Galaxy Clustering Around Nearby Luminous Quasars

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

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aGALAXYCLUSTERINGAROUNDNEARBYLUMINOUSQUASARS1KarlB.Fisher,JohnN.Bahcall,So aKirhakosInstituteforAdvancedStudy,SchoolofNaturalSciences,Princeton,NJ08540andDonaldP.SchneiderDepartmentofAstronomyandAstrophysics,ThePennsylvaniaStateUniversity,UniversityPark,PA16802Received

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

ABSTRACT

Weexaminetheclusteringofgalaxiesaroundasampleof20luminous

<0.30)quasarsobservedwiththeWideFieldCamera-2onlowredshift(z～

theHubbleSpaceTelescope.TheHSTresolutionmakespossiblegalaxy

identi cationbrighterthanV=23.5andascloseas2′′tothequasar.We

ndasigni cantenhancementofgalaxieswithinaprojectedseparationof

1<～100hkpcofthequasars.Ifwemodeltheqso/galaxycorrelationfunction

asapowerlawwithaslopegivenbythegalaxy/galaxycorrelationfunction,

we ndthattheratiooftheqso/galaxytogalaxy/galaxycorrelationfunctions

is3.8±0.8.Thegalaxycountswithinr<15h 1kpcofthequasarsaretoo

>100h 1kpc).highforthedensitypro letohaveanappreciablecoreradius(～

Ourresultsreinforcetheideathatlowredshiftquasarsarelocatedpreferentiallyingroupsof10–20galaxiesratherthaninrichclusters.Weseenosigni cant

di erenceintheclusteringamplitudesderivedfromradio-loudandradio-quiet

subsamples.

Subjectheadings:quasars;galaxyclustering

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

1.INTRODUCTION

Overthelasttwodecades,ithasbeenwellestablishedthatquasarsareassociatedwithenhancementsinthegalaxydistribution.Historically,thisprovidedthe rstdirectobservationalevidencethatquasarswereindeedcosmologicalinorigin(Bahcall,Schmidt,&Gunn1969;Bahcall&Bahcall1970;Gunn1971;Stockton1978).Overtheyears,

<0.4,quasarsresideinsmallconsiderableevidencehasaccumulatedthatlowredshift,z～

tomoderategroupsofgalaxiesratherthaninrichclusters(cf.Hartwick&Schade1990,Bahcall&Chokshi1991,andreferencestherein).Athigherredshifts,thereisamarkeddi erenceintheenvironmentsofradio-loudandradio-quietquasars(Yee&Green1987).

>0.6,radio-loudquasarsareoftenfoundinassociationwithrichclustersAtredshiftsz～

(AbellrichnessR≥1)whileradio-quietquasarsappeartoremaininsmallergroups,orperhapsintheouterregionsofclusters(Boyleetal.1988;Yee1990).

Thegalaxyenvironmentaroundquasarsprovidesmanyimportantcluesastowhattriggersandsustainstheircentralengines.As rstsuggestedbyToomre&Toomre(1972),mergersandinteractionsofgalaxiescanprovideane cientmechanismfortransportinggasintotheinnerregionsofagalaxyorquasar.Therehavebeenattemptstomodeltheinteraction/mergerratesofordinarygalaxiesinordertoexplaintheluminosityfunctionofquasars(DeRobertis1985;Roos1985;Carlberg1990)andtherapidevolutionofthemerger/interactionrateinclusterswithredshiftmayprovideanaturalexplanationofthestrongevolutionofclusteringobservedaroundradio-loudquasars(Stocke&Perrenod1981;Roos1981).Knowledgeofthegalaxyenvironmentaroundquasarsisalsoimportantforunderstandingthelargescaledistributionofquasarsandhowitrelatestothestructureseeningalaxysurveys(Bahcall&Chokshi1991).

<0.3)InthisLetter,weexaminethegalaxyenvironmentaroundtwentynearby(z～

brightquasarsobservedwiththeWideFieldandPlanetaryCamera-2(WFPC2)ofthe

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

HubbleSpaceTelescope(HST).These eldswereimagedaspartofanongoinginvestigationintothenatureofthehostenvironmentofquasars(Bahcall,Kirhakos,&Schneider1994,1995a,1995b,1996a).TheexceptionalresolutionoftheHSTimagesallowscompaniongalaxiestobedetectedatverycloseprojectedseparations,insomecasesr～3h 1kpc<2′′),andgalaxy/starseparationtobeperformeddowntoV～23.5.Thegoalofthe(～

workpresentedhereistoquantifytheexcessofgalaxiesassociatedwiththesequasars.TheoutlineoftheLetterisasfollows.Abriefdescriptionofthequasarsampleusedinouranalysisisgivenin§2.In§3.1wearguethatgalaxycountsareinconsistentwithbeingdrawnfromauniformbackground.In§3.2,westrengthenthisconclusionbycorrectingthecountsforthebackgroundcontamination.Wealsopresenttheexcessgalaxycountsabovethebackgroundinannuliofprojectedseparation.Fromthesecounts,wequantifytheamplitudeofthegalaxyclusteringaroundthequasarsin§3.3intermsofaquasar-galaxyspatialcrosscorrelationamplitude.Wediscussourresultsandtheirrelationtopreviousworkin§4.

2.DATA

ThesampleofobjectsanalyzedinthisLetterconsistsof20oftheintrinsicallymostluminous(MV< 22.9,forH0=100kms 1Mpc 1, 0=1)nearby(z<0.30)radio-quietandradio-loudquasarsselectedfromtheV´eron-Cetty&V´eron(1993)catalog.Table1liststheindividualquasarsandtheirredshifts.Thequasarsspantheredshiftrange0.086≤z≤0.29withamedianredshiftofzmed=0.18.Detailsoftheobservationsandtheresultingimagesforelevenofthetwenty eldshavebeenpresentedinBahcall,Kirhakos,&Schneider(1994,1995a,1995b);theremainingobservationswillbepresentedinBahcalletal.(1996b,inpreparation).Sixofthequasars(denotedwithanasteriskinTable1)areradio-loud,whiletheremainderareradio-quiet(Kellermann1989).

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

Eachquasar eldwasimagedwiththeWFPC2throughtheF606W lter,whichissimilar,butslightlyredderthantheVbandpass(λ=5940 A, λ=1500 A).Thequasarswerepositionedwithin4′′±1.2′′ofthecenterofWideFieldCameraCCD3(WF3).SimultaneousimageswereobtainedintheadjacentCCDchips2and4(WF2andWF4respectively)whichtogetherwithWF3forma“L”shapedimage(see gure1).Eachchiphas800×800pixelsandanimagescaleof0.0996′′pixel 1atthechip’scenter;thiscorrespondstospatialresolutionof2.1h 1kpc/arcsecataredshiftz=0.20( 0=1).Thee ectiveareas(areasnotcoveredbypyramidshadows)ofWF2,WF3,andWF4are1.59,

1.60,and1.59sqarcminrespectively.MoredetailedinformationontheWFPC2anditsphotometricsystemcanbefoundinBurrows(1994),Traugeretal.(1994),andHoltzmanetal.(1995a,b).Therelativelylongexposures(1100or1400seconds),combinedwiththeexcellentspatialresolution,allowedgalaxiestobeidenti edintheimagesdowntolimiting

<2′′fromthecentralquasar.Weperformedmagnitudem(F606W)≤24.5andcloseas～

aperturephotometryonthe eldgalaxies;circularapertureswithradiiof0.3′′to10′′wereusedasappropriate.

3.GALAXYCOUNTSAROUNDQUASARS

3.1.RawCounts:EvidenceforaStrongEnhancement

Ifgalaxiesaredistributedaroundlowredshiftquasarswithapowerlawdistribution,ξ(r)～(r/10h 1Mpc) 1.77(assuggestedby,e.g.,Yee&Green1987),thentherewillbeaverystrongenhancementofthecountswithin30′′(r～60h 1kpcatz=0.2)ofthequasar.Moreover,becausethecentersofWF2andWF4areo setfromthequasar,therewillbeanenhancementofgalaxiesinWF3relativetoWF2andWF4.Thebackgroundgalaxycounts

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

risesteeplywithmagnitudeandthiswilldiluteanysignalofexcessgalaxies.Muchofthebackgroundcontaminationcanberemovedsimplybycountingonlythosegalaxieswithapparentmagnitudesintherangethatislikelytobephysicallyassociatedwiththequasars.Agoodcompromisebetweeneliminatingtoomanyassociatedgalaxiesandminimizingthee ectofforeground/backgroundinterlopers,istocountgalaxiesineach eld,i,whichareinthemagnituderangem (zi) 1tom (zi)+2,wherem (zi)istheapparentmagnitudeofanL galaxyattheredshiftofthequasar,zi.Themean(median)valueofm (zi)foroursampleis18.2(18.1);thetotalrangeofm (zi)is16.4≤m (zi)≤19.2.Incomputingm (z),wehavetakenanL galaxytocorrespondtoanabsolutemagnitudeM (F606W)= 20.75andhaveusedtheK-correctionsbetweenabsoluteandapparentmagnitudesgiveninFukugita,Shimasaku,&Ichikawa(1995).

Figure1showsthepositionsofallgalaxiesinourtwenty eldswith

m (zi) 1≤m≤m (zi)+2;geometriccorrectionswereappliedaccordingto

Holtzmanetal.(1995a).Inthepanelcontainingthequasar(WF3,lowerleft),thereisaclearexcessofgalaxies(50galaxies)relativetoWF2(upperleft)andWF4(18and17galaxiesrespectively).ThehypothesisthatthecountsinallthreechipsaredrawnfromanunderlyingPoissondistributionwithacommonmeanleadstoamaximumlikelihoodmeanperchipof28.4andachi-squaredofχ2=24.5;theprobabilitythatχ2foronedegreeoffreedomisthislargebychanceisonlyP=7×10 7.Thus,withoutanydetailedmodeling,wecanruleoutthepossibilitythatthecountsarerandom uctuationsinthebackgrounddistribution.Inthefollowingsections,weattempttomakethisconclusionprogressivelymorequantitativeby rstmodelingthebackgroundgalaxydistributionandthenintroducingamodelforthespatialdistributionofgalaxiesaroundthequasars.

3.2.CorrectingfortheBackgroundGalaxies

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

Inordertofurtherquantifytheexcessofgalaxycountsaroundthequasars,weneedanestimateofthecontributionfrombackgroundgalaxies.In gure2,weshowthegalaxycounts(d2N/dmd ,persquarearcsec)versusmagnitudeforthe“o quasar”chipsWF2andWF4,inbins m(F606W)=0.5.Thecountsatm(F606W)>21.5magarewellapproximatedbyapowerlaw,log10d2N/dmd = 10.8+0.33m(F606W);thecountsatbrightermagnitudesareinexcessofthoseobtainedbyextrapolatingthefaintpowerlaw t.Becausetheseparationofthecentersofchips2and4isnotlarge(101′′correspondsto213h 1kpcatz=0.2),thecountsinthesechipsareacombinationofbothbackgroundgalaxiesandthe(relativelybright)galaxiesphysicallyassociatedwiththequasars,andhenceyieldanoverestimateofthetruebackground.Wehavecomparedthepower-law twiththecountsderivedfromtheHSTMediumDeepSurvey(MDS)(Gri thsetal.1994;S.Casertano,privatecommunication)whichcoversamuchlargerareaofthesky.Theagreementisgood,andintheremainderofthisLetter,wewilladoptthepowerlawin gure2asourestimateofd2N/dmd .TheagreementwiththeMDSisalsoausefulconsistencycheckforsystematicerrorsinourderivedmagnitudes.

Inthetwenty elds,thereare11galaxiesintherangem (zi) 1≤m≤m (zi)+2withinaprojectedseparationlessthan25h 1kpcofthequasar;thetotalnumberexpectedfromthed2N/dmd powerlawrelationisonly0.99.TheprobabilityoftheobservedcountsbeingaPoissonrealizationofthebackgroundisextremelysmall,P=8×10 9,twoorderofmagnitudessmallerthanourpreviousestimateobtainedbyneglectingthebackground.ThisisalsoamuchstrongerconstraintthantheupperlimitgiveninBahcall,Kirhakos,&Schneider(1995a),P=2×10 2,basedoneight eldsandan(over)estimateofthebackgroundobtainedfromthecountsinWF2andWF4.

Wehavealsocomputedcountswithinprojectedseparationsof5and10h 1kpc.Inthetwenty elds,we nd2/0.039and5/.16(number/numberexpectedinthebackground)

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

withr<5h 1kpcandr<10h 1kpcrespectively;thecorrespondingrandomprobabilitiesareP=7×10 4andP=7×10 7.Thereisasuggestivedi erenceinthecounts

derivedfromtheradio-loudandradio-quietsubsamples.Theradio-quietquasars(14 elds)had0/.027(P=0.97)and2/.11(P=5×10 3)forthe5and10h 1kpccounts;thecorrespondingnumbersfortheradio-loudsample(6 elds)were2/.012(P=8×10 5)and3/.05(P=2×10 5).

In gure3,weshowthefractionalexcessofgalaxiesabovethebackground,

δN/N(r) = Ni(r)/Nb,i(r) 1,obtainedbyaveragingthecountsinthetwenty eldsinbinsof15h 1kpcprojectedseparation.HereNi(r)istheactualcountofgalaxieswithprojectedseparationsbetweenr r/2andr+ r/2intheith eldandNb,i(r)istheexpectedbackgroundcontribution.Again,wehaveonlyconsideredthosegalaxiesintheapparentmagnituderangem (zi) 1tom (zi)+2.

From gure3,wecanimmediatelydrawtwoconclusions.First,thereisasigni cantexcessofgalaxieswithinprojectedseparationsofr<h 1kpcfromthequasars.Second,thereappearstobenosigni cantdi erenceinthegalaxycountsfortheradio-loudandradio-quietsubsamplesfor10h 1kpc<r<100h 1kpc.Inthenextsection,wequantifytheobservedclusteringintermsofthespatialquasar/galaxycross-correlationfunction.

3.3.EstimatingtheSpatialClusteringAmplitude

Adetailedderivationoftherelationbetweenangularcountsandaspatialdistributionofgalaxiesintermsofacross-correlationfunctionisgiveninLongair&Seldner(1979).Brie y,oneassumesthatthegalaxydistribution(abovethebackground)aroundthequasarisspeci edbyaquasar/galaxycross-correlationfunction,ξqg(r,z).Theobservedexcessinprojectedseparationintheith eldisthenobtainedbyintegratingξqg(r,z)

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

overtheredshiftdistributionofgalaxies(dN/dz)iintheapparentmagnituderange

[m (zi) 1≤m≤m (zi)+2],

Ni(r)/Nb,i(r) 1= dθdz(dN/dz)iξqg(s,z)

dzdV,(2)

whereφ(zi)istheintegraloftheluminosityfunctionovertheabsolutemagnitudes

[M (zi) 1≤M≤M (zi)+2]correspondingthetheapparentmagnituderange

[m (zi) 1≤m≤m (zi)+2],

φ(zi)≡ M (zi)+2

M (zi) 1dMΦ(M).(3)

Thecosmologicalmodel( 0,Λ,H0)entersintheaboveequationsimplicitlyinthevolumeelement,angulardistance,andluminositydistance.Weadoptacanonicalmodelwith 0=1,H0=100kms 1Mpc 1,andΛ=0.Thebasicresultsofourstudyarelargelyindependentofthischoice.Theredshiftdistributioniscomputedusinganon-evolvingSchechter(1976)luminosityfunctionwithafaintendslopeofα=0.97(Lovedayetal.1992).TheK-correctionsnecessaryforcomputingtherelationbetweenabsoluteandapparentmagnitudeshavebeentakenfromFukugita,Shimasaku,&Ichikawa(1995).In

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

theresultsthatfollow,theH0dependenceisexplicitlyindicated.Thederivedclusteringamplitudesremainwithinthestatederrorsas 0isvariedfrom0.1to1.0.

Thecross-correlationfunction,ξqg(r,z),isusuallyassumedtoevolvewithredshift.Aconvenientmodelforthisevolutionistoassumethatonsmallscalestheclusteringisconstantinphysicalcoordinates,i.e.thenumberofexcessgalaxiesaroundthequasar,n(z)ξqg(r,z),isaconstant(n(z)isaveragenumberdensityattheepochz).Thusforanyassumedshapeofthecorrelationfunction,F(r),ξqg(r,z)evolvesas

ξqg(r,z)=1

1+z .(4)

Thefactorof(1+z)3compensatesforthechangeinthemeannumberdensity,whiletheredshiftfactorintheargumentofF(r)issimplyamatterofconvention;oneusuallyspeci estheshapeintermsofphysicalseparationswhilein(1),wehavespeci edξqg(r,z)intermsofcomovingseparation.

Weconsiderthreedi erentmodelsforF(r).The rstmodelisapowerlaw,F(r)=Bqgr 1.77,withaslopeequaltothatofthegalaxy/galaxycorrelationfunction(Davis&Peebles1983)andtheamplitudetakenasafreeparameter.Thesecondmodelisanexponentialsurfacedensityofgalaxies,µ(r)=µ0exp( r/rc)which,afterdeprojectionbyastandardAbelinversion,correspondstoF(r)=µ0/πrcK0(r/rc)(K0(x)beingthemodi edBesselfunction).ThismodelwasproposedbyMerri eld&Kent(1989)asatypicalgalaxypro learoundcentrallydominantclustergalaxies.Weadopttheirbestestimateofacoreradiusofrc=100h 1kpc,andvarytheamplitudeµst,weconsideramodi edHubblepro leF(r)=A[1+(r/rc)2] 3/2(Binney&Tremaine1987,eqn.2-37)withrc=100h 1kpcandAafreeparameter.Thesethreemodelshavedi erentbehavioratsmallr.Boththepowerlawandexponentialmodelsdivergeasr→0(althoughthelaterdoessoonlyweakly,K0(x)～ ln(x)),whilethemodi edHubblemodelasymptoticallyapproachesaconstant.

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

Wevariedtheamplitudeofthemodelstoachieveamaximumlikelihood ttotheexcessgalaxycountsinbinsofprojectedseparationof15h 1kpc.Thelimitednumberof eldspreventedusfromvaryingmorethanoneparameterpermodel.Figure3showstheresultingbest- tmodels;theamplitudesofthepower-lawmodelaregiveninTable2.At

>30h 1kpc,theexcesscountsarerelatively atandallthreemodelsfortheseparationsr～

correlationfunction tthedata.However,thecountsintheinnermostbinr<15h 1kpcliesigni cantlyabovethecountsatlargerseparations;theriseinthecountsatsmallradiiisparticularlystrikinginthe veradioloud elds.Thepower-lawistheonlymodelforξqg(r)consideredwhichrisessteeplyenoughtoaccountforthisexcess.

4.DISCUSSION

Ifthequasarsweredistributedliketypicalgalaxies,thenthederivedvalueof Bqg wouldbeequaltotheamplitudeofthegalaxy/galaxycorrelationfunction, Bgg ～20(Davis&Peebles1983).Ahighervalueof Bqg suggeststhatthequasarsliepreferentiallyinregionsofaboveaveragegalaxydensity.FollowingBahcall&Chokshi(1991),wecanconvertourvaluesofBqgintoanestimateofthetypicalrichnessofquasargalaxyenvironment.Here,wede nerichnessasthenumberofL galaxiesassociatedwiththequasar;thisisgivenbyasimpleintegraloverthecorrelationfunction,N=4πn ξqg(r)r2dr,wherethelimitsofintegrationarer=0to1.5h 1Mpc(thetraditionalAbellradius),andn ≈1.5×10 2h3Mpc 3isthenumberdensityofL ingthevaluesof Bqg inTable2,we ndthequasarstypicallyresideingroupsof16–25galaxies.ThesenumbersshouldbecomparedtothetypicalrichnessofAbellclusterswhichhave30–49and50–79membersforrichnessclassesR=0andR=1respectively.Moreover,thisestimateismostlikelyanupperlimitsincethereisevidencethatthegalaxypro learoundquasarsfallso

>0.25h 1Mpc(Ellingson,Yee,&Green1991).Inordertomoresteeplythanr 1.77forr～

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

testtherobustnessofourresultstoasteepeningofthegalaxypro le,we ttedadoublepowerlawmodelwithslopeof 1.77forr≤0.25h 1Mpcand 3forr>0.25h 1Mpc.Thebest tamplitudes,Bqg,forthismodelincreasedbyabout10%(stillwellwithinthequoted1 σerrors),yet,thenumberofinferedbrightgalaxiesassociatedwiththequasarsdecreasedto8duetothesteeperpro leatlargeseparations.

Thederivedamplitudes, Bqg ,inthepurepowerlawcasearesomewhatlargerthantheestimatesbyYee&Green(1987).Theyexaminedtheclusteringof9radio-loudand16radio-quietquasarsintheredshiftrange0.15<z<0.30.Onscalesof～20–500h 1kpc,theyestimated Bqg ≈60±20and Bqg ≈42±14forradio-loudandradio-quietquasarsrespectively.Hayman(1990)derivedgalaxycountsaroundlowredshift(z<0.3)quasarsfromthePalomarSkySurveyprints.Hefoundthattheratioofthequasar/galaxyandgalaxy/galaxyangularcorrelationfunctionswas3.1±0.6;ifthequasarsandgalaxieshavesimilarselectionfunctions,thistranslatestoanestimatesimilartoYee&Greenof Bqg ～61±12.French&Gunn(1983)analysedasample25low-redshiftquasars<0.35)selectedfrom1.2-mPalomarSchmidtplatesandconcluded B =25±12;(z～qg

theyalsoanalysedthedatasetofStockton(1978,27quasarswithredshiftsz≤0.45selectedfromtheredSkySurveyprints)andderived,viathesameanalysis,Bqg=79±40.Ourmeasurementsare,withtheexceptionofYee&Green’svalueforradio-quietquasars,consistentwithinthequotederrors.Theslightlyhigherclusteringamplitudewederivefortheradio-quietsubsamplemaybearesultofoursamplebeingthesubsetofthemostluminousquasars.

Yee&Green(1987)foundthattheclusteringamplitudeofgalaxiesaroundradio-loudquasarsincreasedbyafactor～3betweenz～0.4–0.6andatz～0.6radio-loudquasarsarefoundinenvironmentsasrichasAbellclassR=1.Opticalquasarsdonotevolveasrapidly(Boyleetal.1988),perhapsindicatingadi erentformationscenario.Ithas

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

beensuggestedthequasarsandactivegalacticnucleimaybetriggeredbyinteractions(e.g.,Toomre&Toomre1972;Stocke&Perrenod1981;Roos1981,Yee1987).Thiso ersasimpleexplanationforwhythequasarsaretypicallynotfoundinrichclustersatlowredshifts;thehighvelocitydispersionofsuchclustersleadstoalowinteractionrate.

TheHSTWFPC2isanexcellentinstrumentforextendingthepresentanalysistofainter,low-redshift,quasars.Thisextensionwouldimprovethecountingstatisticswhilealsoprovidinginformationregardingpossiblecorrelationsofthequasarenvironmentwith

<0.6)couldbeaccomplishedluminosity.Theimagingofofmoderateredshiftquasars(z～

byHSTwithsingleorbitexposures.Thisimagingwouldprovideamoredirectcomparisonwithpreviousgroundbasedworkandwouldincreaseourknowledgeoftheevolutionaryhistoryofthequasarenvironment.Anincreasedknowledgeofthequasarenvironmentwouldbeusefulinusingquasarobservationstoprobelargescalestructureathigherredshifts.

WewouldespeciallyliketothankStefanoCasertanoandthemembersoftheMediumDeepSurveyforusefulcommentsandforallowingustocompareournumbercountsandmagnitudeswiththeirmeasurements.Wehavealsobene tedfromdiscussionswithNetaBahcall,MichaelStrauss,OferLahav.Wethanktheanonymousrefereeforhelpfulsuggestions.ThisworkwassupportedinpartbyNASAcontractNAG5-1618,NASAgrantnumberNAGW-4452,andgrantnumberGO-5343fromtheSpaceTelescopeScienceInstitute,whichisoperatedbytheAssociationofUniversitiesforResearchinAstronomy,Incorporated,underNASAcontractNAS5-26555.KBFacknowledgesthesupportoftheNSF.

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

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We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

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We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

TABLE1

quasarsample

NameDateExp.(sec)MVz

Radioloud

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

TABLE2

ClusteringAmplitudes

Sample Bqg 1 σRange Bqg / Bgg

inunitsof[h 1Mpc]1.77

Bgg =19.8(h 1Mpc)1.77

We examine the clustering of galaxies around a sample of 20 luminous low redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble Space Telescope. The HST resolution makes possible galaxy identification brighter than V=23.5 and as clos

FIGURECAPTIONS

Fig.1.—MosaicofthegalaxycountsinWF2,WF3,andWF4(upperleft,lowerleft,andrightpanelsrespectively)foralltwenty elds.Inagiven eldi,wecountonlythosegalaxiesinthemagnituderangem (zi) 1tom (zi)+2wherem (zi)istheapparentmagnitudeofanL galaxyattheredshiftofthequasar,zi.

Fig.2.—Numbercounts,d2N/dmd ,versusm(F606W).The lledcirclesrepresentthecountsdetermineddirectlyfromtheo chipsineachQSO eld.Poissonerrorsareshown.Thelineisaleastsquares ttothefaintcountswithm(F606W)>21.5.

Fig.3.—Averagegalaxycounts δN/N(r) ofthefullquasarsampleandradioandradio-quietsubsets.Thesolidpointsarethecountscomputedusingapowerlaw ttothefaintcountsofchips2and4forthebackground(cf. gure2);theopensymbols(shifted.025inthelog)showthecountsderivedusingthebackgroundcountsfromtheMediumDeepSurvey.Theerrorbarsshowthescatterbetweenthedi erent elds.Thecurvesarethepredictedcorrelationfunctionsifthegalaxy/quasarcrosscorrelationfunctionisapowerlawwithindexγ=1.77(solid),anexponentialsurfacedensity(dashed),oramodi edHubblepro le(dotted).Allcalculationsassume 0=1andΛ=0.

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