The close pair fraction of BCGs since $z=0.5$: major mergers dominate recent BCG stellar mass growth

8.9 MB·

by  Danièl N. Groenewald

MNRAS000,1–18(2017) Preprint1February2017 CompiledusingMNRASLATEXstylefilev3.0
The close pair fraction of BCGs since z = 0.5: major mergers
dominate recent BCG stellar mass growth
Danièl N. Groenewald1,2(cid:63), Rosalind E. Skelton1, David G. Gilbank1
and S. Ilani Loubser2
1SouthAfricanAstronomicalObservatory,ObservatoryRoad,7925,CapeTown,SouthAfrica
2North-WestUniversity,Potchefstroom,2520,SouthAfrica
7
1
0 AcceptedXXX.ReceivedYYY;inoriginalformZZZ
2

n
ABSTRACT
a
UsingtheredMaPPerclustercataloguebasedontheSloanDigitalSkySurvey(SDSS)pho-
J
tometry,weinvestigatetheimportanceofmajormergersinthestellarmassbuild-upofbright-
1
estclustergalaxies(BCGs)between0.08(cid:54)z(cid:54)0.50.WeusetheSDSSspectroscopy,supple-
3
mentedwithspectroscopicobservationsfromtheSouthernAfricanLargeTelescopeathigher

] redshifts,toidentifywhichBCGsandnearbycompanionsarepotentialmajormergercandi-
A dates.Weusethepairfractionasaproxyforthemergerfractioninordertodeterminehow
G much stellar mass growth the BCGs have experienced due to major mergers. We observe a
weaktrendoftheBCGpairfractionincreasingwithdecreasingredshift,suggestingthatma-
.
h jormergersmaybecomemoreimportanttowardsthepresentday.Majormergersarefoundto
p contribute, on average, 24±14 (29±17) per cent towards the stellar mass of a present day
- BCGsincez=0.32(0.45),assumingthathalfofthecompanion’sstellarmassisaccretedonto
o
theBCG.Furthermore,usingourmergerresultsinconjunctionwithpredictionsfromtwore-
r
t cent semi-analytical models along with observational measurements from the literature, we
s
a findthatmajormergershavesufficientstellarmaterialtoaccountforthestellarmassgrowth
[ oftheintraclusterlightbetweenz=0.3andz=0.

1 Keywords: galaxies:clusters:general–galaxies:clusters:intraclustermedium–galaxies:
v ellipticalandlenticular,cD–galaxies:evolution–galaxies:interactions.
2
1
0
9
0 1 INTRODUCTION their host clusters, these massive galaxies are expected to expe-
1. rience multiple mergers during their lifetime, making them ideal
Brightestclustergalaxies(BCGs)arethemostmassiveandlumi-
0 probes with which to study galaxy formation. Observational evi-
nousgalaxiesintheUniverse.Thesegalaxiesaretypicallyfound
7 dence,forexampletidaltailsanddistortedisophotes,indicatesthat
closeto,orat,thecentresoftheirhostclusters(Jones&Forman
1 some BCGs are experiencing mergers (e.g. Bernardi et al. 2007;
1984,1999;Beers&Tonry1986;Rhee&Latour1991).Inthehier-
: Laueretal.2007;vonderLindenetal.2007;McIntoshetal.2008;
v archicalformationscenarioBCGsarethoughttoformthroughthe
Liuetal.2009,2015;Rasmussenetal.2010;Broughetal.2011),
Xi accretionofsmallergalaxies(e.g.Ostriker&Hausman1977;Rich- butitisnotcleartowhatextenttheirstellarmassisbeingbuiltup
stone&Malumuth1983).Thesemassivegalaxiesarealsoknown
r bymergersandatwhatrate.
a tohavepropertiesthatareverydifferentfromthatofotherearly-
type galaxies (ETGs). In comparison to other ETGs of the same Thereiscurrentlynoconsensusintheliteratureregardingthe
mass,BCGsarefoundtobelarger(Bernardietal.2007,andref- stellar mass evolution of BCGs between z=1 and the present
erencestherein)andhaveextendedlightprofiles(Matthewsetal. day.Observationalstudieshavefoundthatthesemassivegalaxies
1964;Tonry1987;Schombert1988;Gonzalezetal.2000,2003). changedtheirmassbyfactorsthatrangebetweenone(equivalentto
Theseuniquepropertieshavebeenattributedtotheirspecialloca- nogrowth;e.g.Aragon-Salamancaetal.1998;Whileyetal.2008;
tionattheclustercentres(Hausman&Ostriker1978).Theseprop- Collinsetal.2009;Stottetal.2010;Oliva-Altamiranoetal.2014)
erties may also suggest that the formation of BCGs are different and 1.4±0.2 (Lin et al. 2013) or 1.8±0.3 (Lidman et al. 2012)
fromthatofotherETGs(e.g.Burkeetal.2000;Stottetal.2008, sincez(cid:46)1.
2010). Predictions from numerical simulations and semi-analytical
Due to the privileged positions of BCGs at the centres of models(SAMs)suggestthatBCGsformintwo‘phases’.Theysug-
gestthatstarformationdominatestheBCG’smassgrowthatz(cid:62)2,
whilemultipledrymergersofsmallergalaxiesdominatethemass
(cid:63) E-mail:[email protected] assemblyatz(cid:54)1(e.g.DeLucia&Blaizot2007;Naabetal.2009;
(cid:13)c 2017TheAuthors
2 D.N.Groenewaldetal.
Laporte et al. 2013). The simulations, however, differ in the stel- 2 DATA
larmassgrowththeypredictforBCGs.Forexample,DeLucia&
2.1 OverviewoftheredMaPPerclustercatalogue
Blaizot(2007)findthatBCGshavechangedtheirmassbyafac-
torof4fromz=1−0,mainlythroughminor1 mergers.Laporte Throughout this paper we use version 5.2 of the red-sequence
et al. (2013) on the other hand predict a BCG mass growth fac- Matched-filterProbabilisticPercolationclustercatalogue(redMaP-
tor closer to 2 over the same redshift range, however this model Per3; Rykoff et al. 2014). This catalogue covers roughly 10 000
findsthatbothmajor2 andminormergerscontributesignificantly deg2 of the sky and consists of more than 25000 galaxy clusters
towardsthemassgrowthofthesemassivegalaxies. which span a redshift range of 0.08(cid:54)z(cid:54)0.55. The clusters in
The discrepancies between the BCGs’ observed stellar mass thiscataloguehavebeenopticallyidentifiedusingphotometryfrom
growthandthatpredictedbysimulationsarequiteapparent.This the Eighth Data Release (DR8; Aihara et al. 2011) of the SDSS.
maybebecausemergersdonotcontributesignificantlytothestel- Wherepossible,wesupplementthecataloguewithadditionalspec-
larmassgrowthoftheBCGsifasignificantfractionofthemerging troscopic redshifts from the SDSS DR12 (Alam et al. 2015). A
massendsupinthecluster’sintraclusterlight(ICL).TheICLcan detaileddescriptionoftheredMaPPerclustercatalogue(andcon-
bedescribedasstarsthatarenotgravitationallyboundtoanysin- struction)canbefoundinRykoffetal.(2014).Here,wesummarize
glegalaxy,butrathertotheclusterpotential.Althoughtheorigin themostimportantfeatures.
oftheICLisnotknown,itisthoughtthattidalstrippingofsatellite Briefly,theredMaPPerclusterdetectionalgorithmphotomet-
galaxies and merger events (involving BCGs) contribute towards rically identifies clusters by searching for overdensities of red-
thestellarmassgrowthoftheICL.TheICLisoftenfoundtobe sequence galaxies. This relies on a set of galaxies with spectro-
more concentrated around the central galaxy in a cluster (Mihos scopicredshiftsthatareusedtoconstructaredshiftdependentred-
etal.2005;Rudicketal.2011).Thisinturnimpliesthatthefor- sequence model. The spectroscopic redshifts needed for the cal-
mationandevolutionofBCGsandtheICLareconnectedtoone ibration of the red-sequence model are retrieved from the SDSS
another. Main Galaxy Sample (MGS; Strauss et al. 2002), Luminous Red
TherecentsimulationofLaporteetal.(2013)predictsthat30 Galaxy(LRG;Eisensteinetal.2001),andBaryonOscillationSpec-
percentofacompaniongalaxy’smasswillbedistributedintothe troscopicSurvey(BOSS;Ahnetal.2012).Thismodelisthenused
ICLduringamergerwiththeBCG(seealsoConroyetal.2007b; tophotometricallygroupredgalaxies(withluminosities(cid:62)0.2L(cid:63))
Puchweinetal.2010).Thisbringsthemodel’spredictedBCGmass atsimilarredshiftsintoclusters,assumingaradialfilterthatcorre-
growthover0<z<1intobetteragreementwiththeobservedstel- spondstotypicalclustersizes.Thealgorithmiterativelydetermines
larmassgrowthestimatesofLidmanetal.(2012);Linetal.(2013); aphotometricredshiftforeachclusterbasedonthecalibratedred-
Burke&Collins(2013)inthesameredshiftrange.Ifwearetobe- sequencemodel,asmeasuredfromthephotometricallyidentified
lievethatsomefractionofthemergingmassendsupintheICL, candidateclustermembers.
then this presents us with a scenario that the ICL is being built-
up through galaxies that are interacting with the BCGs. DeMaio
etal.(2015)howeverarguesagainstthisidea,statingthattheICL 2.1.1 Probabilityofbeingaclustermember
is being built-up by the stripping of satellite galaxies with lumi-
ThecandidateclustermembersareeachassignedtoaredMaPPer
nosities>0.2L(cid:63) (alsoseeContinietal.2014).Consequently,itis
clusterbasedonamembershipprobability(P ).P indicates
MEM MEM
clear that the growth and stellar mass build-up of BCGs and the
theprobabilityofagalaxybeingared-sequencegalaxythatbelongs
ICLarelinked,howeveritisnotcleartowhatextentmergerevents
toaspecificcluster.Thisfollowsdirectlyfromthemethodusedby
contributetowardsthis.
theclusterdetectionalgorithmtoidentifyclusters,i.e.lookingfor
Inthispaperweinvestigatetheimportanceofmajormergers overdensitiesofred-sequencegalaxies,withtheluminositycutand
inthestellarmassbuild-upofBCGsbetween0.08(cid:54)z(cid:54)0.50.We radialfilteringmentionedabove.redMaPPerprovidescataloguesof
selectBCGsfromaphotometricclustercatalogue,whichhasbeen (candidate)clustermembersforeachcluster,basedonthegalaxies’
constructedfromtheSloanDigitalSkySurvey(SDSS;Yorketal. individual properties, including PMEM, and it is these catalogues
2000).WeusespectroscopicinformationfromtheSDSStoidentify whichweusethroughoutouranalysis.
which BCGs and nearby companions are potential major merger In Fig. 1 we show the colour-magnitude diagrams (g−r vs.
candidates.Theclosepairfractionisusedasaproxyforthemerger mi)oftheredMaPPerclustersouttoz=0.35(theredshiftcutwill
fractiontodeterminehowmuchstellarmassgrowththeBCGswill be explained in the next section). All candidate cluster members
experienceduetomergers. and those with PMEM>0.9 are shown in grey and green respec-
Thispaperisstructuredasfollows.Section2containsdetails tively.IneachpanelitisapparentthatmemberswithlargePMEM
pertaining to the redMaPPer catalogue; in Section 3 the methods values form a tighter sequence, as expected, since these galaxies
aredescribed.Theresultsonthepairfractionandmerger-inferred aremorelikelytobered-sequencegalaxies.Wewillreturntothis
stellarmassgrowthoftheBCGsareshownanddiscussedinSec- ideainSection4.1.
tion4.ConclusionsaredrawninSection5.Throughoutthepaper For each cluster in the redMaPPer catalogue, the central
weassumeaflatΛCDMcosmologywithΩM=1−ΩΛ=0.3and galaxy identification algorithm of redMaPPer assigns five galax-
H =70 kms−1Mpc−1.MagnitudesaregivenintheABsystem ies probabilities of being the central galaxy (CG) of the cluster
0
andtheChabrier(2003)initialmassfunctionisused. (PCEN).Thesegalaxiesarerankedaccordingtoprobabilityandare
hereafter referred to as the CG candidates. The central probabili-
ties are defined by using a luminosity filter, photometric redshift
1 Typicallydefinedintheliteratureasmergerswithstellarmassratiosof filterandalocalgalaxydensityfilter(discussedbelow).Theprod-
>1:4−1:20(e.g.Edwards&Patton2012;Burke&Collins2013;Burke uct of these three filters produces the overall centering filter that
etal.2015).
2 Typicallydefinedasmergerswithstellarmassratiosof1:1to1:4(e.g.
Jogeeetal.2009;López-Sanjuanetal.2012;Robothametal.2014). 3 http://risa.stanford.edu/redmapper/
MNRAS000,1–18(2017)
RoleofmajormergersinstellarmassgrowthofBCGs 3
is used to determine the central probabilities (see equation 67 of The redMaPPer catalogue does not supply any stellar mass
Rykoffetal.2014).Theredshiftfilterthatisusedinthisequation (M(cid:63)) estimates for the galaxies. We determine stellar mass esti-
isslightlybroaderthantheclusterred-sequencefilterinordertoal- matesusingversion4.1ofthekcorrectcode(Blanton&Roweis
lowgalaxieswithslightcolouroffsetsfromthered-sequencetobe 2007)whichusestemplatesbasedontheBruzual&Charlot(2003)
consideredasCGcandidates,asitispossiblefortheCGstohave models.Briefly,spectralenergydistributionfittingisusedtoderive
experiencedresidualamountsofstarformationandthereforehave theM(cid:63) ofthegalaxiesbyfittingtheirobservedmodelMagmagni-
bluerphotometriccolours(∼2percentoftheCGsinredMaPPer tudes(intheu,g,r,iandz-bands)againstarangeofspectraltem-
areblue;Rykoffetal.2014). platesfromBruzual&Charlot(2003)assumingaChabrier(2003)
ThegalaxywiththehighestP isnotnecessarilythebright- initialmassfunction.Thebest-fittingstellartemplateisdetermined
CEN
est galaxy. This is because, apart from the luminosity, the local throughχ2-minimization,whereafteramass-to-lightratioisdeter-
galaxy density around the CG candidate is also considered. Cen- minedfromthistemplateandusedtoconverttheluminosityofthe
tralgalaxiesareexpectedtobefoundinthehighestdensitycentral galaxytostellarmass.
regionsofclusters.Consequently,theredMaPPeralgorithmgives
higher preference to galaxies in denser regions than those in less
denseregions.Alessluminousgalaxylocatedinadenserenviron-
2.2 TheSALTsample
mentthanthemostluminousgalaxymaythereforehaveahigher
PCEN. WeextendtheBCGpairfractionanalysisouttomoderatelyhigh
ThetotalmagnitudesofthegalaxiesintheredMaPPercata- redshifts (0.4 (cid:54) z (cid:54) 0.5) using the Gaussian Mixture Bright-
logue are given by the i-band cModel_Mag (mi; Abazajian et al. est Cluster Galaxy catalogue (GMBCG7; Hao et al. 2010) with
2004),whilethecoloursofthegalaxiesaredeterminedusingmod- follow-upspectroscopyfromtheSouthernAfricanLargeTelescope
elMag(Abazajianetal.2004)intheu,g,r,iandz-bands.Allmag- (SALT;Buckleyetal.2006;O’Donoghueetal.2006).Werequirea
nitudesandcolourshavebeencorrectedforGalacticextinctionus- samplewithahighspectroscopiccompletenessinordertoidentify
ingthedustmapsofSchlegel,Finkbeiner&Davis(1998). pairsthatarelikelytomerge.Thespectroscopiccompletenessof
the SDSS drops significantly around z=0.4, so we have supple-
mentedtheSDSSdatawithadditionalredshiftsfromSALT.
2.1.2 RichnessoftheredMaPPerclusters
TheGMBCGclustercatalogueisaphotometrically-identified
The catalogue contains clusters with richnesses4 of λ (cid:62)20S(z), cluster sample similar to redMaPPer and was used in the early
where S(z)5 is a correction factor that is used to account for the stages of this work. When redMaPPer became available, we
surveydepthoftheSDSS.Weshowtheeffectofthesurveydepth switched to using this due to several advantages it offered, such
onthemeasuredcompletenessoftheredMaPPerclustersasafunc- asexplicitBCGprobabilities,candidatemembergalaxyprobabil-
tionoftheirhalomasses(M )inFig.2.TheM havebeenderived ities,etc.However,spectroscopicfollow-upwasalreadywellun-
h h
using the halo mass-richness relation of Rykoff et al. (2012), as- derway for the GMBCG sample. In this work, we treat them as
sumingrichness6isaproxyforM .ThevariouslinesinFig.2are two independent cluster samples, in the same way as comparing
h
constructedfromthevaluesinfigure22ofRykoffetal.(2014).The ourredMaPPerworkwiththoseofothersintheliterature,andwill
redMaPPeralgorithmusedfiveredshiftbinstodeterminethecom- showthatourresultsarecompatible.
pleteness of the catalogue as a function of richness. The lines in We refer the reader to Hao et al. (2010) for details on the
thisfigurejointhemeanredshiftvalueofeachbin.Atz(cid:54)0.35,the GMBCG catalogue. Briefly, the catalogue spans a redshift range
galaxycatalogueisvolumelimitedandthesurveydepthisbrighter of0.1<z<0.55andcontains55000clusters,opticallyidentified
thanthefiducialluminositycutof0.2L(cid:63) (thereforeS(z)=1).At fromtheSDSSDR7(Abazajianetal.2009)asoverdensitiesofred-
theseredshiftstheclustercatalogueis50percentcompletedown sequencegalaxies.ThepositionsoftheBCGsandthephotometric
to clusters with Mh (cid:38)0.2×1015M(cid:12). At z>0.35, however, the redshiftsoftheirhostclustersareprovidedinthecatalogue.Here
magnitudelimitofthesurveycausesonlythemostmassiveclus- weareinterestedinthe4814BCGsthatarefoundintheredshift
terstobedetected.Thiscausestherichnessdetectionthresholdto range0.4(cid:54)z(cid:54)0.5.
increasewithredshift. WeusethemethodoutlinedinSection3.2toselecttheclose
Inthiswork,werestrictouranalysistoonlyconsiderclusters pairs that are included in the SALT sample. Briefly, all compan-
in the volume limited sample at z(cid:54)0.35. We further only con- ionswithinaphysicalseparationdistanceof50kpcand1.5mag-
siderredMaPPerclusterswithhalomassesabovethe50percent nitudes(inthei-band)oftheBCGswereretrievedfromtheDR7
M completeness limit. This is done to maximize the number of database.Thespectroscopicredshifts(ifavailable)ofthesegalax-
h
clustersintheevolutionarysequence(asdiscussedinSection3.1). ies were retrieved from the SDSS DR12 (Alam et al. 2015). We
furtherrestrictedtheSALTsampletoonlyincludepairswhereei-
ther the BCG or companion (in each pair) had a SDSS spectro-
2.1.3 BCGs-Identificationandstellarmasses scopicredshift(zspec).SALTobservationswereusedtodetermine
InourworkweselectthefirstCGcandidate(thegalaxywiththe theremaininggalaxy’szspec todeterminewhetherthegalaxiesare
potentialmergercandidatesornot.Only16oftheclosepairssat-
highestP )ineachclusterastheBCGcandidate.
CEN isfiedthesecriteria.Twelvepairsweresuccessfullyobservedover
twosemestersspanning2013November-2014April(proposalID:
2013-2-RSA-008,PI:Groenewald)and2014May-October(pro-
4 Richnessisdefinedasthenumberofred-sequencegalaxieswithinaclus-
terthatarebrighterthan0.2L(cid:63). posalID:2014-1-RSA_OTH-009,PI:Groenewald).
5 Givenbyequation23ofRykoffetal.(2014).
6 WecorrecttheredMaPPerrichnessestimatesforthesurveydepthusing
λ/S(z).Weusetheseintheremainderofthepaperandrefertothemasthe
correctedrichnesses. 7 http://home.fnal.gov/∼jghao/gmbcg_sdss_catalog.html
MNRAS000,1–18(2017)
4 D.N.Groenewaldetal.
Figure1.Thecolour-magnitudediagramoftheredMaPPerclustersinvariousredshiftbins.AllcandidateclustermembersandthosewithPMEM>0.9are
showningreyandgreenrespectively.Thesolidanddashedlinesrespectivelyindicatethelocationofthered-sequenceandthe1σwidth.Onlyhighprobability
clustermembers(PMEM>0.9)havebeenusedtofindthered-sequence,sincethesegalaxiesaremorelikelytobelongtothered-sequence.Thered-sequence
formsatightersequenceasPMEMincreasesandbecomesredderwithredshift.
2.2.1 Observationsandreductions SDSSgalaxyspectraltemplate(hereafterreferencespectrum).The
restwavelengthsoftheCalciumIIHandKabsorptionlinesinthe
Using the Robert Stobie Spectrograph (RSS; Burgh et al. 2003; referencespectrumwereshiftedtomatchthoseinthegalaxy’sob-
Kobulnickyetal.2003)onSALTweobtainedlongslitspectroscopy servedSALTspectrum.
for the close pairs. A slit with a width of 2(cid:48)(cid:48) was centred on the InTable1wepresentasummaryofthegalaxiesintheSALT
BCGineachpairandalignedinsuchawaythatboththeBCGand samplealongwiththeirSDSSspectroscopicredshifts(ifavailable).
companionwereobservedinasingleobservation.Thisallowedus Foreachclosepairweindicatethevelocitydifferenceasderived
todeterminetherelativevelocitiesbetweenthesetwogalaxiesus- from the SALT spectroscopy. The BCG and companion in each
ing the same wavelength calibration. The RSS observations used pairwereobservedinthesameslitandsharethesamewavelength
thePG900gratingwhichcoversthemainopticalfeaturesthatwe calibration. Thus the velocity differences, which are our ultimate
areinterestedinover4500−7500Å.EachclosepairintheSALT goal,aremorerobustthanifwehadusedacombinationofSALT
samplewasobservedforatotalof106minutes,splitovertwoob- andSDSSredshiftsforeachpair.
servationblocks(eachobservationblockconsistedof2×20minute
exposures).
Basic data reductions, i.e. gain and cross-talk correction as 3 METHOD-THEPAIRFRACTIONAND
wellasbiassubtractionwereperformedaspartoftheautomated MERGER-INFERREDSTELLARMASSGROWTHOF
reductionpipelineofSALT(Crawfordetal.2010).Weperformed THEBCGS
cosmicrayrejectiononthescienceimagesbyusingtheLACosmic
3.1 Constructinganevolutionaryclustersequence
package (van Dokkum 2001). Wavelength calibrations were then
performedwithstandardIRAF(Tody1986,1993)tasks.Wedeter- ThestellarmassesofBCGsareknowntocorrelatewiththehalo
minedthespectroscopicredshiftofeachgalaxyintheSALTsam- massesoftheirhostclusters,withmoremassivehaloshostingmore
plebyfittingtheobservedSALTspectrumagainstthe‘Early-type’ massive BCGs (e.g. Edge 1991; Burke et al. 2000; Brough et al.
MNRAS000,1–18(2017)
RoleofmajormergersinstellarmassgrowthofBCGs 5
Figure2.CompletenessoftheredMaPPercatalogueasafunctionofMh andredshift.ThegreypointsrepresenttheMh-distributionoftheclustersinthe
catalogue(derivedusingthecorrectedrichnesses).Thevariouslinesareconstructedfromthevaluesinfigure22ofRykoffetal.(2014).TheredMaPPer
algorithmusedfiveredshiftbinstodeterminethecompletenessofthecatalogueasafunctionofrichness.Thelinesjointhemeanredshiftvalueofeachbin.
Atz(cid:54)0.35(verticaldashedline),thegalaxycatalogueisvolumelimitedandcompletedowntoclusterswithMh∼0.2×1015M(cid:12).Atz>0.35,however,the
algorithmisonlyabletodetectthemostmassiveclustersduetothemagnitudelimitofthesurveyandisthereforeincompleteforlowmass(lowrichness)
clusters.Seetextfordetails.
Table1.SummaryoftheclosegalaxypairsintheSALTsample.ThepairIDofeachpair(takenfromtheGMBCGcatalogue)isgiveninColumn1while
thegalaxies’coordinatesaregiveninColumns3and4.TheSDSSspectroscopicredshiftsofthesegalaxies,ifavailable,aregiveninColumn5.Thevelocity
differenceofeachclosepair,determinedfromSALTspectroscopy,isgiveninColumn6.
PairID RA(J2000) DEC(J2000) zSDSS ∆vSALT
(deg) (deg) (kms−1)
BCG 190.404850 −0.668280 0.4587±0.0001
337 21±85
Companion 190.406416 −0.667039 —
BCG 128.317079 0.108010 —
5501 1017±85
Companion 128.316370 0.108480 0.4748±0.0001
BCG 153.259460 0.812003 0.4063±0.0001
5919 4517±85
Companion 153.259030 0.809660 —
BCG 27.261019 −0.639814 0.3584±0.0001
22105 685±42
Companion 27.261605 −0.639844 —
BCG 346.949880 0.948270 0.3686±0.0001
22258 307±85
Companion 346.949133 0.947189 —
BCG 186.754036 0.765942 —
23941 2612±42
Companion 186.753290 0.767780 0.5153±0.0002
BCG 148.316850 1.272180 0.3648±0.0001
24097 198±42
Companion 148.316048 1.272079 —
BCG 206.032182 1.948250 —
431 39±42
Companion 206.031830 1.948720 0.5432±0.0001
BCG 223.098640 0.949810 0.4830±0.0001
5680 40±85
Companion 223.099826 0.950909 —
BCG 210.260650 0.275680 —
5905 61±42
Companion 210.258767 0.275656 0.4757±0.0001
BCG 218.763280 3.109750 0.3829±0.0001
24726 152±85
Companion 218.763793 3.111125 —
BCG 357.091165 0.741055 —
52685 106±85
Companion 357.088710 0.741300 0.4113±0.0001
MNRAS000,1–18(2017)
6 D.N.Groenewaldetal.
2008; Stott et al. 2008, 2010, 2012; Whiley et al. 2008; Collins Kitzbichler & White 2008). We have chosen this cut, which is
etal.2009;Hansenetal.2009;Lidmanetal.2012).Consequently, somewhat stricter than many other observational works (e.g. Lin
it is important to take the halo mass growth of the clusters into et al. 2004, 2008; López-Sanjuan et al. 2012; Robotham et al.
account when the stellar mass growth of BCGs is investigated, 2014), in order to use the results of Kitzbichler & White (2008)
to ensure that that BCGs at high redshifts are compared to their to estimate merging timescales for each pair. These field merger
likely descendants at lower redshifts. This idea has already been timescalescanbeappliedtoclosepairsinclusters(seesection4.2
exploredbyotherworksintheliterature(Lidmanetal.2012;Lin ofLidmanetal.2013,formotivation).
etal.2013;Oliva-Altamiranoetal.2014;Zhangetal.2016).Inour WeuseacompilationoftheSDSSspectroscopicgalaxysam-
workweuseanapproachsimilartothatimplementedbyLidman plesofwhichtheBOSSsurveysuppliesthedeepestspectroscopy,
et al. (2012) to construct an evolutionary cluster sequence using uptoamagnitudelimitof19.9magnitudesinthei-band.Wethere-
evolvingM limits.WeassumethattheBCGsinthisevolutionary forefurtherrestrictourclosepairsampletoonlyincludepairswith
h
sequenceareprogenitors/descendantsofoneanothertoderivethe galaxiesbrighterthantheBOSSmagnitudelimit.Ourfinalsample
merger-inferredmassgrowthoftheBCGs. consistsof1016photometricpairs,ofwhich320(∼31percent)
Briefly,weconstructanevolutionaryclustersequencebyiden- havespectroscopy.Wecorrectforthespectroscopicincompleteness
tifying the low redshift descendants of the redMaPPer clusters in of our sample when the BCG pair fraction is measured (Section
our highest redshift bin (z=0.35). This is done by evolving the 3.3).Toensurethatwearerelativelycompleteforcompanionsto
M oftheseclustersforwardintimeusingthemeanmassaccretion theBOSSmagnitudelimit,weonlyconsiderneighbouringgalaxies
h
rates (MMAR) from the Fakhouri, Ma & Boylan-Kolchin (2010) withastellarmassratiowithin1:4oftheBCG(seepanelbofFig.
model(seetheirequation2).ThroughtheseMMARsweareable 4).Thismaximizestheredshiftrangeoverwhichwearecomplete.
todeterminewhatthecorrespondinghalomassesofthesehighred- Wedefineallpotentialmergercandidateswithstellarmassratios
shift clusters will be at later times, allowing us to construct an between 1:1−1:4 as major mergers. For illustrative purposes, we
evolving M limit as a function of redshift. We divide the clus- showtheSDSScutoutsofeightmajormergercandidatesfromour
h
tersintofourequalsizedredshiftbinswithawidthof0.067.The closepairsampleinFig.5.
binwidthischosentobelargerthanthetypicaluncertaintyonthe
photometric redshift of the clusters (∼0.02), which reduces the
chancesthatclusterswillbescatteredinandoutofadjacentred- 3.3 Definingthepairfraction
shiftbins.Secondly,itissmallenoughtoensurewehavemultiple
WeultimatelywanttodeterminethefractionofBCGsthatarein
bins,eachwitharobustnumberofclusters,withwhichtostudythe
boundpairs,i.e.BCGsthatwillmergewiththeircompanionsby
BCGs’merger-inferredstellarmassgrowth.
z=0(hereafterlooselyreferredtoas‘boundcompanions’).Thisis
Wefindatotalof5432clustersthatformpartoftheevolution-
definedasfollows:
arysequence.Theseclusters(alongwiththeevolvingM limit)are
h
showninFig.3.InTable2wepresentasummaryofthenumberof NBCGs,BC
f = (1)
clustersineachredshiftbinalongwiththeirMhranges.InSection pair NBCGs
4.1wetesthowthepairfractionoftheBCGsisinfluencedwhen
whereN isthetotalnumberofBCGsthatformpartoftheevo-
BCGs
thehalomassgrowthoftheclustersisnottakenintoaccount.
lutionarysequence(asderivedinSection3.1)andNBCGs,BCrepre-
sentsthenumberofBCGswithboundmajormergercompanions.
3.2 Theclosepairselection WecanexpandEq.1tothefollowing:
Webeginbyonlyconsideringgalaxiesthatarebrighterthan21.5 f = NBCGs,C×NBCGs,BC (2)
magnitudesinthei-band.ThisisthemagnitudewheretheSDSS pair NBCGs NBCGs,C
is 95 per cent complete for galaxies. This limit was determined
wherethefirsttermisthefractionofBCGswithcompanionsand
by comparing the galaxy number counts in a region on the ce-
thesecondtermisthefractionofthesecompanionsthatarebound
lestial equator from the SDSS to the Stripe 82 survey (Adelman-
(contamination correction). NBCGs,C is the number of BCGs that
McCarthyetal.2007),whichis∼2magdeeper.
haveoneormorecompanion(s)asdescribedinSection3.2.Inthe
From the redMaPPer catalogue we construct a close galaxy
casewhereonlyphotometryisavailable,thesecondterm(contam-
pairsampleof1336pairsbysearchingforallphotometricgalaxies
inationcorrection)inEq.2maybesettoaconstantvalue(e.g.0.5
within a physical separation distance (rsep) of 7(cid:54)rsep(cid:54)50 kpc in Edwards&Patton2012).
fromtheBCGs.Galaxieswithinthissearchradiusarereferredtoas
We obtain a measurement for the BCG pair fraction in two
companions.Thelowerlimitisimposedsincethisistheminimum
cases. We start with the simple case, where only photometry is
separationdistancedowntowhichtheSDSScanresolveindividual
used.Thisisusedinstudieswhichdonothavespectroscopyavail-
galaxiesoverourredshiftrange.
able(suchasEdwards&Patton2012).Thereafterweconsiderthe
Aclosepairsamplethathasonlybeenconstructedusingpho-
casewherethereisspectroscopicinformation.
tometricinformationwillinevitablysufferfromcontaminationdue
Where spectroscopic information is available for some of
to line-of-sight projections. Although the contamination in clus-
the sample, we have more information from which to determine
tersishigherthaninthefield(includingcontaminationfromclus-
whethercompanionsareboundtotheBCGs.Ratherthanassum-
termembersthemselves),thesametechniquescanbeusedtoob-
ingaconstantvalueforthecontaminationcorrectioninEq.2,we
tain the close pair fraction. In this work we use spectroscopy to
deriveamoredetailedcorrectionbygroupinggalaxiesintobinsof
correctforthiscontaminationbydeterminingwhetherthephoto-
colours,magnitudeandseparationdistance.Inthei-thbinthepair
metrically identified companion galaxies are bound to their host
fractioninredshiftbin jis
BCGs. In order for close pairs to be considered potential merger
coafn∆dvid(cid:54)ate3s0,0wkemrse−q1ui(rseetehee.gg.alBauxriebsidtgoeh1a9v7e5;aEvlelliosocnityetdaiflf.e2re0n1c3e; (fpair)j = ∑iC∑i−i1NNBBCCGGssi,SCi (3)
MNRAS000,1–18(2017)
RoleofmajormergersinstellarmassgrowthofBCGs 7
Figure3.TheevolutionaryclustersequenceoftheredMaPPerclustersatz=0.35thathavebeenconstructedusingtheMMARsofFakhourietal.(2010).
TheMh-distributionoftheredMaPPerclustersisrepresentedwiththegreypoints.Theclustersthatformpartofthesequenceareindicatedinblue,whilethe
redlinerepresentsthe50percentMhcompletenesslimit.Seetextfordetails.
whereNBCGs,SC isthenumberofBCGswithspectroscopiccom- of the BCGs, denoted (cid:104)M(cid:63)(cid:105), in each redshift bin. The uncertain-
panions andC is the applied correction for spectroscopic incom- tieson(cid:104)M(cid:63)(cid:105)aredeterminedusingbootstrapresamplingwith1000
pleteness(derivedinAppendixA). realizations.
In our high redshift bin 0.4(cid:54)z(cid:54)0.5, due to the low spec- We estimate average merger timescale, denoted (cid:104)tmerge(cid:105), for
troscopiccompletenessoftheSDSSattheseredshifts,weusedthe each close pair using the results of Kitzbichler & White (2008).
SALT sample to determine the contamination correction (second UsingtheMillenniumSimulation(Springeletal.2005),theyfind
terminEq.2)i.e: thattheaveragetmergeforgalaxieswithrsep(cid:54)50kpcand∆v(cid:54)300
kms−1canbegivenbythefollowing:
NBCGs,C NBCGs,BC(SALT)
fpair = N × N (4) rsep (cid:16) M(cid:63),com (cid:17)−0.3(cid:16) z(cid:17)
BCGs BCGs,C(SALT) (cid:104)tmerge(cid:105)=2.2Gyr50kpc 5.5×1010M(cid:12) 1+8 (5)
Wehaveassumedherethatthefractionofpotentialmergerpairsin
theSALTsampleisrepresentativeofthosefoundinthephotomet- whereM(cid:63),com isthetotalstellarmassofthecompanionatitsob-
ricpairsample(selectedfromtheGMBCGcatalogue).TheSALT servedredshift(determinedusingkcorrect),rsep isthephysical
separationdistance(inkpc)andzisthephotometricredshiftofthe
pairs are not different from those in the photometric pair sample
sincethesamersep and∆mi criteriahavebeenusedtoselectpairs cluster.Theuncertaintieson(cid:104)tmerge(cid:105)aregivenby1σ standardde-
viationandarepropagatedthroughtothefinalmeasuredfractional
inbothsamples.Theonlydifferencebetweenthesetwosamplesis
massgrowths.VariousworkshavecommentedthattheKitzbichler
theredshiftrequirementusedfortheSALTpairselection.Sincethe
&White(2008)mergertimescalesaresignificantlylongerthanthe
BCGsarebright,redgalaxies,theyareverylikelytobeobservedby
estimatefromdynamicalfrictionortheorbitalperiod(e.g.Conroy
theSDSSspectroscopicsurveys,sowedonotexpectabiasagainst
et al. 2007a; Bertone & Conselice 2009; Conselice 2009; Kauff-
spectroscopyforthesegalaxies.Thenumberofpairsobservedwith
SALTshouldthereforeberepresentativeofthenumberofpairsin mannetal.2010;Lotzetal.2011).Theuncertaintieson(cid:104)tmerge(cid:105)in
alltheredshiftbinsarelargeenoughtoencompassdifferenceswith
thephotometricsample.
othermethods(seeTable3).
Theuncertaintyon f representsthe68.3percent(1σ)bi-
pair
nomialconfidencelimitandiscalculatedusingthebetaconfidence Themergerrate,denotedRmerge,ofthesample,i.e.thenum-
berofmergersperBCGperGyr,isdefinedasfollows:
intervalasdescribedbyCameron(2011).
f
pair
Rmerge= (6)
(cid:104)t (cid:105)
merge
3.4 ThemassgrowthoftheBCGs
TheuncertaintyonRmergeisdeterminedusingthestandardpropa-
Wenowcontinueontothemainaimofthepaper,measuringthe gationoferrors.
merger-inferredstellarmassgrowthoftheBCGsasafunctionof Semi-analytical models, for example Conroy et al. (2007b);
redshift.Inordertodothiswefirstcalculateaveragestellarmasses Puchwein et al. (2010); Laporte et al. (2013) have shown that
MNRAS000,1–18(2017)
8 D.N.Groenewaldetal.
Figure4.ToensurewehaveasamplewithahighspectroscopiccompletenesswithwhichtheBCGpairfractioncanbedetermined,werequireallthegalaxies
inourclosepairsampletobebrighterthantheBOSSmagnitudelimitofmi=19.9magnitudes(dashedline).Wefurtherrestrictoursampletoonlyinclude
closepairswithstellarmassratiosof1:1−1:4.Thiswasdonetomaximizeboththeredshiftrangeandnumberofclosepairsinoursample.Seetextfordetails.
30−80 per cent of the companion’s stellar mass ends up in the toz=0.Wedonottakethistermintoconsiderationbecausewe
ICLduringamergerwiththeBCG.Duringamerger,weassume areunabletoestimatethecontributionfromminormergersinthis
that 50 per cent of the companion’s stellar mass is transfered to work,asweareincompleteforthesesystems8.Secondly,weex-
theBCG(fmass=0.5;alsousedbyLiuetal.2009,2015;Burke pectthecontributionfromstarformationtobenegligibleforthese
&Collins2013;Lidmanetal.2013).Toaccountforthepossible lowredshiftBCGs.ItisraretofindBCGsinthelocalUniversethat
rangeof fmassvalues,weassignanuncertaintyof20percenttoour areactivelyformingstars(lessthan1percent;e.g.Liuetal.2012;
assumed fmass valueof50percent(i.e. fmass=0.5±0.2),which Fraser-McKelvieetal.2014).Thesestarburstshavebeenfoundto
ispropagatedthroughtothemeasuredfractionalmassgrowths.In contributeonly∼1−3percent(e.g.Sarazin&O’Connell1983;
order to calculate how much mass (∆M) major mergers add to a Cardiel et al. 1995; Pipino et al. 2009; Liu et al. 2012; Loubser
BCGfromredshiftzidowntoz=0,weusethefollowing: etal.2016)tothestellarmassoftheBCGs.TheuncertaintyonF
isdeterminedusingthestandardpropagationoferrors.
∆M(z=zi−0)=Rmerge×TLB×(cid:104)M(cid:63)(cid:105)com(zi)×fmass (7)
where TLB and (cid:104)M(cid:63)(cid:105)com(zi) respectively gives the lookback time
andtheaveragestellarmassofthecompanionsatzi. 4 RESULTSANDDISCUSSION
Thefractionalcontribution(F)madebymajormergerstothe
stellar mass of a present day BCG, denoted (cid:104)M(cid:63)(cid:105)BCG(z=0), is 4.1 Theredshiftevolutionofthepairfraction
definedasfollows:
4.1.1 TheredMaPPerpairfraction
∆M(z=zi−0)
F= (8) Theredshiftevolutionofthepairfractionsfrom0.08(cid:54)z(cid:54)0.35for
(cid:104)M(cid:63)(cid:105)BCG(z=0)
BCGsselectedfromtheredMaPPercatalogueareshowninFig.6
with
andsummarizedinTable2.Thepairfractionmeasuredwithin50
(cid:104)M(cid:63)(cid:105)BCG(z=0)=(cid:104)M(cid:63)(cid:105)BCG(zi)+∆M(z=zi−0)+∆Mother (9) kpcusingthreedifferentmethodstocorrectforprojectioneffectsis
where (cid:104)M(cid:63)(cid:105)BCG(zi) is the average stellar mass of the BCGs at
redshift zi and ∆Mother accounts for other sources of mass accre- 8 Between0.08(cid:54)z(cid:54)0.20,wearecompleteformergerswithmassratios
tion,i.e.starformationandminormergers,fromredshiftzi down downto1:6(seeSection3.2andpanelcofFig.4).
MNRAS000,1–18(2017)
RoleofmajormergersinstellarmassgrowthofBCGs 9
Figure5.Forillustrativepurposes,weshowtheSDSScutoutsofeightpotentialmergercandidates(twoperredshiftbin)fromoursample.Eachcutoutis
centredontheBCG(circledinblue)andis68(cid:48)(cid:48)onaside,whichroughlycorrespondsto100kpcatz=0.08.TheobjectscircledinredaretheredMaPPer
candidateclustermembersthatarebrighterthantheluminositycutof0.2L(cid:63)(imposedduringtheconstructionofthecatalogue).Thedashedwhitelineindicates
the50kpcphysicalsearchradius(atthecluster’sphotometricredshift).Foreachcutout,weshowtheredMaPPerIDandrichnessalongwiththecluster’s
photometricredshift.
showninthetoppanelofFig.6,whilethebottompanelcompares above (grey symbols and line). We find that the spectroscopicC-
thespectroscopically-correctedresultsmeasuredwithin30and50 correction reduces the photometric pair fraction to ∼6 per cent
kpc. whilethesimpleassumptionof50percentcontaminationreduces
Aclosepairsampleconstructedusingonlyphotometrywith thephotometricpairfractionto∼10percent.
nobackgroundcorrectionwillinevitablysufferfromprojectionef- Several studies in the literature, e.g. Le Fèvre et al. (2000);
fects. The resulting pair fraction can therefore be considered as Kartaltepeetal.(2007);deRaveletal.(2009);López-Sanjuanetal.
anupperlimit(redsymbolsandlineinthetoppanel).Tocorrect (2012);Keenanetal.(2014)parameterisedtheevolutionofthepair
forthiscontamination,wehaveapplieda50percentcontamina- fractionwiththefollowingpower-lawfunction:
tioncorrection(asusedbyEdwards&Patton2012,bluesymbols f (z)= f (z=0)×(1+z)m (10)
andline).Wecomparethispairfractiontothatderivedusingthe pair pair
availablespectroscopy,correctedforincompletenessasdescribed The power law fits to each of our three measurements are
MNRAS000,1–18(2017)
10 D.N.Groenewaldetal.
shown by the dashed and dotted lines in the figure. The negative ters by using a fixed9 Mh cut of ∼0.3×1015M(cid:12). The resulting
powerindicesineachcasesuggestthattheindividualpairfractions pair fractions of these two subsamples are also consistent within
increaseslightlywithdecreasingredshift.Thepairfractionderived theuncertainties.Theseresultsmaysuggestthatthepairfraction
usingthespectroscopicC-correctionhasasteeperredshiftevolu- of BCGs involved in major mergers does not depend on the halo
tionthanphotometricandthe50percentcontamination-corrected massoftheirhostclusters.
pairfractions.
The bottom panel of Fig. 6 compares the spectroscopic- 4.1.2 TheSALTpairfraction
corrected f asmeasuredwithin30kpc(blacksymbolsandline)
pair
Inorderforaclosegalaxypairtobeconsideredapotentialmerger
tothatwithin50kpc.Hereagain,thenegativepowerindicessug-
gestthatthepairfractionsareincreasingwithdecreasingredshift. candidate,werequirersep(cid:54)50kpcand∆v(cid:54)300 kms−1.Wefind
that seven of the 12 close pairs in the SALT sample satisfy this
Thepairfractionwithin50kpcisafactorof∼2largerthanthat
criteria (see Table 1). Using Eq. 4, we find the measured bound
measuredwithin30kpc.Thisisclosetotheexpecteddifferencein
BCGpairfractiontobe0.21+0.04between0.4(cid:54)z(cid:54)0.5.Ifinstead
normalizationfordifferentradiifoundinpreviousworks(e.g.from −0.05
we consider a separation radius of 30 kpc, we find that three of
thetwo-pointcorrelationfunction,Belletal.2006).Althoughthe
thepairsintheSALTsamplecanbeconsideredpotentialmerger
observedevolutionof f isslightlysteeperwithinthesmallerra-
pair candidates.ThisresultsinaboundBCGpairfractionof0.09+0.06
dius,itisconsistentwithintheuncertaintytothatmeasuredwithin −0.03
(asderivedusingEq.4).
thelargersearchradiusof50kpc.
RecallthattheclustersinredMaPPerhavebeendetectedby
4.1.3 Literaturecomparison
looking for overdensities of red-sequence galaxies (Section 2.1).
This may tempt the reader to think that the given cluster mem- In Fig. 7 we show the comparison of our measured pair fraction
bersareonlyred-sequencegalaxies.Ifthisisthecasethenwemay againstvaluesobtainedfromtheliterature.Wecompareagainstthe
miss potential merger candidates that are not located on the red- resultsofstudiesconductedinclustersandthefield.Themajority
sequence,causingustopotentiallyunderestimatethepairfraction ofthestudiesdiscussedhereafterhavederivedpairfractionswithin
oftheBCGs.InFig.1weshowedthatgalaxieswithhighprobabil- rsep(cid:54)30kpc.Wehavethereforechosentoonlyshowthespectro-
ities(PMEM>0.9)formatightersequence,asexpected,sincethey scopicallycorrectedpairfractionweobtainedbyconsideringclose
are more likely to be red-sequence galaxies. Those with smaller pairswithinthesamephysicalseparationdistance(blackpoints).
P values,ontheotherhand,aremorescatteredincolour.This
MEM
suggeststhat somegalaxiesthat arenot onthered-sequence (i.e.
galaxiesinthebluecloud)arealsoincludedinthecatalogue.We Clusterstudies:
investigatethisfurtherusingamethodsimilartothatofLuetal.
BothMcIntoshetal.(2008)andLiuetal.(2009)havestudiedthe
(2009)toseparatered-sequencegalaxiesfromtheirbluecounter-
BCG pair fraction (over 0.01(cid:54)z(cid:54)0.12) by searching for close
parts.WereferthereadertoLuetal.(2009)fordetails.Briefly,we
pairs within 30 kpc. They further restricted their samples to only
obtainedthewidthofthered-sequencebyfittingasingleGaussian
consider close pairs where both the BCG and companion show
againstthegalaxieswithhighprobabilitiesofbeingred-sequence
signsofmorphologicaldistortions(i.e.diffusetailsandasymme-
cluster members (PMEM >0.9). Galaxies within 2σ of the red-
tries in the inner isophotes). These distortions indicate that the
sequenceareconsideredtobepartofthered-sequencewhilethose
galaxiesareintheprocessofmerging.Byonlyconsideringmajor
below this limit are classified as blue galaxies. We find that a
mergers(luminosityratios(cid:54)1:4),McIntoshetal.(2008)foundthat
smallfraction(7percent)ofthegalaxiesinredMaPPerareblue.
38oftheir845closepairsweremorphologicallydistorted,giving
Querying the SDSS DR8 database, we find an additional ∼200
f =0.04±0.01,assumingPoissonerrors(redpoint).Similarly,
bluegalaxiesneighbouringourBCGsthatarenotincludedinthe pair
Liuetal.(2009)found f =0.03±0.01(18/515)byassuming
redMaPPer catalogue. Including these ‘missed’ galaxies into our pair
Poissonerrors(bluepoint).
close pair sample and remeasuring the photometric pair fraction,
Only our first redshift bin overlaps with the redshift range
resultsinanincreaseoflessthanonepercent.Itisthereforeclear
used in the above-mentioned studies (z(cid:54)0.12). We measure a
thatthese‘missing’bluegalaxieshaveanegligibleinfluenceonthe
pair fraction of 0.05±0.01 at z∼0.11. The main difference be-
averageBCGpairfraction.
tweenourstudyandthatofMcIntoshetal.(2008);Liuetal.(2009)
Severalstudiesintheliteratureuseluminosityratherthanstel- isthetechniqueusedtoselectmergers:morphologicaldistortions
lar mass to identify potential major merger candidates (e.g. Liu vs. close pairs. Mergers identified through morphological distor-
etal.2009,2015;Edwards&Patton2012;Burke&Collins2013; tionsareinthefinalstagesofmerging.Galaxiesinthesepairsare
Keenanetal.2014;Burkeetal.2015,andothers).Wefindthatthe typically expected to merge within ∼0.2 Gyr (e.g. Patton et al.
selectingpairsbasedontheirluminosityratioratherthantheirmass 2002;Hernández-Toledoetal.2005;Lotzetal.2011).Theclose
ratiohasnoperceivableeffectonourindividuallymeasuredBCG pair technique on the other hand selects mergers that are in vari-
pairfractions. ousstagesofthemergingprocess(earlytofinalstages).Formajor
merger pairs with a separation distance of ∼50 kpc, Lotz et al.
Recallthatthepairfractionsquotedherehavebeenderivedby (2011) estimate a merger timescale of roughly 0.6 Gyr. For pairs
onlyconsideringtheBCGsthatformpartoftheevolutionaryclus- withrsep∼30kpc,tmergedecreasesto0.33Gyr(Lotzetal.2011).
tersequence(i.e.thehalomassesoftheclustershavebeentaken The merger timescales of the pairs in our sample are therefore
intoaccount).Wefindthesepairfractionstobeconsistentwithin longer(onaverage)thanthoseinMcIntoshetal.(2008);Liuetal.
theuncertaintiestothatmeasuredinthecasewherewedidnottake
thehostclusters’M growthintoaccount.Additionally,wedivide
h
the cluster sample into a subsample of low and high mass clus- 9 Thiscuthasbeenvaried,howevertheresultsremainunchanged.
MNRAS000,1–18(2017)