Mon.Not.R.Astron.Soc.000,1–21(2007) Printed25May2007 (MNLATEXstylefilev2.2) Environmental dependence of AGN activity in the supercluster A901/2 R. Gilmour1,2? , M. E. Gray3, O. Almaini3, P. Best1, C. Wolf4, K. Meisenheimer5, 6 5 C.Papovich and E. Bell 1ScottishUniversitiesPhysicsAlliance,InstituteforAstronomy,RoyalObservatory,BlackfordHill,Edinburgh,EH93HJ,UK 2EuropeanSouthernObservatory,AlonsodeCordova3107,Vitacura,Casilla19001,Santiago19,Chile 3SchoolofPhysicsandAstronomy,UniversityofNottingham,UniversityPark,Nottingham,NG72RD,UK 4DepartmentofPhysics,DenysWilkinsonBldg.,UniversityofOxford,KebleRoad,Oxford,OX13RH,UK 5Max–Plank–Institutfu¨rAstronomie,Ko¨nigstuhl17,D-69117Heidelberg,Germany 6StewardObservatory,TheUniversityofArizona,933NorthCherryAvenue,Tuscon,AZ85721,USA Accepted.Received;inoriginalform ABSTRACT WepresentXMMdataforthesuperclusterA901/2,atz ∼0.17,whichiscombinedwith deepimagingand17-bandphotometricredshifts(fromtheCOMBO-17survey),2dFspectra andSpitzer24µmdata,toidentifyAGNinthesupercluster.The90ksecXMMimagecontains 139 point sources, of which 11 are identified as supercluster AGN with LX(0.5−7.5keV) > 1.7×1041erg cm−2s−1. The host galaxies have M < −20 and only 2 of 8 sources with R spectra could have been identified as AGN by the detected optical emission lines. Using a largesampleof795superclustergalaxieswedefinecontrolsamplesofmassivegalaxieswith nodetectedAGN.ThelocalenvironmentsoftheAGNandcontrolsamplesdifferat>98per ∼ cent significance. The AGN host galaxies lie predominantlyin areas of moderate projected galaxydensityandwithmorelocalbluegalaxiesthanthecontrolsample,withtheexception ofoneverybrightTypeIAGNverynearthecentreofacluster.Theseenvironmentsaresimilar to,butnotlimitedto,clusteroutskirtsandbluegroups.Despitethelargenumberofpotential hostgalaxies,noAGNarefoundinregionswiththehighestgalaxydensity(excludingsome cluster coreswhere emission from the ICM obscuresmoderateluminosity AGN). AGN are also absent from the areas with lowest galaxy denstiy. We conclude that the prevalence of clusterAGNislinkedtotheirenvironment. Keywords: Galaxies:clusters:individual:A901/2-Galaxies:active 1 INTRODUCTION etal. (2003)findthesameresultforstar-formationrate.Wolfetal. (2005)findthatdustystar-forminggalaxiesaregenerallyfoundin Thepropertiesandevolutionofgalaxiesareknowntobestrongly moderatedensityenvironments. linked to their external environment. In particular, populations of galaxiesinclustersarestrikinglydifferenttothoseinthefield,as Variousprocesseshavebeensuggestedtoaccountfortherapid shownbythemorphology–densityrelation(Oemler1974,Dressler transformationofclustergalaxies,fromlocaleffectssuchasmerg- 1980)anddramaticchangesinstar-formationrates(e.g.Lewisetal. ers(e.g.Mihos&Hernquist1996)andrepeatedclosegravitational 2002 and Go´mez et al. 2003). It appears that galaxies change encounters(e.g.Mooreetal. 1998) tothelargescaleeffectslike significantlyastheyjoindenserenvironmentssuchasgroupsand the tidal field (e.g. Byrd & Valtonen 1990) and the intra-cluster clusters. medium, viaram-pressure stripping (e.g. Abadi et al. (1999)) or Itisincreasinglyevident thatmanyofthechangesingalaxy ‘strangulation’(e.g.Larsonetal. 1980). propertiesbetweenclustercoresandthefieldaretriggeredininter- The processes which affect galaxy properties may also, di- mediatedensityenvironments,andthatadistinctionbetweenfield rectlyorindirectly,affecttheaccretionontothecentralblackhole and cluster populations is overly simplistic. For example, Wake foundinmost,ifnotall,galaxieswithastellarbulge(e.gMagor- et al. (2005) find that galaxy colour isa function of local rather rianetal. 1998).Bothlocalandlarge-scaleprocesseswhichmay thanextendedgalaxydensity,andLewisetal. (2002)andGo´mez affectclustergalaxiesalsohavethepotentialtoaffectthedistribu- tionofgasinthegalaxies,andhencemaytriggerorsuppressAGN ? e-mailrgilmour@eso.org activity.Recentlylargeandmoderatesizedsurveyshavebegunto (cid:13)c 2007RAS 2 R. Gilmouret al. shed light on the local and extended environments of AGN, and Giventhatmanygalaxytransformationsoccurinintermediate produceobservationalevidenceforsomeoftheseprocesses. densityenvironments, itmaybethatAGNarealsoalteredbythe The first evidence of a suppression of AGN in the cores of hostgalaxyenvironment.TounderstandthelinksbetweenAGNac- galaxy clusters was found in the optical survey of Dressler et al. tivityandtheirextendedenvironmentitisdesirabletolookbeyond (1984), although a large source of bias was suggested by Edge galaxyclustersasasingleentity,andinsteaddeterminetheeffect (1994).Furtheropticalsurveys(suchasColdwelletal. 2002and oflocal(∼100kpc)andlargescale(∼1Mpc)environment,from Kauffmannetal. 2004)havealsofoundadeficitofluminousAGN thefieldthroughgroupsandclusteroutskirtstotheclustercores. indenseregions.HoweverMilleretal. (2003)findthatthefraction Superclustersareidealtestbedsforsuchastudyastheyconsist ofluminousgalaxieswithAGNisindependent ofgalaxydensity, ofalargenumberofgalaxiesinarangeofenvironments,butatthe a conclusion also drawn from the auto-correlations of AGN and sameepoch.ThecorrelationsbetweenenvironmentandAGNprop- galaxies presented by Wake et al. (2005). Surprisingly, some of ertiescanthereforebestudiedinonefield,withoutcomplications these studies use the same datasets, but draw contrasting conclu- duetogalaxyorAGNevolution.Forexample,theAGNpopulation sions,probablyduetodifferentAGNselectiontechniques. ingalaxygroupscanbecomparedtothatinclusteroutskirtsofsim- The picture is further complicated when detections in other ilarlocalgalaxydensitytodistinguishbetweenlocalandlargescale wavebandsareconsidered.Incontrasttotheopticalresults,many environments.Inadditionsuperclusterscontainbothdisturbedand radio studies (e.g. Best et al. 2002, Miller & Owen 2003, Barr relaxedregions,whichmayaffectAGNindifferentways. etal. 2003,Reddy&Yun2004,andBest2004)showanincrease This paper presents the results of investigations into AGN in radio-loud AGN activity in galaxy clusters, at a range of red- inthesuperclusterA901/2,whichhasextensivemulti-wavelength shiftsand in both relaxed and merging systems. Best (2004) find imagingandspectroscopy, summarisedinSection2.Thedatare- thatthemajorityofradio-loudAGNinthedensestregionsarenot ductionandidentificationofthesuperclusterAGNaredescribedin emission-linesources,andsomaybemissedbyopticalstudies. Sections3and4.ThepropertiesoftheAGNandtheirhostgalax- However, radio-loud AGN arenot representative of AGN as iesareinvestigated,alongwiththeenvironmentsinwhichtheyare awhole,andopticalstudiesarepronetoselectioneffects:studies found, and the environments of the AGN hosts are compared to of X-ray emission are an alternative method to remove some se- othersuperclustergalaxiesinSection5.Theresultsaresummarised lectioneffects, andtodetectalargerpopulationof AGN.Indeed, in Section 6. Details of individual AGN candidates are given in Martini et al. (2006) find that only four of at least 35 X-ray de- theAppendix.Throughout thepaperthecosmologicalparameters tectedAGNinasampleofclustershaveopticalspectralsignatures Ωm,ΩΛ andH0aresetto0.3,0.7and70kms−1Mpc−1,andall ofAGNactivity.ThemajorityofstudiesintheX-rayhavefocused COMBO-17absolutemagnitudesareconvertedtothiscosmology. ongalaxyclusters,whichhavetheadvantageofalargenumberof galaxiesandhighdensity,butarecomplicatedbytheX-rayemis- sionfromtheintra-clustermedium.Thistendstomaskanydetec- 2 THESUPERCLUSTERA901/2 tionsofAGNintheverycentreofthecluster,inparticularinthe 2.1 Opticaldata centralgalaxy,ifoneexists,whichisoftenamassiveellipticaland radioloud(e.g.Peresetal. 1998,Bestetal. 2006).Theexclusion ThesuperclusterconsistingofAbell901andAbell902(A901/2), of such galaxies may in fact be an advantage as they are in very firstidentifiedbyAbell(1958),isidealforastudyoftheeffectof different environments fromtheother cluster galaxies, frequently environmentonAGNduetothelowredshift(∼ 0.17)andwealth lyinginthecentreofthepotentialwell.Thisunusualenvironment ofopticaldataavailable.ItisoneofthefieldsintheCOMBO-17 probablyhasaverydifferenteffectonAGNactivitycomparedto survey(ClassifyingObjectsbyMedium-BandObservations in17 theotherclustermembers, anditisthereforepreferabletodistin- Filters, Wolf et al. 2003), and in addition 2dF spectra are avail- guishbetweencentralandnormalgalaxieswhenevaluatingtheen- ablefor282superclustergalaxies,fromobservationswiththetwo vironmentsofAGN. degreefield(2dF)spectrographontheAnglo–Australiantelescope. Statisticalstudiesofpointsourcesinthefieldsofgalaxyclus- TheCOMBO-17surveyusedtheWideFieldImager(WFI)at tershavefoundnumerousclustersthathavemoreluminouspoint theMPG/ESO2.2mtelescopetoobtainimagesofa0.56×0.55 sourcesthanexpectedfromanon-clusterfield(e.g.Henry&Briel degree field with a pixel size of 0.23800. Images were taken in 5 1991,Lazzatietal. 1998,Molnaretal. 2002,Cappietal. 2001, broadand12narrowbandfiltersandmatchedtoasetoftemplate Johnsonetal. 2003,Cappellutietal. 2005andRuderman&Ebel- spectratodeterminephotometricredshifts(z ).Reliablephoto- phot ing2005andGilmouretal.inprep.)andwhicharethereforelikely metricredshiftswerefoundforthe∼18000objectswithm <24, R tocontainAGN.Othersstudiesfindclusterswhichappeartohave with errors of order σ /(1 + z) < 0.01 (which is comparable z noexcessofsources(e.g.Molnaretal. (2002),Kimetal. 2004a to the velocity dispersion of the supercluster) at m < 20, and R and2004b),andthereforenodetectable(generallymoderatelumi- σ /(1 + z) < 0.02 for m < 23 (Wolf et al. (2005), here- z R nosity)AGN. after WGM05). The accuracy of the photometric redshifts when Martinietal. (2006)confirmspectroscopicallythateightlow- comparedtotheavailablespectroscopicredshiftsisshownbyWolf redshiftclusterseachcontainbetween2and10X-raysourceswith et al. to be good, such that it is possible to select a magnitude- L > 1041ergs−1,themajorityofwhichareAGNwithoutopti- limitedsupercluster samplewithminimal contamination fromin- X calemissionlines.Thiscorrespondsto5percentofgalaxieswith terlopersandonlyafewpercentlossoftruesuperclustergalaxies. M < −20 hosting AGN with L > 1041erg s−1, which is a Acutof0.155 0.2 counts s−1 for A901b.Atailofsmaller,bluergalaxiesextendssouthofA901bto- MOS1andMOS2detectorsand>0.67countss−1forPNresulted wardsA902,whichisamoreopticallydiffusecluster.Thereisalso inanexposuretimeof∼ 67ksecforMOSand∼ 61ksecforPN, agroupofgalaxiesinthesouth-westcorner,andopticaldataand andremovedallepisodesofsignificantflaring. 3Dweaklensinganalysis(Tayloretal. 2004)hasidentifiedaclus- teratredshift∼0.5almostdirectlybehindA902.Itisclearthatthe superclustercontainsawiderangeofenvironmentswithdiffering 3.2 Sourcedetection ratiosofdarktoopticallyvisiblematter.Theeffectoftheseenvi- ronmentsonthegalaxystar-formationratehasbeeninvestigatedby SourcesweredetectedusingWAVDETECT(Freemanetal. 2002) Grayetal. (2004),whofoundthattheproportionofgalaxiesthat on600x600pixelunvignettedfull-bandimagesforeachdetector. arestar-formingisastrong function of local dark-matter density, Theimagesandthecorrespondingexposuremapshadapixelsize withfarlessstar-formationingalaxiesindenserregions.Inaddi- of4.100.Amaskwascreatedforeachdetector,whichremovedareas tion,theWGM05studyfound apopulationof dustystar-forming withlessthan25percentofthemaximumexposureoranexposure galaxieswhichpreferentiallyexistinmediumgalaxy-densityenvi- mapgradientofover0.4forMOSor0.03forPN.Threeareasof ronments,avoidingboththefieldandtheclustercores. streakingwereremovedbyhandinthePNmask. 2dF spectra are available for 282 of the brightest galaxies 102sourcesweredetectedintheMOS1image,96inMOS2 in the supercluster, and cover the wavelength range 3900–6000A and128inPN.Thetotalnumberofuniquesourcesdetected,with- (Grayetal.inprep.).TheCOMBO-17SEDsand2dFspectraare outapplyinganycutonsourcesignificance,was150(ofwhich64 notsufficienttocompileasampleofAGNasmanyAGNareop- were detected in all three images, 33 intwo and 53 in one). The ticallyobscured.Incomparison,X-raysamplesarefarmorecom- vast majority of those missed inone or twoimages wereoutside plete(seeforexampleMartinietal. 2006andSzokolyetal. 2004) the fieldof view of those detectors, or only detected in the more butsufferfromconfusionwithheavilystar-forminggalaxies.Com- sensitivePNimage. bininganX-raysourcelistwiththe2dFspectraandCOMBO-17 Apointsourcecataloguewasconstructedforeachdetectorby data can help identify supercluster X-ray sources and distinguish removingalldetectionsofextendedsuperclusteremission.Asthe between X-ray emission from AGN and that from other sources sizeandshapeofthePSFisnotwelldefinedinXMMtwomethods suchasstar-formationandpopulationsoflowmassX-raybinaries. wereusedtodeterminewhichwerepointsources: Comparing the positions of AGN hosts with the other identified • The FWHM was found for each object. As the sources be- supercluster galaxies will determine whether AGN activity isen- comeincreasinglyellipticaltowardstheedgeoftheimage,itwas hancedorsuppressedinarangeofenvironments. requiredthatthesemi-minoraxishadaFWHMof<3pixels.This includesallbrighton-axispointsources,whichhaveaFWHMof 2.2pixels,andallowssomemarginoferrorforthefaintersources. 2.2 Infra-reddata Thismethodwasonlyusefulformoderatetobrightsources. This field is currently being surveyed using MIPS (Multi-band • The catalogue was compared to the results for this field ImagingPhotometerforSpitzer)onSpitzer.Anearlyreleasecat- from the XCS survey (private communication, see M. Davidson, alogueofthe24micronsourcesinthisfield(1/7ofthefinaldata, inprep.). Thissurvey uses asophisticated wavelet reconstruction (cid:13)c 2007RAS,MNRAS000,1–21 4 R. Gilmouret al. methodtofindextended emissioninXMMimages.Duetoprob- intwoimages;howeverithasaverylargepositionalerror,andfor lemswiththerawdatasetthismethodcouldonlyusetheMOSdata consistencyandaccuracywasnotincludedinthecatalogue. intheNEquarteroftheimage,whereasalldataareusedintherest Thefinallistof139significantpointsources,includingposi- of the image. Detections of extended emission are therefore less tionalerrors,isgiveninTable2andshowninFigure2. accurateintheNEquarter. Theresultsofthesemethodsarebroadlyinagreement,within the errors described, and identified eight areas of possible super- 4 FINDINGTHESUPERCLUSTERACTIVEGALACTIC cluster emission, shown in Figure 2. Of these, the smallest three NUCLEI arelikelytobeartifactsastheyalloccurnearthechipboundaryof 4.1 MatchingX-rayandOpticalcatalogues oneimageonly.Allofthesesourceswereremovedfromthecata- logue.AnalysisoftheextendedemissionwillbecoveredinGray TheCOMBO-17catalogueconsistsof63776objectsdetectedus- etal.,inprep. ingSExtractorontheR-bandimage(Wolfetal. 2003).Thesewere A further consideration is the brightest source in the field, matchedwiththeXMMpointsourcestoidentifytheX-raysources whichhastheFWHMofapointsource.Asthissourceissobright in the supercluster. Some saturated stars and fainter objects near (similar influx to theX-ray emission from A901b) and liesvery diffraction spikes are not included in the COMBO-17 catalogue, closetothecentreofA901a,itcouldbeconcentratedclusteremis- sotheareasaroundeachX-raysourcewereexaminedmanuallyfor sionoracoolingflow.Thesescenarioswereruledoutbyanalysis missingobjects,andfoursuchopticalobjectswhichcouldpossibly ofthespectrum,whichisapowerlawratherthanthermalandthe matchanX-raysourcewereaddedtothecatalogue. fact that the X-ray emission is centred on a galaxy which is not A maximum-likelihood technique was used to match the X- thebrightestclustergalaxyandwhichhasradioemission(fromthe ray sources to the COMBO-17 optical catalogue. Matching was NVSS,Condonetal. 1998).Itisthereforeconcludedthatthisob- performed by comparing the value LR (a measure of the as- i,j jectisanAGN. sociation between two sources, i and j, see Equation 1) with the distributionofthisvalueforX-raysourcesplacedrandomlywithin the field. Because LR depends on the error on the source co- i,j 3.3 Pointsourceproperties ordinates, which varies significantly in the X-ray sample, the ex- pecteddistributionofLR wascalculatedforeachX-raysource, Thereality,positionandpositionalerrorofthesourceswerefound i,j j,byrandomlyplacing14000X-raysourceswitherrorσ overthe bycomparingthesourcesfoundineachofthethreedetectors.For j opticalcatalogue.Theresulting(normalised)distributionN(LR) sources that were detected in more than one image the position j givestheprobabilityofobtainingeachlikelihoodratiobychance was defined as the midpoint of the two closest positions for that (ifsourcejhadnoopticalcounterpart),asshownforonesourcein source.(Inthemostcommoncaseofsourcesdetectedinallthree Figure3. imagesthisremovederrorsduetoonedetectionbeingnearachip Thelikelihoodratiowasdefined(followingMannetal. 1997 boundary.)Forsinglydetectedsourcesthegivenpositionwasused. andTaylorE.L.etal. 2005,whouseamethoddescribedindetail TheWAVDETECT1-sigmapositionalerrorswerefoundtobe inSutherland&Saunders1992)as generallylessthanonearcsecondevenforthefaintestsources.In comparison,theseparationsbetweendetectionsofthesamesource e−ri2,j/2σj2 in different images were on average 3 arcseconds, which is just LRi,j = σ2N( 3inatleastoneimagewereremoved,wherethesignifi- larmagnitude.TheseissuesareimportantwhenafaintQSOisthe canceisgivenby C ,forsourcecountsC andbackgroundcounts onlymatchandisassignedtoolowaprobability.Thiswillleadto σB B. (Thismeans that all sources arenot random fluctuations to at incompletenessintheX-raymatching,butwillnotaffectthesuper- least3σsignificance,butinmostcasesisoverlyconservativeasit clustersample.Inaddition,theprobabilitieswillnotbeaccurateif doesn’ttakeintoaccountthespatialdistributionofphotonswithin morethanonepossiblematchisidentified,andoneofthematches thesourcearea).Thiscutremoved11sources,andalsoagreeswell isaQSOorbrightstar.Thiscanaffectthesuperclustersample,and with the reality determined by eye. At least one of these sources inthesecasestheopticalclassifications,locationsanderrorsofall is real (at ∼09:56:24 -10:01:52, probably matching a z =2.2 thepossiblematcheswereexaminedindetailtodeterminethetrue phot quasarintheCOMBO-17catalogue),asitwasmarginallydetected sourceoftheX-rays. (cid:13)c 2007RAS,MNRAS000,1–21 EnvironmentaldependenceofAGNactivityin thesuperclusterA901/2 5 Figure2.Theidentifiedsourcesinthesuperclusterfield.Sourcesmarkedwitharectanglearepossibleextendedemission.Pointsourceswithsignificance C >3aremarkedwithanellipse,andthosedrawninboldarepossiblesuperclustermembers.TheemissionintheregionofA901aisaverybrightpoint σB sourceratherthanextendedemission.Thesourcesareplottedoveracombined,vignettingweightedimage,andsomesourcesarenotvisiblebyeyeduetothe imagecombinationandscale.Theimageis∼300indiameterandthetopleftcornerisNorth-East. In addition this technique compares likelihoods to the aver- ageoverthefieldandignoresanyclustering,whichgivesslightly higher likelihood values for X-ray sources in the line of sight to cluster centres, andslightlylower for those inareaswithfew superclustergalaxies.Thisisveryunlikelytochangeanyresults,es- peciallyasthecatalogueisdominatedatallopticalmagnitudesby non-supercluster objects(> 80percentofopticalobjectsarenot inthesuperclusteratm <20,and>93percentatm <24). R R Thestandardmethodofcalculatingthereliabilityofamatch requiresalargesampleofsources,soinsteadthereliabilityforeach X-ray–optical pairi,j wasdefinedastheprobability ofnot ob- tainingLR randomly, i,j ΣN(LR >LR ) R =1− j i,j . (2) i,j 14000 Most X-ray sources have more than one potential optical counterpart,aswellasasignificantprobabilityofhavingnomatch, Figure3.TheexpecteddistributionoflikelihoodratiosforoneX-raysource suchthatthereliabilitiessumto> 1.Theprobabilitythatoptical basedon14000trials(solidline),cumulativeexpecteddistribution(dashed objectiisthetruecounterparttoj,(P )andtheprobabilitythat i,j line) and thehistogram ofactual likelihood ratios forthe optical objects thereisnocounterpart (P ),givenasetofpossiblecounter- none,j nearthissource(thevastmajorityofoptical objects havelog(likelihood) parts,k,arecalculatedfollowingRutledgeetal. (2000), <1).Onepossiblematchisidentified,withareliabilityof∼0.8. P = Ri,jQMk6=i(1−Rk,j) (3) i,j S Aliberalcatalogueofpossibleandsecurematcheswascon- P = QMk=1(1−Rk,j) (4) structed.Thethresholdsusedweredeliberatelylooseasmanyfac- none,j S torscouldincreasethelikelihoodofamatchonacasebycasebasis. whereSisanormalisationfactorsothattheprobabilitiessumto1, Theseincludematchingwithotherwavelengths,visualinspection andM isthenumberofpossibleopticalcounterpartstotheX-ray oftheshapeandcentringoftheX-raypointsource,andcharacter- source. isticsoftheopticalcounterpart,forexampleifitisafaintquasar. (cid:13)c 2007RAS,MNRAS000,1–21 6 R. Gilmouret al. Inaddition,someoftheX-raysourceshavesuchlargeerrorsthat identifyopticalmatchesinpreviouslyambiguouscases.For97X- thechanceofarandomassociationwillalwaysbehigh,evenifthe raysourcestheadditionofthe24µmdataconfirmedtheresultof matchappearstobeexcellent.Thereforeallmatcheswhichfulfill thepreviousmatching,including49caseswhereneitherthe24µm thecriteriadescribedbelowareincludedinTable2,andtheaddi- data nor the optical catalogue gave a good match. This includes tionalfactorswhichmayaffecttheprobabilityofamatcharelisted caseswherethe24µmdataconfirmtheexistenceoftwopossible inthe‘Notes’ column. Thepossible supercluster AGN areevalu- matches, and cases where two 24µm sources correspond well to atedonacasebycasebasis. oneX-rayandoneopticalobject(andappeartobemultipledetec- Forsourceswithonlyonelikelyopticalcounterpart,amatch tionsofnearbygalaxies).Forafurther14X-raysourcesthe24µm isdefinedasP > 0.8,whichisconfirmedbyvisualinspection. matching identifies at least one of the optical counterparts but at i,j Forsourceswithmorethanonepossiblecounterparttheconditions a lower probability (between 0.65 and 0.8). Four X-ray sources for object i to be a unique counterpart given a set of options, k, hadasecure 24µmmatchwithnooptical counterpart, and20X- werePkPk,j > 0.8andPi,j/Pk6=iPk,j > 4.Thisresultedin raysourceshadanopticalcounterpart butnolikely24µmmatch. 66secureidentificationsoutof139sources,ofwhichfewerthan6 ForfourX-raysourcesthe24µmdatachangedtheassignedoptical areexpectedtoberandomassociations.Forsourceswithmultiple match, by eliminatingor confirming possible optical objects, and possible counterparts (PkPk,j > 0.8 and Pi,j/Pk6=iPk,j 6 thesesourcesareflaggedinTable2. 4)allopticalobjectswithP > 0.15wereincludedaspossible ThefinallistofX-raysourceswithuniqueandmultiplecoun- k,j matches.Thisresultedin17sourceswithtwopossiblecounterparts terparts is given in Table 2, which lists the X-ray IDs, positions, and3sourceswiththreeoptions. positional error and count rates, and the possible COMBO-17 matches, optical position and photometric redshift. Theprobabil- itiesand reliabilitiesof theoptical matches aregiven to allow an 4.2 Matchingwiththe24-microndata evaluation of the accuracy and uniqueness of each match. The TheSpitzer24microncatalogueof1194sourcesintheX-rayfield 24µmfluxofanymatchingSpitzersourcesandthecombinedprob- ofviewwasusedtoimprovetheX-raytoopticalmatching.These abilityoftheX-ray–SpitzermatchandtheSpitzer–Opticalmatch dataareuseful asAGNoftenhaveinfra-redemission,butalsoin bothbeingtruearealsogiven. a purely statistical sense there are far fewer Spitzer sources than opticalobjectsinthefieldofview,sotheprobabilityofarandom 4.3 CriteriaforidentifyingsuperclusterAGN association is far lower. As the 24µm sources have smaller posi- tionalerrorsthantheX-raysources,andarerarerthantheoptical To identify the AGN in the supercluster it is necessary to use objects, amatch between an X-ray and 24µm source can signifi- thephotometric and, if possible, spectroscopic redshifts from the cantlyimprovetheaccuracyoftheopticalmatch. COMBO-17 survey. A cut of 0.155 < z < 0.185 was used phot Asimplelikelihoodratiotest,followingthemethodfortheop- toensurethatallAGNassociatedwiththesuperclusterwereiden- ticalcatalogue,produced81uniquematchesbetweentheX-rayand tified(seeWGM05fordetailsoftheredshiftcut).Thisrangealso 24µmsources,and20X-raysourceswithmorethanonepossible allowsfortheerrorsinthephotometricredshifts,whichmayalso 24µmcounterpart.ThesameprobabilitycutsastheX-raymatch- beaffectedbytheAGNemission.Itisfoundthatadding/subtract- ingwereapplied,andthelowsurfacedensityof24µmandX-ray ingtheCOMBO-17redshift errorforeachX-rayemittingsource sourcesmeanthatveryfewfalsematchesareexpected.Examina- does not reveal any more possible supercluster X-ray sources. In tionoftheopticalimagesforthefewX-raysourceswithmorethan addition some galaxies have bimodal photometric redshift distri- onepossible24µmcounterpartshowsthatinmostcasesthe24µm butions,sothesecondchoiceredshiftswerecheckedandnoextra ‘pointsources’arelikelytobemultipledetectionsofnearbygalax- superclusterX-raysourceswerefound. ies,whicharesignificantlylargerthanthe24µmPSF. The presence of an AGN may cause the template fitting in The24µm sources werethen matched withtheoptical cata- theCOMBO-17surveytogivewrongphotometricredshifts,asthe logue.Theerrorsforthe24µmcatalogueweretakenas1.200,which COMBO-17templatesdonotincludeSeyfert-likespectrawithboth is half a pixel. This will underestimate the errors on faint 24µm AGN and host galaxy contributions. To check for missed super- sources,andgiveamoreconservativecatalogue.Theadvantageof cluster X-ray sources we examined all optical counterparts with takingthesameerrorsforallsourcesisareductionincomputing 21>m >17.75(betweenthefaintestsuperclusterX-raysource R timeasonlyoneexpecteddistributionneedstobecalculated.Inad- and the brightest supercluster galaxy), and B −R < 2.3 (on or dition,asthesamplewaslargeenough,atruereliabilitywascalcu- bluerthanthesuperclusterredsequence)whichwereclassifiedas latedbycomparingthelikelihoodratiodistributionforthesample galaxiesaccordingtotheirtemplatespectra.1.Thephotometricdata (Ntrue(LR))withthatof10randomcatalogues(Nrandom(LR)) forthesegalaxiesweremanuallycomparedtospectraltemplatesat followingthemethodofTaylorE.L.etal. (2005).Thereliability thesuperclusterredshift.Twoopticalcounterparts(COMBOcata- isdefinedasafunctionoflikelihoodratio, loguenumbers12953and41435, matchingX-raysources#3and N (LR)−N (LR) #135) were found to fit well with templates at z ∼ 0.16 despite R(LR)= true random . (5) having different photometric redshifts in the COMBO catalogue. N (LR) true ThesearediscussedindetailintheAppendix. ProbabilitiesforeachpossiblematchwerecalculatedusingEqua- X-rayemissionoftheluminositiesseeninthissamplecould tions 3 and 4, and the criteria for unique and multiple matches be caused by a large population of low mass X-ray binaries usedintheX-raymatchingwereapplied.The24µmsourcesthat (LMXBs),hot coronae of massive galaxies, or highlevelsof star matched the X-ray catalogue were examined by eye to identify those which had good optical matches, but were rejected due to underestimatedpositionalerrorsinthe24µmdata. 1 Photometricredshiftsofobjectsclassedashighredshiftquasarsareac- The combined probability of the X-ray – 24µm – optical curateasthechanceofagalaxyatmR <24beingmistakenforaquasar match was used to identify the 24µm counterparts and to help isverysmall(Wolfetal. 2004). (cid:13)c 2007RAS,MNRAS000,1–21 EnvironmentaldependenceofAGNactivityin thesuperclusterA901/2 7 formation,aswellasAGNactivity.EmissionfromLMXBsisruled • X-ray Hardness Ratio – A good indication of the spec- out followingthe method of Martini et al. (2006), who compare tral properties and absorption of X-ray sources is given by the theirobservationstothetightrelationbetweenB-bandgalaxylumi- luminosity hardness ratio, HR = H−S, where H = L H+S 2−8keV nosityandthetotalX-raybroadbandluminosityfromallLMXBs and S = L . Sources with HR > 0.8 are unlikely be 0.5−2keV (Kim&Fabbiano2004).EvenwithoutcorrectingforthewiderX- star-formingduetotheverylargeamountsofabsorptionrequired raybandusedinthisrelation(0.3−8keVcomparedto0.5−7.5keV (Mainieriet al. 2002) (unlesstheemissionisdominatedbyhard fortheA901/2sources)theX-rayemissionfromthepossiblesu- X-raybinaries),andsourceswithHR > −0.2aremorelikelyto perclusterX-raysourcesisatleastafactorof6,andmedianfactor beAGNthanstar-forming(Szokolyetal. 2004).Hardnessratios of 32 higher than the Kim & Fabbiano average relation. This is werecalculatedfromthebackgroundsubtractedimageswhichwill significantlyhigherthanthescatterintheirobservations.Emission be described in detail in the paper on extended emission in this fromhotcoronaeisalsohighlyunlikelytobethecauseoftheX- field. rayemission,astheupperlimitontherelationshipbetweenB-band • OpticalLineRatios– Lineratiosinopticalspectracandis- and X-ray luminosity for such sources is very similar to that for tinguishbetweenstar-forminggalaxiesandAGN.Asthe2dFspec- LMXBs(Sunetal. 2007)andtheA901/2sourceshavefarhigher traarenotfluxcalibratedonlythe[OIII]andHβlineswereused,as X-raytoB-bandluminosityratios. theyareclosetogetherinwavelengthsoconsideringthelineequiv- To distinguish between X-ray emission from high levels of alentwidths,andassumingaflatcontinuumspectrum,willnotin- starformationandthatfromAGN,variousmethodswereusedde- troduceoverlylargeerrors.Mostofthe2dFspectraoftheoptical pendingontheinformationavailableforeachsource.Usedalone counterpartshaveveryfaintornolinessoonlyupperlimitscanbe mostofthesemethodscannotdistinguishabsolutelybetweenstar- measured. Lamareille et al. (2004) compare the classification of forminggalaxiesandthosewithAGN,butcombiningtheavailable emissionlinegalaxiesusingthetraditionalBaldwinetal. (2003) datacangiveareliableindicator ofthenatureoftheX-rayemis- diagnosticmethod,andamethodbasedononlyblueemissionlines sion. (λ > 3700). Their results show that if [OIII]λ5007/Hβ > 5.5 ∼ then the object is almost certainly an AGN. However if the ratio • X-ray Luminosity – Star forming galaxies generally have is< 5.5 theobject could stillcontain someAGN activity, and if low X-ray luminosities. A source in the local universe with no lines are visible it may be an obscured AGN, so this method L0.5−8keV >3×1042ergs−1isextremelyunlikelybepurelystar canconfirmthepresenceofanAGNbutcannotruleoutanyAGN forming (Bauer et al. 2004), and any source with L0.5−8keV ∼> activity. 1×1041ergs−1islikelytobeanAGN(seeFigure7ofBaueretal. –mostsourceswithL > 1041 whicharenotnearthefluxlimit X are securely identified as AGN by their column density or hard- 4.4 DetailsofsuperclusterAGNcandidates ness ratio). Luminosities were calculated using aperture photom- Afterapplyingtheredshiftcut,thecandidatesforsuperclusterAGN etry on images with the mean background subtracted. The back- arereduced toelevenoptical matcheswithsecure photometricor ground subtraction process followed the method of Arnaud et al. spectroscopicsuperclusterredshifts,twomatcheswithrevisedpho- (2002),andwillbedescribedintheforthcomingpaperontheex- tometricredshiftsandthreelesssecurematcheswithconfirmedsu- tended emission (Gray et al. in prep.). The redshift of the super- perclustergalaxies.ElevenofthecandidatesuperclusterAGNhost clusterAGN(∼0.17)meansthattheobserved0.5–7.5keVcounts galaxieswereobservedwith2dF,andfivedonothavespectra. canbeconvertedintoanemitted0.58–8.7keVluminosity.Asthis The details of the candidates are given in the Appendix and studydoesnotrequireveryaccurateluminosities,andastheerrors inTable1,includingtheirX-rayandopticalproperties,imagesof onthecountratesarelarge,thisistakenasanapproximationtothe thehostgalaxiesandspectrawhereapplicable.Thespectralenergy 0.5–8keVluminosity.Thetrueluminositiesmaybeslightlyhigher, distribution(SED)ofthesuperclustergalaxiesisgivenineachcase butthisdependsontheX-rayspectrum. aseither‘oldred’,‘dustystar-forming’or‘blue-cloud’,asdefined • [OII] Star-Formation Rates – If the X-ray emission is intheWGM05samplefromphotometricfitstogalaxytemplates. purelyduetostar-formation,withnoAGNpresent,thenthestar- The morphologies of the host galaxies which are included in the formation rate (SFR) can be estimated from the soft band X-ray sampleofLaneetal. (2007)arealsogiven. luminosityas ThecandidateswereassignedtobesuperclusterAGN,possi- SFR(M(cid:12)/yr)=2.2×10−47L0.5−2keV(W) (6) blesuperclusterAGN,orrejectedoutright.ThesuperclusterAGN aresources #20, #24, #34, #37, #71, #79, #81, #104, #105, #135 (Ranallietal. 2003).IfthereisnoAGNandminimalabsorption and#139.ThepossibleAGNis#3.Thefinalsamplethereforecon- thenthismustbeneartotheSFRderivedfromthe[OII]λ3727line sistsof 11 likely supercluster AGN and 1 possible member. Two flux(Hopkinsetal. 2003); sources,#3and#135,showsignificantvariabilityinthebroadband photometry. Thesearealsothetwomost luminous sourcesinthe L SFR(M(cid:12)/yr)= 2.97×[O1I0I3]3W (7) X-ray,andduetotherapidvariabilitymustbeopticalTypeIAGN. AlloftheAGNhostsappeartobemassivegalaxiesandsome whereL canbeestimatedforthoseobjectswith2dFspectra appear to be morphologically disturbed. The morphologies as- [OII] fromtheequivalentwidthofthelineandtheCOMBO-17magni- signedtotheAGNhostsareindistinguishablefromtheparentpop- tude in the rest-frame Johnson U band. (This method assumes a ulationofluminousclustermembers.Detailedstudyofthesmaller flatspectrumintheUband,butwillgiveanestimateofthefluxat scalemorphologies of theAGNhostsisbeyond thescopeofthis 3727A˚ towithinatleastafactoroftwo,astheUbandmagnitude work,andwillbeleftuntilrecentlyobtainedHubbleSpaceTele- hasminimalcontaminationfromfluxabovethe4000A˚ break). scopeimagesofthisfieldareanalysed. Thefractionof ‘oldred’, • X-ray/OpticalFluxRatio– Sourceswithf /f > ‘dustystar-forming’ and‘blue-cloud’ host galaxies(asdefined in 0.5−8keV R 1areverylikelytobeAGN,andthosewithf /f > 0.1 theWGM05samplefromphotometricfitstogalaxytemplates,and 0.5−8keV R arelikelytobeAGN(seeBaueretal. 2004andreferencestherein). excludingtheverybrightAGNasitissignificantlycontaminated (cid:13)c 2007RAS,MNRAS000,1–21 8 R. Gilmouret al. 2.5 secure AGN 10-2 possible AGN 2.0 n) o gt B-R colour 1.5 X) / L(X,eddin 10-3 L( 10-4 1.0 secure AGN AGN in excluded area possible AGN 0.5 10-5 -24 -23 -22 -21 -20 -19 -18 -23.5 -23.0 -22.5 -22.0 -21.5 -21.0 -20.5 -20.0 R band magnitude MR Figure4.Thecolour–magnitude plot forall supercluster galaxies (small Figure5.Approximate accretion efficiency (fraction ofEddingtonX-ray dotswitherrorbars)andAGNhosts.Totalabsolutemagnitudesareplotted, luminosity)asafunctionofhostgalaxyabsolutemagnitude.Thesolidline witherrorsinthecolourandfluxmeasurement.Theerrorsdonotinclude shows the approximate minimum detectable accretion efficiency at each uncertainties intheredshifts, whichincrease theerrorbarsto±0.1.The point. The X-ray bright source #135 is excluded from this plot, and has areaoftheAGNsymbolsisproportionaltolog(LX(0.5–8keV)).TheX-ray anaccretionefficiencyof∼4percentatMR=−23. source#135ismarkedwithastarasitisnotinthecontrolareadescribed inSection5.2. Thecolour–magnitudediagramforthesuperclusterisshown withAGNopticallight)are0.5,0.2and0.3,comparedtofractions inFigure4,withthesuperclusterAGNhostgalaxiesindicated(it intheintermediatedensitygalaxypopulation of ∼ 0.54, ∼ 0.26 is important here to note that the R band magnitudes of the host and∼0.20,sotheAGNhostsareagainindistinguishablefromthe galaxiesarenotsignificantlychangedbythepresenceofanAGN, parentpopulation. ascanbeseendirectlyfromthe2dFspectra).AlloftheAGNand ItisworthnotingthatallofthesuperclusterAGNcandidates possibleAGNlieinbright(M <−20)galaxies.TheX-raylumi- R areclassedasgalaxiesintheCOMBO-17surveyasthephotomet- nosityoftheAGN,asindicatedbythesizeofthesymbolsinFig- ricmethodisnotsensitivetolowluminositySeyfert-likeAGNand ure4,showsthatthelackofAGNinfaintergalaxiesisnotdueto obviouslymissesopticallyobscuredAGN.Inaddition,ofthe8su- fainterX-rayAGNbeingfoundinopticallyfaintergalaxies,which perclusterAGNwithopticalspectraonly5haveemissionlines,and wouldleadtoAGNinlowerluminositygalaxiesfallingbelowthe onlytwohave[OIII]/Hβratioswhichwouldleadtoaclassification detectionthreshold. Infact,ifanycorrelationexistsitislikelyto asanAGNusingopticaldataalone.Thishighlightsagaintheneed be theopposite –there isan 84 per cent chance that more X-ray forX-raystudiestoinvestigatetheAGNpopulation. luminousAGNarefoundinopticallyfaintergalaxies,accordingto aSpearmanranktest. To find the range of accretion rates covered by this sample, L /L wascalculated for eachAGN, whereitwasas- 5 ANALYSISOFAGNPROPERTIESAND X X,eddington sumedthat10percentofthebolometricluminosityisemittedin ENVIRONMENTS the0.5–8keV band. Therelationlog(MBH/M(cid:12)) = −0.5MR − ThesuperclusterA901/2isverydiverseandcontainsawiderange 2.96ofMcLure&Dunlop(2002)wasusedtocalculatetheblack ofenvironmentswhichmayhaveaneffectonAGNactivity.Eleven hole mass from the rest frame R-band absolute magnitude (M ) R X-raysourcesinthisfieldarelikelytobesuperclusterAGN.Using given in the COMBO-17 catalogue (as derived from the galaxy theCOMBO-17datasetthepropertiesofthehostgalaxiesofthese template,correctedtothecosmologyofthispaper).Theresulting AGNcanbefound,andthenumberofgalaxieshostingAGNcan plot (Figure5, excluding thebright source #135), although crude becalculated.Thisinformationcanbeusedtoconstructacontrol duetotheapproximationsmade,appearsatfirstglancetoshowa sampleofgalaxieswhichappearsimilartoAGNhosts,butdonot correlationbetweenM andL /L (whichwouldnot R X X,eddington havesignificantX-rayemission.Bylookingatnearbygalaxydistri- bechangedbyalteringmanyoftheassumptionsabove,forexample butions,theenvironmentsoftheAGNhostscanthenbecompared the10percentemittedintheX-ray,astheywouldonlychangethe tothose of thecontrol sample to determine whether environment scaleoftheplot).However,therearemoregalaxiesatM ∼−21 R andAGNactivityarelinked. thanM ∼−23andforthefaintergalaxieslowerefficiencyAGN R mayfallbelowthedetectionlimit.Tocalculatewhetherthelackof moreefficientAGNinmoreluminousgalaxiesisduetothesmaller 5.1 PropertiesoftheAGNhosts samplesizeoraphysicaleffectitisnecessarytoknowthenumber TheWGM05superclustersamplecontains795galaxies,whereall ofpossibleAGNhostsasafunctionofMR. galaxies with 0.155 < z < 0.185 and absolute Johnson V phot magnitude < −17 were identified as supercluster members. The 5.2 ThefractionofgalaxiescontainingadetectedAGN largeredshift range (the same asapplied for the AGN inSection 4.3)allowsfortheerrorsinthephotometricredshifts,andthemag- Todetermine theproportion of galaxieswithAGN and theprop- nitudecutremovesfaintobjects,whichhavefarlessaccuratepho- ertiesoftheAGNhostsitisnecessary todefineacontrolareain tometricredshifts. which AGN could have been detected, and compare the galaxies (cid:13)c 2007RAS,MNRAS000,1–21 EnvironmentaldependenceofAGNactivityin thesuperclusterA901/2 9 0.4 tiniet al. (2006), whofounda∼5percent X-raydetectedAGN fractionforgalaxieswithM < −20inlowredshiftclustersata R 1.2) fluxlimitofLX = 1041ergs−1.Figure6alsoshowsanincrease g(L)>4X0.3 isnivAeGgaNlaxfriaecst,ibount(tahbeosvmeatlhlesalmumplienosisziteymliemanits)tihnatinthcreeearsrionrgblyarmsaarse- o N (l verylarge.InthissuperclusterAGNareonlyfoundingalaxieswith G M < −20.4.Asstatedpreviously, theluminositiesoftheAGN A R with 0.2 showthatthisisnotduetothefluxlimitofthesample.Rather,it es appearsthatfainterAGNaremorelikelytobefoundinmorelumi- xi a nousgalaxies,brighterAGNinmoderatelyluminousgalaxies,and al g of 0.1 noAGNintheleastluminousgalaxies. op. Returning to Figure 5, it is now clear that the lack of AGN Pr withL /L >∼3×10−4andM <−22.5isprob- X X,eddington R ablyduetothelackof brightgalaxies,ratherthanatendencyfor 0.0 -23 -22 -21 -20 -19 more luminous host galaxiesto havelower efficiencies: there are MR only19possibleAGNhostgalaxiesabovethisluminosity,andas ∼3percentoffainter(−22.5 10−3onlycorresponds #135andgalaxieswhereAGNactivitycouldnotbeeasilydetectedarenot to∼0.5percentofthegalaxiesinthat0.5magnitudebin,explain- included. ingthelackofbrightergalaxieswithsimilarefficiencies.However theremustbeasignificantdecreaseintheefficiencyofanyAGNin galaxieswithM >−20asthenumberofsuperclustergalaxiesis R withandwithoutanAGNinthisarea.Toselectthecontrolareathe verylargeyetnoAGNareobservedabovetheX-rayfluxlimit. COMBO-17 catalogue wascut toremove objectswithin16000 of thetopandsidesand30000ofthebottomoftheimage.Thiscuten- 5.3 Definingacontrolsample suresthereturnedareais97percentcoveredbytheX-rayimage, andalsoensuresthattheedgeofthecataloguedoesnotaffectprop- To compare the AGN environments and properties, control sam- ertiessuchaslocaldensity.Inadditionareaswherethesensitivity pleswerecreated,consistingofgalaxiessimilartotheAGNhosts, topointsourcesdecreasedsignificantlyduetoextendedsourcesor whereAGNactivitycouldhavebeendetectedbutwasnotfound. very bright X-ray emission, wereremoved fromthecontrol area: Whereasarandomlyselectedcontrolsamplewouldcontainmany three areas were bright enough to obscure moderate luminosity faintgalaxies,itisinsteadpreferabletodefineacontrolsamplewith AGN:a3000circlearoundtheextendedemissiontothenorth-west asimilardistributionofgalaxypropertiesastheAGNhosts.Any of A901a (marked with a rectangle in Figure 2), 6700 around the difference betweenthe AGNhosts’ environments andthe control AGNinA901a(#135)and8300aroundA901b.Intheseregionsthe sampleenvironmentswouldthereforebeduetoanenvironmental noise level is at least 50 per cent higher than for the rest of the effectonAGN. field.Attheedgeoftheexcludedregionsthefaintestdetectedsu- 100controlsamplesweremade,eachconsistingof65ofthe perclusterAGN(1.8×1042erg/sec)wouldbedetectableatlowsig- 183 supercluster galaxies with aperture magnitude m < 20 ap,R nificance,butthesensitivitytowardsthecentreof#135andA901b whichlieinthecontrol areadescribed inSection5.2andarenot decreasestoalevelwhereonlyAGNbrighterthan∼ 1044erg/sec AGNhosts.Thesesampleswereselectedatrandomsuchthatthere wouldbedetectable.Nosourcesaredetectedintheseareas(except areequalnumbersofgalaxiesineach0.5aperturemagnitudebin for#135itself),evenatlowsignificance.Thesmallchangesinde- toreplicatethedistributionofAGNhostmagnitudes.Thismethod tection sensitivity due to emission from A902 and the SWgroup ignores the possible increase in the number of AGN in brighter andthechangesinPSFwerenotincludedastheyonlyaffectvery galaxies(Figure4)butthisislesssignificantwhenaperturemag- marginaldetections,suchthatthefaintestsuperclusterAGNwould nitudesareused,andduetothesmallnumberofgalaxiesselected stillbedetected at low significance, and asmall change insensi- shouldnotaffectanyresultssignificantly. tivity is not significant in a sample of this size. The applied cuts The 100 samples are identical at m < 18.5, due to the ap,R alsoremovetheAGNinA901a(#135)fromthesample,whichis small number of available galaxies, but at 18.5 < m < 20 ap,R necessaryasitobscurespossibleAGNactivityfromallofthesur- different sets of galaxies were chosen, although the samples still roundinglargegalaxiessowillbiasthesample.Inadditionitshigh haveconsiderableoverlapwitheachother.Eachofthe100control accretionefficiencyandluminosityandopticalvariabilityshowthat sampleswascomparedtotheAGNsampleandthemedianstatis- it,andtheonepossiblesuperclusterAGN,aretheonlyX-rayType- tictakentoreducetheerrors.AKolmogorov–Smirnov(K–S)test IAGNinthesample.Thecontrolsamplecontains604supercluster (toidentifychangesinthemean)andKuipertest(aK–Stestusing galaxies,as149wereremovedintheedgecutand42wereinre- Kuiper’sstatisticwhichisbetteratidentifyingchangesinspread) gionswhereAGNcouldnotbedetected. confirmthatthecontrolsamplesandAGNhostsaredrawnfromthe ThenumberofAGNhostsinthesuperclustercanbedirectly sameR-bandmagnitudedistribution(atatleast56percentconfi- compared to the number of possible host galaxies in the control denceforthesuperclusterAGN). samplearea.ThenumberofAGNperpossiblehostgalaxyisshown Thecolour–magnitudediagraminFigure4alsoshowsthatat inFigure6 for arange of host luminosities. Thetotal fractionof leastthreeAGNhostsaresignificantlybluerthantheredsequence. galaxies with M < −20 hosting AGN with L > 1041.2erg K–SandKuiper’stestsonthedeviationfromtheredsequencegive R X s−1is10/253,or∼4percent.ThisissimilartotheresultsofMar- 0.59 and 0.48 probabilities that the supercluster AGN hosts and (cid:13)c 2007RAS,MNRAS000,1–21 10 R. Gilmouret al. -10.0 C E D -10.2 -149.2 RA -149.0 Figure8.Regionswithaclearenvironmentalcategoryinthesupercluster. Regionsareidentifiedbyeye,usingdeepimagesandgalaxypropertiesas Figure 7. Distribution of Σ10 for the control samples (solid line, mean wellasthegalaxypositionsshown.Galaxiesinthe80percentcomplete valueofall100samplesisgiven)andsuperclusterAGNhostgalaxies(filled superclustersamplearemarkedwithasmallcross,andforcomparisonthe histogram).Thecontrolsampledoesnotincludegalaxiesinthreeareasof AGNarealsomarkedbysquares(superclusterAGN)anddiamonds(pos- high X-ray emission, where moderate luminosity AGN could not be de- siblesuperclusterAGN).Thefigureisaround300 by240,withnorth-east tected. TheAGN hostgalaxies havefarlowervalues ofΣ10,andanar- tothetopleft.Thedashedcircleinthetopright-handcorneris1.50inra- rowerspreadofvalues,thanothersimilargalaxies.Thedifferencebetween dius,thesizeusedtodeterminethelocalenvironmentalpropertiesofeach the control samples andAGN (taking the median value) issignificant to galaxy. >0.98usingKuiperstestand>0.99usingaK–Stest. 5.5 ThetypesofenvironmentsthatcontainAGN Thesuperclustercontainsawiderangeofenvironmentswhichare controlgalaxiesaredrawnfromthesamedistribution.Martinietal. evidentbyeye,suchasclusters,groupsandfilamentsofgalaxies. (2002)foundapropensityforAGNhoststobebluerthansimilar InordertobetterclassifytheenvironmentsofAGN,itisdesirable galaxiesintheclusterA2104,butthissampleissosmallthatsuch tofindamethodofdistinguishingbetweentheseenvironmentsin aneffectcannotbeconfirmedhere. termsoftheirproperties.Asafirststep,areaswhichfallintoaclear environmental categorywereselectedbyeye,asshowninFigure 8.Althougharbitrarilydefined,thepropertiesofgalaxiesinthese areascanhelptoworkoutwhatpropertiesdistinguishdifferenten- vironments, andhence todefinetheenvironments ofother super- 5.4 ThelocalgalaxydensitiesofAGNhostgalaxies clustergalaxiesandtheAGNhosts. TheenvironmentsoftheAGNwereevaluatedbycomparingthelo- Theregionsweredefinedasfollows:clusterandgroupregions caldensitiesoftheAGNhostgalaxiestothoseofthecontrolsam- arecirclesof10 radius,centredonthebrightestclustergalaxy.As ples. Thesurface massdensityfromthe weak-lensing analysisin A902doesnothaveaclearcentre,aradiusof1.50isusedtoinclude Grayetal. (2002)wasnotusedasthesampleofAGNissmalland the whole cluster region. For the rich cluster A901a and the SW theylieinareas(outsidecluster cores)wheretheerrorsarequite groupasecondregionoutto30 radiusisconsidered,markingthe large,such thatany resultswould haveverylow significance. In- clusterandgroupedgeenvironments. Thefilamentregion,witha stead,theprojectedgalaxydensitywasused,followingthemethod largenumberofbluegalaxies,ismarkedwitharectangle. for the WGM05 sample, where Σ is defined as the number of Therearemanypossiblewaystodefinegalaxyenvironments, 10 superclustergalaxiesper(Mpch−1)2 withinacirclewitharadius suchasmeanlocalluminosity,galaxycolourordistancefromthe given by the average distance to the 9th and 10th nearest neigh- restframeU-Vcolour-magnitudemainsequence(asusedbyGray bours.Thesuperclustersampleusesonlythebrightestgalaxiesas etal. 2004),butnotallofthesewerefoundtobeusefulindistin- definedinSection5.1. guishingbetweenthepredefinedareasinFigure8.Itwasfoundthat The distributionsof Σ for thecontrol samples and thesu- thegalaxiesindifferentenvironmentscouldnotbeseparatedwell 10 perclusterAGNareshowninFigure7.TheAGNhostgalaxieslie purelyintermsoflocalprojecteddensity,andthattwootherfactors predominantlyinareasofmoderatedensitycomparedtothecontrol are also important: firstly the number of less luminous galaxies, sample,withasignificanceof98.4percentand99.0percentfrom and secondly the colour of the local galaxies. The distance from Kuipers and theK–S test respectively. It isparticularly clear that the colour-magnitude relation was investigated as a measure of AGNhostsavoidthedensestregionsofthesupercluster,evenac- colour,buttheenvironmentswerebetterseparatedusingthemea- countingforthefactthattwooftheclustercoresareremovedfrom sured colour, as the latter combines colour with a distinction be- thisstudy.Theexception,ofcourse,istheveryluminousAGNin tweenbrightandfaintgalaxies.Themeanlocalcolourwasusedas thecoreofA901a,whichwasexcluded,alongwiththesurround- thismeasureissensitivetothepresenceofafewveryblueoractive inggalaxies,astheydonot lieinthecontrol area. Thissource is galaxies,wherasthemediancolourisdominatedbyredgalaxieson differentinbothpropertiesandpositionfromtheotherAGNinthe thecolour-magnitudemainsequence(Figure4),andismoreamea- supercluster. sureoflocalmagnitude.Toincludefaintergalaxiesthesupercluster (cid:13)c 2007RAS,MNRAS000,1–21

Environmental dependence of AGN activity in the supercluster A901/2

Similar Environmental dependence of AGN activity in the supercluster A901/2