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 (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.

To appear in ApJ Letters

References in corpus (23)

• A systematic variation of the stellar initial mass function in early-type galaxies
• The Structure of the Milky Way's Bar Outside the Bulge
• No large population of unbound or wide-orbit Jupiter-mass planets
• Improved GRAVITY astrometric accuracy from modeling of optical aberrations
• The Field Substellar Mass Function Based on the Full-sky 20-pc Census of 525 L, T, and Y Dwarfs
• Stellar Proper Motions in the Galactic Bulge from deep HST ACS/WFC Photometry
• OGLE-IV: Fourth Phase of the Optical Gravitational Lensing Experiment
• Variations of the stellar initial mass function in the progenitors of massive early-type galaxies and in extreme starburst environments
• Evidence for a Turnover in the IMF of Low Mass Stars and Sub-stellar Objects: Analysis from an Ensemble of Young Clusters
• A rich population of free-floating planets in the Upper Scorpius young stellar association
• OGLE-III Microlensing Events and the Structure of the Galactic Bulge
• The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis
• The age-chemical abundance structure of the Galactic disc II: $\alpha$-dichotomy and thick disc formation
• The Bulge Radial Velocity Assay for RR Lyrae stars (BRAVA-RR) DR2: a Bimodal Bulge?
• Microlensing constraints on the Galactic Bulge Initial Mass Function
• Microlensing as a probe of the Galactic structure; 20 years of microlensing optical depth studies
• Exploring the stellar age distribution of the Milky Way Bulge using APOGEE
• The Initial Mass Function of the Inner Galaxy Measured From OGLE-III Microlensing Timescales
• The Milky Way's bulge star formation history as constrained from its bimodal chemical abundance distribution
• VVV Survey Microlensing Events in the Galactic Center Region
• Chemical evolution of the Galactic Center
• Microlensing optical depth as a function of source apparent magnitude
• An MCMC Approach to the Three-dimensional Structure of the Milky Way Bulge using OGLE-IV $\delta$ Scuti Stars

Cited by in corpus (1)

• The origin of free-floating planets