Galaxy Clustering Around Nearby Luminous Quasars We examine the clustering of galaxies around a sample of 20 luminous low redshift (z0.30) 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 6 9 9 1 eb F 5 1 1 v 87 2 6 9 / h p - o rt sa :v i X r 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 (z0.30) 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=23.5andascloseas2tothequasar.We ndasigni cantenhancementofgalaxieswithinaprojectedseparationof 1100hkpcofthequasars.Ifwemodeltheqso/galaxycorrelationfunction asapowerlawwithaslopegivenbythegalaxy/galaxycorrelationfunction, we ndthattheratiooftheqso/galaxytogalaxy/galaxycorrelationfunctions is3.80.8.Thegalaxycountswithinr15h 1kpcofthequasarsaretoo 100h 1kpc).highforthedensitypro letohaveanappreciablecoreradius( Ourresultsreinforcetheideathatlowredshiftquasarsarelocatedpreferentiallyingroupsof1020galaxiesratherthaninrichclusters.Weseenosigni cant di erenceintheclusteringamplitudesderivedfromradio-loudandradio-quiet subsamples. Subjectheadings:quasars;galaxyclustering We examine the clustering of galaxies around a sample of 20 luminous low redshift (z0.30) 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;BahcallBahcall1970;Gunn1971;Stockton1978).Overtheyears, 0.4,quasarsresideinsmallconsiderableevidencehasaccumulatedthatlowredshift,z tomoderategroupsofgalaxiesratherthaninrichclusters(cf.HartwickSchade1990,BahcallChokshi1991,andreferencestherein).Athigherredshifts,thereisamarkeddi erenceintheenvironmentsofradio-loudandradio-quietquasars(YeeGreen1987). 0.6,radio-loudquasarsareoftenfoundinassociationwithrichclustersAtredshiftsz (AbellrichnessR1)whileradio-quietquasarsappeartoremaininsmallergroups,orperhapsintheouterregionsofclusters(Boyleetal.1988;Yee1990). Thegalaxyenvironmentaroundquasarsprovidesmanyimportantcluesastowhattriggersandsustainstheircentralengines.As rstsuggestedbyToomreToomre(1972),mergersandinteractionsofgalaxiescanprovideane cientmechanismfortransportinggasintotheinnerregionsofagalaxyorquasar.Therehavebeenattemptstomodeltheinteraction/mergerratesofordinarygalaxiesinordertoexplaintheluminosityfunctionofquasars(DeRobertis1985;Roos1985;Carlberg1990)andtherapidevolutionofthemerger/interactionrateinclusterswithredshiftmayprovideanaturalexplanationofthestrongevolutionofclusteringobservedaroundradio-loudquasars(StockePerrenod1981;Roos1981).Knowledgeofthegalaxyenvironmentaroundquasarsisalsoimportantforunderstandingthelargescaledistributionofquasarsandhowitrelatestothestructureseeningalaxysurveys(BahcallChokshi1991). 0.3)InthisLetter,weexaminethegalaxyenvironmentaroundtwentynearby(z brightquasarsobservedwiththeWideFieldandPlanetaryCamera-2(WFPC2)ofthe We examine the clustering of galaxies around a sample of 20 luminous low redshift (z0.30) 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,insomecasesr3h 1kpc2),andgalaxy/starseparationtobeperformeddowntoV23.5.Thegoalofthe( workpresentedhereistoquantifytheexcessofgalaxiesassociatedwiththesequasars.TheoutlineoftheLetterisasfollows.Abriefdescriptionofthequasarsampleusedinouranalysisisgivenin2.In3.1wearguethatgalaxycountsareinconsistentwithbeingdrawnfromauniformbackground.In3.2,westrengthenthisconclusionbycorrectingthecountsforthebackgroundcontamination.Weals