeterminingwhethertheclose
0.8 DEEP2 (n = 3) at zavg = 0.99 pairswillmergeornot,buthavelittleinfluenceonthemerger
time-scale if those pairs are going to merge. When we an-
0.7
alyze those halo-halo pairs that do not merge, we find that
y 0.6 themajorityofthesepairsareactuallywidelyseparatedin3-
t
li 0.5 Dspaceandhavelargedifferencesin3-Dvelocity,andsuch
i
b projectioneffectsaremorepronouncedforpairsinoverdense
ba 0.4
o environments. Intheremainderofthecases, onecomponent
r
P 0.3 ofthehalopairsmaybetidallystrippedandfailstobeiden-
tifiedasahaloiftheydonotsatisfythevirialconditioninthe
0.2
next snapshot (Jian et al. 2008). In such cases, if the most-
0.1
bounded particles do not belong to any halo, we then stop
0 tracingtheir historiesand countthem as non-mergingcases.
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
log10(1+d n) We caution that this might underestimateCmg in a way that
the galaxy component may still survive temporarily until it
FIG. 4.—Theoverdensity(1+δn)distributions ofthemockgalaxycat- mergeswith its companions,even thoughthe dark matter of
alog(solidcurve)andoftheDEEP2sample(dashedcurve)atz∼1. The itshostinghalosisstripped.However,ifthisistrue,theywill
projecteddistancefromthe3rd-nearestneighborisadoptedtocomputethe
contributetothetailoftheT distributionandhenceshiftT
overdensityfortheDEEP2sample;thatofthe6th-nearestneighborisusedfor mg mg
towardahighervalue.Becausethemergerrateisproportional
overdensitymeasurementsinthemockgalaxycatalog.Thedifferentchoices
ofnth-nearestneighborsbetweentwosamplesareduetothespectroscopic toCmgandinverselyproportionaltoTmg,suchaneffectwillbe
incompleteness oftheDEEP2survey. Empiricallywefoundthattheadop- roughlycanceledout. Tomodeltheenvironmentdependence
tionofn=6inthesimulatedgalaxycatalogbestreproducestheoverdensity ofC ,wefitthecurvesinpanel(a)ofFig. 5bytwolines:
(1+δ3) distributionfunctionoftheobservedDEEP2galaxies. mg
(0.01z- 0.08)x- (0.16z- 0.85), ifx<0
Cmg=(cid:26)(0.22z- 0.45)x- (0.16z- 0.85), ifx≥0, (5)
observedDEEP2galaxies,asdemonstratedinFig. 4.
Foreachhalo-halopairidentifiedwiththecriteriarp ≤50 wherex=log10(1+δn).
h- 1kpcand|∆v|≤500kms- 1 ,wetracetheirmost-bounded Inthesimulations, thevalueofTmg forpairswith rp < 50
10particlesidentifiedatagivenepochinthenextadjacentfew h- 1kpcisapproximately1Gyratz∼1,whichisalmosttwice
redshift frames. If 60% of these most-bounded10 particles thetypicalvalueof∼0.5Gyradoptedinpreviousstudies(Lin
fromthetwopaircomponentscanbefoundinasinglehaloin etal.2004,2008)thatusedamorestringentcriteriumrp<30
the sequentialframe, the pairs are then called to be merged. h- 1kpc. Thelongertime-scaleforsuchwiderpairshasalso
C isthuscomputedasthefractionofpairsthatwillmerge been suggested in earlier works by Lotz et al. (2008b) who
mg
intoasinglehalo. Amongthosemergedhalos,werecordthe studiedthe mergertime-scaleusingN-body/hydrodynamical
time-scaleT overwhichthehalo-halopairsmerge. simulations. We notice that T increases slightly when go-
mg mg
Fig.5displaysC andT asafunctionoflocaldensity.It ing to lower redshifts. Because the simulations were stored
mg mg
is interesting that while T varies little with the local den- atdiscreteepochs,thevalueofT canonlybeestimatedby
mg mg
8 Linetal.
(a) (b)
1 z = 1.15 z = 0.91 z = 1.12
fitting fitting z = 0.91
z = 0.70
0.8 z = 0.50
g 0.6
m
C
0.4
0.2
yr]109
0 [g
z = 0.70 z = 0.50 m
1 T
fitting fitting
0.8
g 0.6
m
C
0.4
0.2
0 108
-1 0 1 2 -1 0 1 2 -1.5 -1 -0.5 0 0.5 1 1.5 2
log (1+d ) log (1+d ) log (1+d )
10 n 10 n 10 n
FIG.5.—(a)Cmg(fractionofkinematicpairstobemerged)asafunctionofoverdensity(1+δn),determinedusingthemockgalaxycatalogconstructedfrom
theN-bodysimulations. Thedashedlinesrepresentthebest-fittingformulaofthedatapoints. Overtheentireredshiftrangewehaveprobed,Cmg isastrong
functionoflocalenvironment. Thisislargelyduetothestrongerprojectioneffectsinoverdenseregionsthanthatinunderdenseregions. Thefittingformulaof
CmgisgiveninEq. 5. (b)Tmg(themergingtime-scale)asafunctionofoverdensity(1+δn)forthosepairsthatwilleventuallymerge. Theerrorbarsrepresent
theroot-mean-squaredofthescatterineachbin.
summingthetimeintervalofseveraladjacentframesuntilthe Eq. (10) and (11) in Kitzbichler & White 2008). In their
last frame in which the halos are identified as merged. In analysis, every pair will eventually merger; in other words,
this way, T is likely to be overestimated. However, since the effect of C is absorbed into the quantity of T (i.e.,
mg mg mg
the time interval is typically ∼ 200 Myr at z∼1, which is equivalenttosetC =1). IfadoptingtheirEq. (10)withh=
mg
mpauircshwsmithalrlper<th5a0nht-h1ektpycp(iLcaoltztimetea-ls.c2a0le0s8nbo),rwmealblyelfioevuendsufcohr 0w.h7i,crhpi<sth5e0thy-p1ikcpalcsatenldlatrhmeastseslelsarinmoausrspMa∗ir∼sam3×ple1,0w10eMg⊙et,
uncertainty is negligible. Such an effect, on the other hand, T ∼ 2.7 Gyr. The ratio of C /T is thus 0.37, which is
mg mg mg
becomesmoreapparentatlowerredshiftasthetimeinterval about half of our value of 0.7. This leads to a potential un-
betweentwoadjacentredshiftframesofthesimulationsrises certaintybyaslargeasafactoroftwointheestimatesofthe
to400Myratz∼0.4.Thismightexplainthetrendofincreas- fractionalmergerrate,dependingontheadoptedmodelingof
ingTmg withredshift. In thiswork, we adoptTmg=1 Gyrin CmgandTmg.
allenvironmentandC derivedwithEq. 5whenconverting
mg
thepairfractionintothefractionalmergerrateaspresentedin
thenextsection. 3.4. TheFractionalMergerRate fmgasaFunctionof
Environment
Beforeproceedingtocomputethemergerrateinferredfrom
the pair fraction, it is worth discussing how our derived T In this section, we present our results on the fractional
mg
andC arecomparedtopreviousworksbyothergroups,in mergerrate,definedasthefractionofgalaxiesintherangeof
ordermtog assess possiblesystematic errorsin ourestimatesin - 21<Me <- 19thatmergeperGyrwithanothergalaxysuch
B
the fractional merger rate. As we will see in §3.4, the frac- that the luminosity ratio of the pair is between 4:1 and 1:4.
tionalmergerrateisproportionaltoC /T ,hereweusethe Thisquantitycanbederivedfromthepairfractioncomputed
mg mg
ratioCmg/Tmgasacomparisonquantity.Inourcase,thetypical in§3.2withtheknowledgeofCmg andTmg wehaveobtained
valueofC isapproximately0.7andT is∼1Gyraveraged in §3.3, but keeping in mind that the pair fraction in §3.2 is
mg mg
in all kind of environments,leading toC /T = 0.7. A re- computedusing pairs drawnfrom within a luminosityrange
mg mg
centstudybyKitzbichler&White(2008)usetheMillennium of two magnitudes. Some true companionsmay fall outside
Simulation (Springel et al. 2005) to determine the averaged the absolutemagnituderangeofour sample, while some se-
merger time-scale T as a function of stellar mass and red- lectedcompanionshaveluminosityratiosoutsidetherangeof
mg
shiftofclosepairsselectedwithvariousselectioncriteria(see 4:1to1:4.Toaccountforbothoftheseeffects,weusethefol-
lowingequationtoconvertthepairfractionintothefractional
EnvironmentofWET,DRY,ANDMIXEDGALAXYMERGERS 9
mergerrate f : N-body simulations (Fakhouri & Ma 2009; Hester & Tasit-
mg
siomi 2009) or based on the Monte-Carlo merger trees that
fmg=(1+G)×CmgNc(z)Tm-g1, (6) areconstructedwiththeextendedPress-Schechter(EPS)and
excursionset models (Kauffmann& Haehnelt2000). Using
whereGisthecorrectionfactorthataccountsfortheselection
theMillenniumsimulation(Springeletal.2005),Fakhouri&
effect of companions due to the restricted luminosity range
Ma (2009)measuredthe mergerrate of darkmatterhalosas
(seeLinetal. 2008forthedetailedcomputationofG). Itis
afunctionofthelocalmassdensitywithinasphereofseveral
worthnotingthatthefactor(1+G)inEq. 6isdifferentfrom
Mpcusingafriends-of-friends(FOF)algorithm. Theyfound
(0.5+G)thatisshownintheEq.(5)ofLinetal.(2008)dueto
a strongdependenceofspecific halo mergerratesonthe en-
differentdefinitionsbetweenthemergerrate(Linetal.2008)
vironment,beinggreaterinthedensestregionsthaninvoids
andthefractionalmergerrateadoptedinthiswork.
byafactorof∼2.5. Thelevelofenhancementofthespecific
InFig. 6,weshowthefractionmergerrate, f ,asafunc-
mg halomergerratesindenseregionsisinbroadagreementwith
tion of overdensity for wet (blue points), dry (red points),
whatwemeasureforobservedgalaxies. Veryrecentworkby
mixed (green points), and all (black points) mergers. Those
Hester & Tasitsiomi (2009) has also explored similar issues
valuesarealsolistedinTable1. OwingtothedecreasingC
mg butfor subhalosextractedfromthe Millenniumsimulations.
withoverdensity,theincreaseof f withrespecttotheover-
mg Incontrast,theyfindthatingroupenvironments,thesubhalos
densityisnotassteepasN . However,westillfindthatthe
c are often tidally stripped and hence the chance of subhalo-
fractionalmergerrateintheoverdenseregionsis,onaverage,
subhalomergersis low. As a consequence,the specific halo
3-4 times greater than that in the underdense regions for all
mergerrateingroupsisnormallysuppressed,whichseemsto
mergersregardlessoftheirtypes,shownasblacksymbolsin
be in contradiction to our finding that the fractional merger
Fig. 6 (also see Table 1). Such enhancementin dense envi-
rateisenhancedinoverdenseenvironments.
ronmentsis inbroadagreementwith recenttheoreticalwork
However,we cautionthat directcomparisonsbetweenour
byFakhouri&Ma(2009)whomeasuredthemergerratesof
results and simulations could be limited by several factors.
friends-of-friends(FOF) identified mock matter halos in the
Forexample,thestudiesbyFakhouri&Ma(2009)utilizethe
Millenniumsimulation(Springeletal.2005)asafunctionof
merger trees constructed from distinct FOF halos which do
local mass density. When dividing the merger sample into
notcorrespondaswelltoobservedgalaxiesasdothesubhalos
subcategories(wet,dry,andmixedmergers)wefindasignif-
(substructures). Thereforetranslatingtheirsimulationresults
icantenhancementof the frequencyof dryand mixedmerg-
of halo merger rates into the actual merger rate of galaxies
ersbetweenunder-andoverdenseenvironments. Incontrast,
is not straightforward. On the other hand, the halo environ-
there is only a weak environmentdependence between den-
mentadopted in Hester & Tasitsiomi (2009)is based on the
sityextremesforwetmergers(Fig. 6). Thisimpliesthatthe
size/mass of the groups, characterized by the maximum of
group-likeandcluster-likeenvironmentarepreferredenviron-
theirrotationvelocitycurve,V . AsshowninFig. 2,there
mentsfordryandmixedmergerstotakeplace. max
isspreadin(1+δ ) foragivennumberofgroupmembers,and
Atz∼1,thefractionaldrymergerrateinhigh-densityre- 3
viceversa, despitethatin generalthe localdensityincreases
gionsisfoundtobe16±4%(Table1),whichisabout3times
withtheglobalenvironment(velocitydispersion,thenumber
largerthantheglobalfractionaldrymergerratederivedfrom
of group members, etc.). This suggests that at a given high
earlier studies (Lin et al. 2008; Bundy et al. 2009) regard-
localdensityinoursample,therearecontributionsfromboth
lessoftheirenvironments.Anenhancementofdrymergersin
the’dense’field-likeenvironments,aswellasgroup-likeand
overdenseenvironmentsisalsoobservedinthelocaluniverse.
possiblyevencluster-likeenvironments. Thereforemoread-
UsingdatadrawnfromtheSloanDigitalSkySurvey(SDSS),
equate comparisonsshall await largersurveyswhich sample
McIntoshetal.(2008)findthatthefrequencyofmergersbe-
mergersinvariousscalesofgalaxygroupsandclusters.
tweenluminousredgalaxies(LRGs) ishigheringroupsand
clusters compared to that of overall population of LRGs by 4.2. AreK+AGalaxiesFormedThroughMajorMergers?
a factor of 2- 9 at z< 0.12. Our work similarly suggests
There have been several mechanisms proposed to quench
thata greaterprobabilityfordrymergersin highdensityen-
the star formation in galaxies and lead to the formation of
vironmentswas already in place by at least z∼1. If we as-
K+A galaxies. These mechanisms include galaxy-galaxy
sume an average stellar mass ratio of 1:2 in our dry merger
mergers (Mihos & Hernquist 1994), ram-pressure stripping
sample, a constant fractional dry merger rate in dense en-
(Gunn & Gott 1972), high speed galaxy encounters (galaxy
vironments at 0< z<1, and that all stellar mass involved
harassment;Mooreetal.1996),and’strangulations’inwhich
in each merger is deposited into the final merger remnant,
thewarmandhotgasisremoved(Larsonetal.1980;Balogh
then we estimate that on average every local massive red-
et al. 2000). Except for galaxy mergers, many of those are
sequencegalaxyinadenseenvironmentisassembledthrough
stronglyassociatedwiththeclusterenvironment. Severalen-
0.16±0.04(merger/Gyr)×7.7(Gyrs)∼1.2±0.3 majordry
mergers,leadingto∼(38±10)%(=1.2×0.5/(1+1.2×0.5)) vironmentstudiesofpoststarburstshavefoundahigherfrac-
tion of K+A galaxies in clusters than in the field (e.g. Tran
massaccretionsincez∼1.
et al. 2003, 2004; Poggianti et al. 1999). In contrast, other
studies using large low-redshiftsampleshave suggestedthat
4. DISCUSSION
poststarbursts are preferentiallyfound in the low density re-
4.1. ComparisonoftheEnvironmentDependenceofMerger
gion(Baloghetal.2005;Goto2005;Hoggetal.2006). Re-
RatesBetweenObservationsandSimulations
cently,Yanetal.(2009)studiedtheenvironmentdistribution
In this subsection, we discuss how our results are com- of 74 K+A galaxies found at z∼0.8 in the DEEP2 redshift
paredto previoustheoreticalpredictionson the environment survey.Theyfoundthatatthisredshiftrange,thereisverylit-
dependence of the merger rate of dark matter halos. There tleenvironmentdependenceoftheK+Afraction. Puttingall
havebeenseveralattemptstoinvestigatetherelationbetween these resultstogether, it is suggestedthatthe galaxymerger,
halo merger rates and underlying environments either using which is not a cluster-specific mechanism, is potentially an
10 Linetal.
0000000.......33333335555555
0000000.......3333333 lllllllllllllloooooooooooooowwwwwwwwwwwwww mmmmmmmmmmmmmmeeeeeeeeeeeeeedddddddddddddd hhhhhhhhhhhhhhiiiiiiiiiiiiiigggggggggggggghhhhhhhhhhhhhh lllllllllllllloooooooooooooowwwwwwwwwwwwww mmmmmmmmmmmmmmeeeeeeeeeeeeeedddddddddddddd hhhhhhhhhhhhhhiiiiiiiiiiiiiigggggggggggggghhhhhhhhhhhhhh
0000000.......22222225555555 00000000000000..............7777777777777755555555555555 <<<<<<<<<<<<<< zzzzzzzzzzzzzz <<<<<<<<<<<<<< 11111111111111..............00000000000000 ((((((((((((((aaaaaaaaaaaaaa)))))))))))))) 11111111111111..............00000000000000 <<<<<<<<<<<<<< zzzzzzzzzzzzzz <<<<<<<<<<<<<< 11111111111111..............22222222222222 ((((((((((((((bbbbbbbbbbbbbb))))))))))))))
all
)))))))
1111111
-------yryryryryryryr 0000000.......2222222 wet
GGGGGGG
( ( ( ( ( ( (ggggggg 0000000.......11111115555555 dry
mmmmmmm
fffffff 0000000.......1111111 mix
0000000.......00000005555555
0000000
-------1111111.......5555555 -------1111111 -------0000000.......5555555 0000000 0000000.......5555555 1111111 1111111.......5555555 2222222 -------1111111.......5555555 -------1111111 -------0000000.......5555555 0000000 0000000.......5555555 1111111 1111111.......5555555 2222222 2222222.......5555555
llllllloooooooggggggg (((((((1111111 +++++++ ddddddd ))))))) llllllloooooooggggggg (((((((1111111 +++++++ ddddddd )))))))
11111110000000 3333333 11111110000000 3333333
FIG.6.—Thefractionalmergerrate, fmg,asafunctionofoverdensity(1+δ3).Hereweshowtheresultsforfourtypesofmergers:wetmergers(bluetriangles),
drymergers(redsolidcircles),mixedmergers(greensolidsquares),andallpairsregardlessoftheircolors(blackopensquares).Theabovefourquantitieshave
beennormalizedtothenumbersofblue,red,blue+red,andblue+redgalaxiesrespectively. Theerrorbarsarecalculatedbybootstrapping. Weset fmg=0and
puttheerrorstobethePoissonerrorsfor5objectswhennopairisfoundatagivenenvironment. Whilethefractionalwetmergerrateshowsweakdependence
onlocaldensity,thefractionalrateofdryandmixedmergersstronglydependsontheoverdensityintheredshiftrangeprobed.
TABLE1. THEFRACTIONALMERGERRATE(fmg)ASAFUNCTIONOFDIFFERENTEN-
VIRONMENT
MergerTypes z Na Nb Nc G Ca Cb Cc Ta fa fb fc
c c c mg mg mg mg mg mg mg
(Gyr) (Gyr- 1) (Gyr- 1) (Gyr- 1)
All 0.88 0.025±0.006 0.079±0.010 0.233±0.026 1.04 0.76 0.68 0.39 1.0 0.039±0.009 0.109±0.014 0.187±0.021
1.08 0.058±0.012 0.084±0.015 0.032±0.054 1.10 0.73 0.65 0.42 1.0 0.088±0.018 0.115±0.020 0.281±0.047
Wet 0.88 0.024±0.007 0.051±0.009 0.090±0.018 1.19 0.76 0.68 0.39 1.0 0.039±0.011 0.076±0.014 0.077±0.016
1.08 0.057±0.012 0.073±0.014 0.157±0.031 1.27 0.73 0.65 0.42 1.0 0.094±0.019 0.109±0.021 0.149±0.030
Dry 0.88 0.009±0.005 0.081±0.024 0.212±0.047 0.40 0.76 0.68 0.39 1.0 0.010±0.005 0.077±0.022 0.116±0.026
1.08 0.000±0.092 0.043±0.023 0.347±0.129 0.38 0.73 0.65 0.42 1.0 0.000±0.093 0.039±0.021 0.199±0.074
Mixed 0.88 0.004±0.002 0.021±0.005 0.108±0.018 0.71 0.76 0.68 0.39 1.0 0.005±0.003 0.025±0.005 0.072±0.012
1.08 0.007±0.004 0.015±0.008 0.128±0.038 0.72 0.73 0.65 0.42 1.0 0.009±0.005 0.017±0.008 0.092±0.027
alowdensity(-1.0<log10(1+δ3) <-0.2)
bchmiegdhiadnendseitnysi(t0y.5(-<0.2lo<g1l0o(g11+0(δ13+)δ<3)2.<0)0.5)
importantoriginofK+Agalaxiesfoundinthefield.Oneway parametricstatisticaltests,theKolmogorov-Smirnovtest(K-
totestthishypothesisistocomparetheenvironmentdistribu- Stest),theAnderson-Darlingtest(A-Dtest;Anderson&Dar-
tionsofK+Asamplestothatofthemergersamples. ling1954;Pettitt1976;Sinclair&Spurr1988),andtheMann-
As shown in Fig. 6, the fractional wet merger rate in the Whitney-Wilcoxontest(MWWtest;Mann&Whitney1947)
DEEP2 sample depends weakly on the local density, simi- asdoneinYanetal.(2009).TheresultsarepresentedinTable
larly to the trend seen in DEEP2 K+A galaxies (see Fig. 6 2. We findthatthep-valuesderivedfromtheK-S,A-D,and
of Yan et al. 2009). On the other hand, the mixed merg- MWWtestsfortheK+Asampleagainstdryandmixedmerg-
ersshowstrongerdependenceontheenvironment,unlikethe ersaloneareallveryclosetotherejectionthreshold0.05.On
K+A galaxies. In order to make more careful comparisons, theotherhand,thep-valuesforthewetvs. K+Asetandthe
we limit the K+A sample selected by Yan et al. (2009)with wet+mixedvs. K+A set are wellbeyondthe threshold0.05.
anadditionalrestframemagnitudecut-21.77<Me <-19.77, Basedontheseresults,weconcludethattheenvironmentdis-
B
whichis 2 × brighterthan the pair sample, to reflectthe as- tributions of K+A galaxies and of wet or wet+mixed merg-
sumptionthattheK+Agalaxiesareproductsoftwomerging ersareindistinguishable. Nevertheless,we cautionthatsuch
galaxies. Wealsoapplytheredshfitcutof0.75<z<0.88to analysisispossiblylimitedbythesmallnumbersofK+Aand
ourpair samplesso theyspan the same redshiftrangeas the pairsamples.
K+Agalaxies. InFig. 7, weplotthe(1+δ ) distributionof Theidea thatK+A galaxiescouldbe formedthroughgas-
3
K+Agalaxiesagainstwet,dry,mixed,andwet+mixedpairs. richmergershasalsobeentestedusingsimulationsbyBekki
Among the four types of pair samples, the density distribu- etal. (2005),whoshowed thatthe propertiesofK+A galax-
tions of dry pairs and mixed pairs are distinct from that of iescouldbereproducedbymergingtwogas-richsystems,al-
theK+Asample,whereaswetpairsorwet+mixedpairsshow though the details depend strongly upon the orbital config-
similaradensitydistributiontotheK+Agalaxies.Toquantify uration. In addition, based on the kinematic study of K+A
the significance of the environment difference or similarity withintegralfieldunit(IFU)spectroscopyfor10nearbyK+A
between K+A samples and mergers, we perform three non- galaxies, Pracy et al. (2009)found that the majority of their