Probing MACHOs Toward the Galactic Bulge

If the massive compact halo object (MACHO) fraction of the Galactic dark halo is f ~ 20% as suggested by some microlensing experiments, then about 1.2% of lensing events toward the Galactic bulge are due to MACHOs. For the 40% of these that lie nearby (D_l

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aProbingMACHOsTowardtheGalacticBulgeAndrewGouldDepartmentofAstronomy,TheOhioStateUniversity,140W.18thAve.,Columbus,OH43210gould@astronomy.ohio-state.eduABSTRACTIfthemassivecompacthaloobject(MACHO)fractionoftheGalacticdarkhaloisf～20%assuggestedbysomemicrolensingexperiments,thenabout1.2%oflensingeventstowardtheGalacticbulgeareduetoMACHOs.Forthe40%ofthesethatlienearby(Dl<4kpc),measurementoftheirdistanceDlwoulddistinguishthemfrombulgelenses,whilemeasurementoftheirtransversevelocityvlwoulddistinguishthemfromdisklenses.Hence,itwouldbepossibletoidentifyabout0.5%(f/20%)ofalleventsasduetoMACHOs.IshowthataplannedexperimentusingtheSpaceInterferometryMission(SIMPlanetQuest)couldtherebydetect1or2suchevents.Thisisatthemarginofwhatisrequiredbecauseofasmall,butnon-negligiblebackgroundfromspheroidstars.Subjectheadings:darkmatter–galaxies:stellarcontent–gravitationallensing–instrumentation:interferometers1.Introduction

FollowingthesuggestionofPaczy´nski(1986),theMACHO(Alcocketal.1993)andEROS(Aubourgetal.1993)collaborationsbegansearchingfordarkmatterintheformofmassivecompacthaloobjects(MACHOs)bymicrolensingobservationstowardtheLargeMagellanicCloud(LMC).ThistargetseemedidealbecauseofthesmallcolumnofknownpopulationsofstarscomparedtothehugevolumeofspacethatwouldbehometotheputativeMACHOs.Themicrolensingopticaldepthduetoknownstarswasestimatedtobeτ 9MWLMC=8×10fortheMilkyWaydisk(Gouldetal.1997)andτLMCLMC=1×10 8fortheLMCitself(Gould1995b).Bycontrast,ifthedarkhalowerecompletelycomposedofMACHOs,theiropticaldepthwouldbeoforder,

τLMCv2rothalo～

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