Substellar mass function and maximum baryonic mass in the halo of the Galaxy

Abstract

We use recently observed luminosity functions of halo M-dwarfs and an accurate mass-luminosity relationship to derive the mass function of Pop II low-mass stars in the galactic spheroid. All these luminosity functions yield rising mass functions near the hydrogen-burning limit (with α ≈ 2). Connecting the microlensing and star count constraints, we show that the mass function in the dark halo steepens significantly (α ≳ 4). The minimum mass is found to be minf ≈ 0.02M⊙, which corresponds to an average mass ≈ 0.03M⊙, strongly suggesting that the dark objects responsible for the microlensing events are genuine brown dwarfs. We determine the expected spheroid and halo brown-dwarf density in the solar neighbourhood. The mass fraction under the form of substellar objects in the halo of the Galaxy is likely to represent only 5 to 15 % of the total dynamic mass.