The impact of spheroid stars for Macho microlensing surveys of the Andromeda Galaxy

Abstract

The Andromeda Galaxy (M31) is an important test case for a number of microlensing surveys looking for massive compact halo objects (Machos). A long-standing theoretical prediction is that the high inclination of the M31 disk should induce an asymmetry in the spatial distribution of M31 Macho events, whilst the distribution of variable stars and microlensing events in the M31 disk should be symmetric. We examine the role of stars in the M31 visible spheroid as both lenses and sources to microlensing events. We compute microlensing event number density maps and estimate pixel-lensing rates and event durations for three-component models of M31 which are consistent with the observed rotation curve, surface brightness profile and dynamical mass estimates. Three extreme models are considered: a massive spheroid model; a massive disk model; and a massive Macho halo model. An important consequence of the spheroid is that, even if Machos are absent in M31, an asymmetric spatial signature is still generally expected from stellar lensing alone. The relative mass-to-light ratio of the spheroid and disk populations controls which of these signatures dominates the overall stellar spatial distribution. We find that the inclusion of the spheroid weakens the M31 Macho spatial asymmetry by about 20-30% over a disk-only asymmetry for the models considered. We also find for our models that Machos dominate over most of the far disk provided they contribute at least ~25% of the halo dark matter density. The presence of the spheroid also has beneficial consequences for M31 lensing surveys. The stellar asymmetry is likely to be important in distinguishing between a spheroidal Macho halo or a highly flattened halo or dark matter dominated disk, since spatial asymmetries of opposing signs are expected in these cases. (Abridged)