Probing the Milky Way stellar and brown dwarf initial mass function with modern microlensing observations

TL;DR

This study uses microlensing data to analyze the initial mass function in the Galactic bulge, finding it consistent with the disk IMF and suggesting a common formation mechanism for stars and brown dwarfs.

Contribution

It demonstrates that the Galactic bulge's IMF aligns with the Chabrier (2005) IMF and clarifies misconceptions about different IMFs in stellar and substellar domains.

Findings

IMF in the bulge matches the Chabrier (2005) IMF

Other proposed IMFs overpredict low-mass objects

Central bulge IMF appears bottom-heavy

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 (2005) IMF characteristic of the Galactic disk. In contrast, other IMFs suggested in the literature overpredict the number of short-time events, thus of very-low mass stars and brown dwarfs, compared with observations. This, again, supports the suggestion that brown dwarfs and stars form predominantly via the same mechanism. We show that claims for different IMFs in the stellar and substellar domains rather 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.