Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo?

Abstract

Models of our galaxy based on dynamical observations predict a spheroid component much heavier than accounted for by direct measurements of star counts and high velocity stars. If, as first suggested by Caldwell and Ostriker, this discrepancy is due to a large population of faint low-mass stars or dark objects in the spheroid, the spheroid could be responsible for microlensing events for sources in the Large Magellanic Cloud (LMC). We show that, although the rate of events is lower than predicted by a galactic halo made of microlensing objects, it is still significant for EROS/MACHO observations. Because of the different matter distributions in the halo and spheroid components, a comparison between microlensing event rates in the LMC, future measurements of microlensing in the galactic bulge and, possibly, in M31 can provide information about the amounts of dark objects in the different galactic components. If the EROS/MACHO collaborations find a deficiency with respect to their halo expectation, when more statistics are available, their detected events could be interpreted as coming from spheroid microlenses, allowing for a galactic halo composed entirely of non-baryonic dark matter.