MACHO versus HST: how bright can dark matter be?

Abstract

Recent results from gravitational microlensing experiments, such as MACHO, indicate that a substantial fraction of the Galactic dark matter (DM) is in compact form, with a typical mass in the range ~0.05 - 1 Mo. This mass range favours the DM being either low-mass hydrogen-burning stars or white-dwarf remnants of an early stellar population. There appears however to be a conflict between microlensing and other results, such as from Hubble Space Telescope (HST), which argue against a high DM fraction in hydrogen-burning stars or their remnants. Here I discuss the possibility that the DM detected by MACHO comprises low-mass stars residing in globular cluster associations. I show that such a scenario can reconcile HST and MACHO results and can also satisfy cluster dynamical constraints.