Rotating Halos and the Microlensing MACHO Mass Estimate

Abstract

We investigate the implications of a bulk rotational component of the halo velocity distribution for MACHO mass estimates. We find that for a rotating halo to yield a MACHO mass estimate significantly below that of the standard spherical case, its microlensing must be highly concentrated close to the Sun. We examine two classes of models fitting this criteria: a highly flattened $1/r^2$ halo, and a spheroid-like population with whose density falls off as $1/r^{3.5}$. The highly flattened $1/r^2$ models can decrease the implied average MACHO mass only marginally and the spheroid models not at all. Generally, rotational models cannot bring the MACHO mass implied by the current microlensing data down to the substellar range.