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Using deep HST/ACS observations, we demonstrate that the sub-solar stellarinitial 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 lowermass 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. Ifformulated 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 $-1.87$ forBo\"otes I. The significance of such deviations depends on the galaxy and istypically 95\% or more. When modeled as a log-normal, the IMF fit results in alarger peak mass than in the Milky Way disk, however a Milky Way disk value forthe characteristic system mass ($\sim0.22$ M$_{\odot}$) is excluded only at68\% significance, and only for some UFDs in the sample. We find that the IMFslope correlates well with the galaxy mean metallicity and, to a lesser degree,with the velocity dispersion and the total mass. The strength of the observedcorrelations is limited by shot noise in the number of observed stars, butfuture space-based missions like JWST and WFIRST will both enhance the numberof dwarf Milky Way Satellites that can be studied in such detail, and theobservation depth for individual galaxies.

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