2018ApJ...855...20G

Using deep observations obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST), we demonstrate that the sub-solar stellar initial mass function (IMF) of six 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 ∼0.45 M_☉, while the upper limit is ∼0.8 M_☉, set by the turnoff 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 Bootes I. The significance of these deviations depends on the galaxy and is typically 95% or more. When modeled as a log-normal, the IMF fit results in a higher peak mass than in the Milky Way disk, but a Milky Way disk value for the characteristic system mass (∼0.22 M_☉) is excluded at only 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 the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope (WFIRST) will enhance both the number of dwarf Milky Way satellites that can be studied in such detail and the observation depth for individual galaxies.