Probing MACHOs Toward the Galactic Bulge(14)

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

8log(dτ/dDl[kpc]) 9 10

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Galactocentric Distance (kpc)8

Fig.1.—OpticalDepthperunitpathlengthdτ/dDlasafunctionofdistancefromtheGalacticcenterforasourceneartheGalacticcenter.Thehalo(assumingaf=20%MACHOfraction)andthespheroidareshownbysolidanddashedcurves,respectively.Forf=20%,spheroidstarsarea20%backgroundatR=4kpcanda10%backgroundatR=7kpc,whichimpliesthat2or3halolensesmustbeidenti edatR>4kpcforareliablehalo“detection”.InsideR<4kpcthespheroidcontinuestogrow(andalsotransformsintothebulge),makingtheidenti cationofhalolenseslesssecure.Hence,theexperimentshouldberestrictedtoR>4kpc,wherethetotalopticaldepthisτ=5×10 8f.

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