On the Incidence of Strong Gravitational Lensing b

arXiv:astro-ph/0210352v115Oct2002Draft:TobesubmittedtoTheAstrophysicalJournalOntheIncidenceofStrongGravitationalLensingbyClustersintheLasCampanasDistantClusterSurveyDennisZaritskyStewardObservatory,UniversityofArizona,933NorthCherryAvenue,Tucson,AZ85721,USAandAnthonyH.GonzalezDepartmentofAstronomy,UniversityofFlorida,211SpaceSciencesBldg.,Gainesville,Fl,32611-2055,USAABSTRACTTheobservedincidenceofstronglylensingclustersexceedsthepredictionsofaΛCDMmodelbyaboutafactorof10.WerevisittheobservationalsideofthisdiscrepancybymeasuringtheincidenceofstronglensinginasubsampleofclustersdrawnfromtheLasCampanasDistantClusterSurvey(LCDCS).Amongclusterswith0.5≤z≤0.7,theredshiftrangeinwhichwefocusoursearch,wefindtwostronglylensedsystemswithinaneffectivesearchareaof69sq.deg.ThereisatleastoneotherstronglylensedsystemsintheLCDCSoutsideofthisredshiftrange,wherewearelesscomplete.Overallredshifts,theΛCDMmodelproducesonelargearcevery146sq.degrees.AssumingPoissonstatistics,theprobabilityoffinding3ormorestronglylensingclustersin69sq.degreesis0.012.ThelensingincidencewithintheLCDCSisinagreementwiththatde-rivedfromanX-rayselectedsampleandwhathasbeenpreliminarilypresentedfromanindependentopticalclustersurvey.Theoriginofthedisagreementbe-tweentheoryandobservations,whichremainsatleastattheorderofmagnitudescalefortheΛCDMmodel,lieseitherintheconcordancecosmologicalmodel,inthecharacteristicsoftheresultingclusterpotentials,orintheadoptedsourcepopulation.–2–1.IntroductionThe“cosmicconcordance”cosmologicalmodel,Ωm=0.3,ΩΛ=0.7,hasquicklybecomethestandardasaresultofnewobservations(TypeIsupernovaebyRiessetal.(1998)andPerlmutteretal.(1999);cosmicmicrowavebackgroundanisotropiesbydeBernardisetal.(2000),Hananyetal.(2000),andPrykeetal.(2002))anditsabilitytoreconcilelong-standingproblems(forexamplesseeOstriker&Steinhardt(1995)).TheonlyprominentpartialholdoutinthisavalancheofevidencehadbeenthedistributionofQSOlensimageseparations(seeFalcoetal.(1998)),buteventhatdiscrepancyisnowthoughttoberesolved(Keeton2001).Onelesswellknown,butperhapsmorestrikingdiscrepancylieswiththeincidenceofgiantgravitationalarcsingalaxyclusters.Bartelmannetal.(1998)foundthatthethecurrentlyfavoredΛCDMunderpredictsthestronglensingincidenceamongasampleofX-rayselectedclustersbyanorderofmagnitude.Becausethisdiscrepancyisoneofthefewpiecesofobservationalevidenceagainstwhathasbecometheacceptedcosmologicalmodel,itwarrantsfurtherexaminationonboththetheoreticalandobservationalfronts.WemeasuretheincidenceoflensingamongclustersdrawnfromtheLasCampanasDistantClusterSurvey(Gonzalezetal.2001)andcompareittoothermeasurementsandtotheexistingtheoreticalexpectations.Thepotentialuseofthegalaxyclustermassfunctiontoprobethecharacteristicsandevolutionofthelargestgravitationallyboundstructures,andthereforeconstrainthecosmo-logicalmodel,hasbeentheoreticallydemonstratedoverthepastdecade(seeforexamplesFrenketal.(1990),Eke,Cole,&Frenk(1996),andEvrardetal.(2002)).Unfortunately,therearevariousreasonswhythestudyofclustershasyettoprovideawhollysatisfyingconstraint.Oneofthemostevidentweaknesseshadbeenthelackoflargenumbersofknownclustersathigher(z0.5)redshifts.Thisweaknessisbeingaddressedwiththemany-foldgrowthinthenumberofknownhighredshift,massiveclustersdueinlargeparttoavarietyofbothpublished(Postmanetal.(1996),Scodeggioetal.(1999),Gonzalezetal.(2001))andongoing(GladdersandYee(2000),Willicketal.(2001))ground-based,opticalsurveys.Asecondmoreperniciousdifficultyinvolvesthemeasurementoftheclustermass.Themeasurementisfraughtwithpossiblesystematicerrorsandisobservationallytime-intensive.Therearefourwaystoconfirmthataclusterismassive:1)computethevelocitydispersionusingalargenumber(30)ofmembergalaxyredshifts(forhigh-zclustersthisrequiresseveralhoursofobservationperspectroscopicmaskon10m-classtelescopes;seeLubinetal.(2002)),2)measurethetemperatureoftheintergalacticmediumusingX-rayobservations(thisrequiresexposuresoftensofksecperclusteronX-raysatellitetelescopes;seeJeltemaetal.(2001)),3)constructaSunyaev-Zel’dovichdecrementmap(thisrequires∼40hoursofon-sourceintegration;seeJoyetal.(2001))or4)measurethedistortionofbackground–3–galaxyimagesintroducedbythecluster’sgravitationalpotential(weaklensingmapsrequireseveralhoursofimagingon10-mclasstelescopes;seeCloweetal.(2000)).Thedemandingobservationalrequirementsexplainwhytherearerelativelyfewhigh-zclusterswithmeasuredmassesandcreateasignificantimpedimenttotakingfulladvantageofthehundredsofpotentialmassivehigh-zclusterspresentedinthenewcatalogs.Aspecialcaseofmethod(4)iswhatisofinteresthere.Stronglensing,wheretheclus-terpotentialdistortstheimageofabackgroundimagesufficientlythatitappearsasanarc(Lynds&Petrosian(1986),Soucailetal(1987)),isararephenomenonthatincreasesinlikelihood,inpart,astheclustermassincreases(Luppinoetal.(1999),Dahleetal.(2002)).Theobservationaladvantagesofusingstronglensingincludethatarelativelymodestin-vestmentoftelescopetimeperclusterisnecessary,thatithighlightstheclusterswiththegreatestrelaxedmass,andthatitisarelativelyunambiguousphenomenonataproperlyselectedmagnitudelimi