Evidence of a non universal stellar Initial Mass Function. Insights from HST optical imaging of 6 Ultra Faint Dwarf Milky Way Satellites

TL;DR

This study reveals that the initial mass function in six ultra-faint dwarf galaxies is more bottom-light than in the Milky Way, with variations linked to metallicity and galaxy properties, challenging the notion of a universal IMF.

Contribution

First direct measurement of the IMF in ultra-faint dwarf galaxies showing deviations from the Milky Way's IMF, highlighting environmental dependence.

Findings

IMF slopes are shallower than Salpeter in all six UFDs.

IMF peak mass is larger than in the Milky Way disk.

IMF slope correlates with metallicity and galaxy mass.

Abstract

Using deep HST/ACS observations, we demonstrate that the sub-solar stellar initial mass function (IMF) of 6 ultra-faint dwarf Milky Way Satellites (UFDs) is more bottom light than the IMF of the Milky Way disk. Our data have a lower mass limit of about 0.45 M$_{\odot}$, while the upper limit is $\sim 0.8$ M$_\odot$, set by the turn-off mass of these old, metal poor systems. If formulated as a single power law, we obtain a shallower IMF slope than the "Salpeter" value of $-2.3$, ranging from $-1.01$ for Leo IV, to $-1.87$ for Bo\"otes I. The significance of such deviations depends on the galaxy and is typically 95\% or more. When modeled as a log-normal, the IMF fit results in a larger peak mass than in the Milky Way disk, however a Milky Way disk value for the characteristic system mass ($\sim0.22$ M$_{\odot}$) is excluded only at 68\% significance, and only for some UFDs in the sample. We find that the IMF slope correlates well with the galaxy mean metallicity and, to a lesser degree, with the velocity dispersion and the total mass. The strength of the observed correlations is limited by shot noise in the number of observed stars, but future space-based missions like JWST and WFIRST will both enhance the number of dwarf Milky Way Satellites that can be studied in such detail, and the observation depth for individual galaxies.