Substructures in WINGS clusters

arXiv:0704.0579v1 [astro-ph] 4 Apr 2007Astronomy&Astrophysicsmanuscriptno.babbagec󰀏ESO2008

February1,2008

SubstructuresinWINGSclusters⋆

M.Ramella1,A.Biviano1,A.Pisani2,J.Varela3,D.Bettoni3,W.J.Couch4,M.D’Onofrio5,A.Dressler6,G.Fasano3,

P.Kjærgaard7,M.Moles8,E.Pignatelli3,andB.M.Poggianti3

1INAF/OsservatorioAstronomicodiTrieste,viaG.B.Tiepolo11,I-34131,Trieste,Italy2IstitutodiIstruzioneStataleClassicoDanteAlighieri,ScientificoDucadegliAbruzzi,MagistraleS.Slataper,vialeXXsettembre11,I-34170Gorizia,Italy3INAF/OsservatorioAstronomicodiPadova,vicoloOsservatorio5,I-35122,Padova,Italy4SchoolofPhysics,UniversityofNewSouthWales,Sydney2052,Australia5DipartimentodiAstronomia,Universit`adiPadova,vicoloOsservatorio2,I-35122Padova,Italy6ObservatoriesoftheCarnegieInstitutionofWashington,Pasadena,CA91101,USA7CopenhagenUniversityObservatory.TheNielsBohrInstituteforAstronomyPhysicsandGeophysics,JulianeMariesVej30,2100Copenhagen,Denmark8InstitutodeAstrof´ısicadeAndaluc´ıa(C.S.I.C.)Apartado3004,18080Granada,Spain

Received/Accepted

ABSTRACT

Aims.WesearchforandcharacterizesubstructuresintheprojecteddistributionofgalaxiesobservedinthewidefieldCCDimagesofthe77nearbyclustersoftheWIde-fieldNearbyGalaxy-clusterSurvey(WINGS).ThissampleiscompleteinX-rayfluxintheredshiftrange0.04

Keywords.Galaxies:clusters:general–Galaxies:kinematicsanddynamics

1.Introduction

Accordingtothecurrentcosmologicalparadigm,largestruc-

turesintheUniverseformhierarchically.Clustersofgalaxies

arethelargeststructuresthathavegrownthroughmergersof

smallerunitsandhaveachievedneardynamicalequilibrium.In

thehierarchicalscenario,clustersarearatheryoungpopulation,

andweshouldbeabletoobservetheirformationprocessevenat

ratherlowredshifts.Asignatureofsuchprocessisthepresence

ofclustersubstructures.Aclusterissaidtocontainsubstruc-

tures(orsubclusters)whenitssurfacedensityischaracterized

bymultiple,statisticallysignificantpeaksonscaleslargerthan

thetypicalgalaxysize,with“surfacedensity”beingreferredto

theclustergalaxies,theintra-cluster(IC)gasorthedarkmatter

(DMhereafter;Buote2002).

Studyingclustersubstructurethereforeallowsustoinvesti-

gatetheprocessbywhichclustersform,constrainthecosmo-

logicalmodelofstructureformation,andultimatelytestthehi-

erarchicalparadigmitself(e.g.Richstoneetal.1992;Mohretal.

1995;Thomasetal.1998).Inaddition,italsoallowsustobetter

understandthemechanismsaffectinggalaxyevolutioninclus-

ters,whichcanbeacceleratedbytheperturbativeeffectsofa

cluster-subclustercollisionandofthetidalfieldexperiencedby2M.Ramellaetal.:SubstructuresintheWINGSclustersRoettigeretal.1997;Barrenaetal.2002;Cloweetal.2006).Hence,itisequallyusefultoaddressclustersubstructureanaly-

sisintheX-rayandintheoptical.

Traditionally,thefirstdetectionsofclustersubstructures

wereobtainedfromtheprojectedspatialdistributionsofgalax-ies(e.g.Shane&Wirtanen1954;Abelletal.1964),incom-

bination,whenpossible,withthedistributionofgalaxyve-

locities(e.g.vandenBergh1960,1961;deVaucouleurs1961).

Increasinglysophisticatedtechniquesforthedetectionand

characterizationofclustersubstructureshavebeendeveloped

overtheyears(seeMolesetal.1986;Pereaetal.1986a,b;Buote2002;Girardi&Biviano2002,andreferencestherein).

Inmanyofthesetechniquessubstructuresareidentifiedasde-viationsfromsymmetry

in

the

spatialand

/

or

velocity

distri-

bution

ofgalaxiesandintheX-raysurface-brightness(e.g.

Westet

al.1988;

Fitchett&Merritt1988;Mohretal.1993;

Schueckeretal.2001).Inothertechniquessubstructuresare

identifiedassignificantpeaksinthesurfacedensitydistribu-

tionofgalaxiesorintheX-raysurfacebrightness,eitheras

residualsleftafterthesubtractionofasmooth,regularmodel

representationofthecluster(e.g.Neumann&B¨ohringer1997;

Ettorietal.1998),orinanon-parametricway,e.g.bythetech-

niqueofwavelets(e.g.Escaleraetal.1994;Slezaketal.1994;

Bivianoetal.1996)andbyadaptive-kerneltechniques(e.g.

Kriessler&Beers1997;Bardellietal.1998a,2001).

Theperformancesofseveraldifferentmethodshavebeen

evaluatedbothusingnumericalsimulations(e.g.Mohretal.

1995;Croneetal.1996;Pinkneyetal.1996;Buote&Xu

1997;Cen1997;Valdarninietal.1999;Knebe&M¨uller2000;

Bivianoetal.2006)andalsobyapplyingdifferentmethods

tothesameclusterdata-setsandexaminetheresultdiffer-

ences(e.g.Escaleraetal.1992,1994;Mohretal.1995,1996;

Kriessler&Beers1997;Faddaetal.1998;Kolokotronisetal.

2001;Schueckeretal.2001;Lopesetal.2006).Generally

speaking,thesensitivityofsubstructuredetectionincreaseswith

bothincreasingstatistics(e.g.moregalaxiesormoreX-raypho-

tons)andincreasingdimensionalityofthetest(e.g.usinggalaxy

velocitiesinadditiontotheirpositions,orusingX-raytempera-

tureinadditiontoX-raysurfacebrightness).

Previousinvestigationshavefoundverydifferentfractions

ofclusterswithsubstructureinnearbyclusters,depending

onthemethodandtracerusedforsubstructuredetection,on

theclustersample,andonthesizeofsampledclusterre-

gions(e.g.Geller&Beers1982;Dressler&Shectman1988;

Mohretal.1995;Girardietal.1997;Kriessler&Beers1997;

Jones&Forman1999;Solanesetal.1999;Kolokotronisetal.

2001;Schueckeretal.2001;Flin&Krywult2006;Lopesetal.

2006).Althoughthedistributionofsubclustermasseshasnot

beendeterminedobservationally,itisknownthatsubclusters

of∼10%theclustermassaretypical,whilemoremassive

subclustersarelessfrequent(Escaleraetal.1994;Girardietal.

1997;Jones&Forman1999).Thesituationisprobablydif-

ferentfordistantclusterswhichtendtoshowmassivesub-

structuresmoreoftenthannearbyclustersclearlysuggesting

hierarchicalgrowthofclusterswasmoreintenseinthepast

(e.g.Gioiaetal.1999;vanDokkumetal.2000;Hainesetal.

2001;Maughanetal.2003;Huoetal.2004;Rosatietal.2004;

Demarcoetal.2005;Jeltemaetal.2005).

Additionalevidenceforthehierarchicalformationofclus-

tersisprovidedbytheanalysisofbrightestclustergalaxies

(BCGshereafter)insubstructuredclusters.BCGsusuallysit

atthebottomofthepotentialwelloftheirhostcluster(e.g.

Adamietal.1998b).WhenaBCGisfoundtobesignificantly

displacedfromitsclusterdynamicalcenter,theclusterdisplaysevidenceofsubstructure(e.g.Beersetal.1991;Ferrarietal.

2006).FromthecorrelationbetweenclusterandBCGluminosi-

ties,Lin&Mohr(2004)concludethatBCGsgrowbymerg-

ingastheirhostclustersgrowhierarchically.Therelatedevo-

lutionofBCGsandtheirhostclustersisalsosuggestedbythe

alignementofthemainclusterandBCGaxes(e.g.Binggeli

1982;Durretetal.1998).BoththeBCGandtheclusteraxes

arealignedwiththesurroundinglargescalestructuredis-

tribution,whereinfallinggroupscomefrom.Theseinfalling

groupsarefinallyidentifiedassubstructuresoncetheyenter

theclusterenvironment(Durretetal.1998;Arnaudetal.2000;

West&Blakeslee2000;Ferrarietal.2003;Plionisetal.2003;

Adamietal.2005).Hence,substructurestudiesreallyprovide

directevidenceforthehierarchicalformationofclusters.

Concerningtheimpactofsubclusteringonglobalcluster

properties,ithasbeenfoundthatsubclusteringleadstoover-

estimatingclustervelocitydispersionsandvirialmasses(e.g.

Pereaetal.1990;Bird1995;Maurogordatoetal.2000),butnot

inthegeneralcaseofsmallsubstructures(Escaleraetal.1994;

Girardietal.1997;Xuetal.2000).Duringthecollisionofa

subclusterwiththemaincluster,boththeX-rayemittinggas

distributionanditstemperaturehavebeenfoundtobesignifi-

cantlyaffected(e.g.Markevitch&Vikhlinin2001;Cloweetal.

2006).Asaconsequence,ithasbeenarguedthatsubstruc-

turecanexplainatleastpartofthescatterinthescalingrela-

tionsofoptical-to-X-rayclusterproperties(e.g.Fitchett1988;

Girardietal.1996;Barrenaetal.2002;Lopesetal.2006).

Asfarastheinternalpropertiesofclustergalaxiesare

concerned,thereisobservationalevidencethatahigherfrac-

tionofclustergalaxieswithspectralfeaturescharacteristic

ofrecentorongoingstarburstepisodesislocatedinsub-

structuresorintheregionsofcluster-subclusterinteractions

(Caldwelletal.1993;Abrahametal.1996;Bivianoetal.1997;

Caldwell&Rose1997;Bardellietal.1998b;Moss&Whittle

2000;Milleretal.2004;Poggiantietal.2004;Miller2005;

Giacintuccietal.2006).

Inthispaperwesearchforandcharacterizesubstructures

inthesampleof77nearbyclustersoftheWIde-fieldNearby

Galaxy-clusterSurvey(WINGShereafter,Fasanoetal.2006).

ThissampleisanalmostcompletesampleinX-rayfluxinthe

redshiftrange0.04

spatial,projecteddistributionofgalaxiesintheclusterfields,us-

ingtheadaptive-kernelbasedDEDICAalgorithm(Pisani1993,

1996).InSect.2wedescribeourdata-set;inSect.3wede-

scribetheprocedureofsubstructureidentification;inSect.4we

useMonteCarlosimulationsinordertotweakourprocedure;

inSect.5wedescribetheidentificationofsubstructuresinour

data-set;inSect.6thecatalogofidentifiedsubstructuresispro-

vided.InSect.7weinvestigatethepropertiesoftheidentified

substructures,andinSect.8weconsidertherelationbetween

theBCGsandthesubstructures.Weprovideasummaryofour

workinSect.9.

2.TheData

WINGSisanall-sky,photometric(multi-band)andspectro-

scopicsurvey,whoseglobalgoalisthesystematicstudyofthe

localcosmicvarianceoftheclusterpopulationandoftheprop-

ertiesofclustergalaxiesasafunctionofclusterpropertiesand

localenvironment.

TheWINGSsampleconsistsof77clustersselectedfrom

threeX-rayfluxlimitedsamplescompiledfromROSATAll-

SkySurveydata,withconstraintsjustontheredshift(0.04

0.07)anddistancefromthegalacticplane(|b|≥20deg).Thecore