Environmental dependence of AGN activity in the supercluster A901/2

21 Pages·2007·3.47 MB·English

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
? [email protected] 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<z <0.185gives795galaxieswithtotal
R X phot
far higher AGN fraction than previously determined fromoptical absoluteVbandmagnitude<−17,whichareusedinWGM05and
surveys. Recent results (Martini et al. 2007) show that the radial Laneetal. (2007).Thislargesamplemakesitpossibletodetermine
distributionof thefainter AGN inthissample followsthat of the very accurately the distribution and properties of the galaxies in
clusterpopulation, butthemoreluminousAGNarefoundprefer- A901/2(Grayetal. 2004).
entially in the central regions. This result is in agreement with a TheopticallyidentifiedstructureofA901/2isshowninFig-
recentstatisticalsurveyof18clusters(Branchesietal.2007). ure 1. A901 consists of two dark matter halos of comparable
(cid:13)c 2007RAS,MNRAS000,1–21
EnvironmentaldependenceofAGNactivityin thesuperclusterA901/2 3
from Bell and Papovich, private communication) will help in the
identificationofsomeAGN,asshowninSection4.2.
2.3 X-raydata
TheA901/2superclusterregionhadpreviouslybeenobservedfor
∼ 0.4 ksecas part of theROSATAllSkySurvey(Ebeling et al.
1996) and in addition with the ROSAT High Resolution Imager
for ∼ 12ksec. Schindler (2000) found seven sources in the field,
twoofwhichcoincidewithA901aandA901b.Thebrightemission
coincidentwithA901awasfoundtobeapointsource.
In this paper a new, deep (90ksec) XMM-Newton image of
thesupercluster,obtainedin2003,ispresented.Bycombiningthe
deep X-ray image and the optical data a sample of AGN in the
superclusterareselectedandanalysed.
3 X-RAYDATAREDUCTION
Figure 1. Galaxy number density in A901 (grey-scale) and supercluster
3.1 Datareduction
darkmatterdensityfrom2DweaklensinganalysisbyGrayetal. (2002)
(contours).Galaxies areselected intherange0.155 < zphot < 0.185. A90ksecXMMimageofA901/2wastakenon6th/7thMay2003
Theopticallyidentifiedclustersandgrouparemarked.Theimageis∼300
usingthethreeEPICcameras(MOS1,MOS2,PN)andathinfilter.
squareandthetopleftcornerisNorth-East.
The level 1 data were taken from the supplied pipeline products,
andreducedwithSASv5.4andthecalibrationfiles.Thedatawere
filteredforbadpixels,thestandardgoodpatternsof0–12andXM-
size,A901aandA901b,eachwithamassivegalaxyinthecentre.
MEA EM or XMMEA EP and energy between 0.5 and 7.5 keV.
A901acontainsfarmoregalaxiesandisfarmoreconcentratedthan
Removing times when the count rate was > 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(<m ) (1)
less than one pixel. This is a random error, rather than astromet- j i
ric,duetothedifficultyoffindingthecentreoffaintobjectswith whereσ isthepositionalerroronX-raysourcej,r thedistance
j i,j
a large pixel size compared to the PSF, and is a better measure- tooptical object i fromX-ray source j, and N(< m ) the num-
i
mentofthe‘true’erroronthestatedsourceposition.TheWAVDE- ber of optical objects brighter than object i in the r band image.
TECTerrorsaredependent onthesourcesizeandcounts,andare This takes account of angular separation and optical magnitude,
correlated with the distance between detections in different im- butmadenodistinctionbetweenobjectclassificationorphotomet-
ages.Thisdistanceisonaverage7timestheWAVDETECTerrorfor ricredshift.Theerrorsonthepositionsoftheopticalobjectswere
low-significancesources(onlychangingto6timeshigherforhigh- small enough to neglect compared to those in the X-ray, and the
significancesources).Therefore,althoughtheWAVDETECTerrors astrometricerrorswerealsofoundtobenegligibleastheminimum
are unphysically low they can be used to estimate the true error, errorontheX-raypositionof0.5pixelsissignificantlylargerthan
especiallyforsinglydetectedsources.Forthisreasontheerroron theastrometricerroronthisimage.
thesourcepositionwasgivenbythelargerofthefollowingthree This method treats optical quasars and galaxies of the same
measurements:thedistancebetweenthetwoclosestdetections(if fluxinthesameway,anddoesnotaccountforthefactthatquasars
theyexisted),7timesthestatedWAVDETECTerror,or0.5pixel(an are rarer and more likely to be X-ray sources. In addition, the
erroroflessthanhalfapixelwasdefinedasunphysical). methoddoesnotdistinguishbetweenthebrightestgalaxies,which
Asafinalstage,allsourcesthatwerenotdetectedatsignifi- arerareand quitelikelytobeX-raysources, and starsof asimi-
cance> 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 su-
perclustergalaxies.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 <M <−21.5)galaxieshaveac-
Figure6.ProportionofgalaxieshostingAGNasafunctionofR-bandab- R
cretionefficienciesabove3×10−4 itisnotsurprisingtofindno
solutemagnitude,with1σerrorbars.Thedashedlinesshowtheresultsif
bright galaxieswithAGN efficienciesabove thislevel. Similarly,
thepossiblesuperclusterAGNisincluded.Thedottedlineinthefinalbin
showsthe95percentconfidencelimitforthefirstemptybin.Againsource theonesuperclusterAGNwithefficiency>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