Detecting compact dark matter in galaxy clusters v

arXiv:astro-ph/0406282v111Jun2004Mon.Not.R.Astron.Soc.000,1–15(2004)Printed2February2008(MNLATEXstylefilev2.2)Detectingcompactdarkmatteringalaxyclustersviagravitationalmicrolensing:A2218&A370A.V.Tuntsov1⋆,G.F.Lewis1†,R.A.Ibata2‡&J.-P.Kneib3§,1A29,SchoolofPhysics,UniversityofSydney,NSW2006,Australia2ObservatoiredeStrasbourg,11,ruedel’Universite,F-67000,Strasbourg,France3ObservatoireMidi-Pyr´en´ees,14Av.E.Belin,31400Toulouse,FranceonleaveatCaltech,Astronomy,105-24,Pasadena,CA91125,USAAccepted2004June10.Received2004June8;inoriginalform2004March31ABSTRACTAfterdecadesofsearching,thetruenatureofdarkmatterstilleludesus.Onepoten-tialprobeoftheformofdarkmatteringalaxyclustersistosearchformicrolensingvariabilityinthegiantarcsandarclets.Inthispaper,asimplemethodisintroducedtocharacterizepixelvariabilityinthelimitofhighopticaldepthtomicrolensing.Expandingonearlierwork,theexpectedmicrolensingsignalfortwomassiveclus-ters,A2218&A370iscalculated.Itisfoundthatthemicrolensingsignaldependssensitivelyuponthemixofsmoothandcompactdarkmatterinthecluster.Com-parisonoftwodeepexposurestakenwithJamesWebbSpaceTelescopeortwohourlongexposurestakenwitha30-metreclasstelescopeintwoepochsseparatedbyafewyearswillpossiblydetectaboutafewdozenpixelswhichshowstrongvariabilityduetomicrolensingat5σlevel,revealingwealthofinformationonthemicrolensingpopulation.Keywords:gravitationallensing:microlensing:highopticaldepth–darkmatter:compact–galaxies:clusters:individual(A370,A2218)1INTRODUCTIONThereissignificantevidencethatmostoftheenergyden-sityintheUniverseresidesintheformsyetunknown,butthephysicalnatureofthisdarkmatterisanissueoflivedebateandisfarfrombeingunderstood(Lahav&Liddle2003;Mu˜nos2003).Typicalexplainationsinvokeeithercom-pactobjects,suchasprimordialblackholes,stellarremnantsorplanets,orcontinuousmaterialsuchasweaklyinteract-ingmassiveparticles,unseenhydrogenormoreexoticex-plainations.PotentialcluestothenatureofdarkmatterhaverecentlybeenuncoveredwiththedetectionofneutrinomassandobservationsofcompactmassiveobjectsthoughttobemostlywhitedwarfsinGalacticneighbourhood(e.g.,Fukudaetal.2000;Alcocketal.2000).However,giventheirmassneutrinoscannotcontainmorethan∼13percentofthedarkmattermassbudget(Hu,Eisenstein&Tegmark1998).Furthermore,thehalomicrolensingprogramsconstraincom-pactobjectsinthemassrangeof(10−7−10−1)M⊙tomak-ingnomorethan10percentoftheGalactichalo(Sadoulet1999).⋆E-mail:[e-mail]†E-mail:gfl@physics.usyd.edu.au‡E-mail:[e-mail]§E-mail:[e-mail]sallowedthedetailedrecon-structionoftheprojectedmassdistributioningalaxyclus-ters(e.g.,Ebbelsetal.1998).Unfortunatelysuchanalysisdoesnotprobethefundamentalnatureofdarkmatter.How-ever,althoughdeflectionanglesduetocompactobjectsinthedarkmatterarenegligibletheirderivativesmaybesub-stantialwhichmeansthatalthoughthegranularityofdarkmattercannotchangethemorphologyoflensedsystems,thismicrolensingcanhavesignificantimpactontheobservedfluxes(Liebes1964;Paczy´nski1986).Asthemutualconfig-urationofsourcesandlenseschangewithtime,thisresultsinvariabilityoftheobservedflux.Severalmicrolensingprogramshavebeenproposedandundertaken,themostsuccessfulofthembeingthemicrolensingexperimentstowardsGalacticbulgeandMagellanicCloudsbyMACHO,EROSandOGLEcollaborations(Alcocketal.2000;Lasserreetal.2000;Udalski,Kubiak&Szymanski1997).Anotherapproachispixellensingwhenthesearchfocusesnotonindividualsourcesbutonalargenumberofstarsseenasasinglepixeloftheimage.Eventsofstronglensingofasinglestarcanbede-tectedinthepixellightthoughtheyneverdominateit.Sucheventshavebeendiscussedinconnectionwiththegalax-iesM31andM87(Crotts1992;Baillonetal.1993;Gould1995),andafewobservationalprogramsmonitoringthese2Tuntsovetal.galaxieshavebeenimplemented(Crotts&Tomaney1996;Ansarietal.1997;Riffeseretal.2001;Baltzetal.2003).FurtherawayfromtheGalaxy,ithasbeenpro-posedtosearchformicrolensing-inducedvariabilityofquasarsseenthroughgalaxyclusters(Walker&Ireland1995;Tadros,Warren&Hewett1998).Thishasbeenun-dertakenfortheVirgocluster,althoughitscloseproximityensuresthattheopticaldepthtomicrolensingislow.Al-thoughanalysisofthequasarvariabilitybasedonalongob-servationseriesbyHawkins(1996)lethimconcludethatthedarkmattermaybedominatedbyJupiter-massmicrolenses,thisideaisnotwidelysupported.Oneofthedifficultieshereisthatquasarsareexpectedtopossessintrinsicvariability(Zackrissonetal.2003)whichsignificantlycomplicatesmi-crolensingstudies.Totani(2003)proposedtoexploretheadvantageoftherecentdiscoveryofagalaxyclusterfoundjustbehindtherichclusterAbell2152butthishasyettoresultinanobservationalprogram.Anotherpossibilityistolookforthevariationsinsur-facebrightnessofstronglylenseddistantgalaxies–e.g.,gi-antgravitationallylensedarcs(Lewis&Ibata2001;Lewis,Ibata&Wyithe2000).Theyarefoundintheregionsoflargemagnificationandthereforehighopticaldepth.As-sumingthatmicrolensesmakeupanappreciablefractionofthelensingmass,itmeansthatinanyinstantallthestarsinapixelaresubjecttostrongmicrolensingandthereforeaninvestigationofthesurfacebrightnessvariabili