Probing the Milky Way Stellar and Brown Dwarf Initial Mass Function with Modern Microlensing Observations

Abstract

We use recent microlensing observations toward the central bulge of the Galaxy to probe the overall stellar plus brown dwarf initial mass function (IMF) in these regions well within the brown dwarf domain. We find that the IMF is consistent with the same Chabrier IMF characteristic of the Galactic disk. In contrast, other IMFs suggested in the literature overpredict the number of short-time events, and thus of very low mass stars and brown dwarfs, compared with observations. This again supports the suggestion that brown dwarfs and stars predominantly form via the same mechanism. We show that claims for different IMFs in the stellar and substellar domains arise from an incorrect parameterization of the IMF. Furthermore, we show that the IMF in the central regions of the bulge seems to be bottom-heavy, as illustrated by the large number of short-time events compared with the other regions. This recalls our previous analysis of the IMF in massive early-type galaxies and suggests the same kind of two-phase formation scenario, with the central bulge initially formed under more violent, burst-like conditions than the rest of the Galaxy.

Keywords

• Stellar mass functions
• Initial mass function