  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.
