The initial mass function in the Coma Berenices dwarf galaxy from deep
  near-infrared HST observations
  We use deep $HST$ WFC3/IR imaging to study the Initial Mass Function (IMF) of
the ultra faint dwarf galaxy Coma Berenices (Com Ber). Our observations reach
the lowest stellar mass ever probed in a resolved galaxy, with 50\%
completeness at $\sim 0.17$ M$_{\odot}$. Unresolved background galaxies however
limit our purity below $\sim 0.23$ M$_{\odot}$. If modeled with a single power
law, we find that the IMF slope is $-1.45^{+0.29}_{-0.3}$ (68\% credible
intervals), compared to a Milky Way value of $-2.3$. For a broken power law, we
obtain a low-mass slope of $-1.18_{-0.33}^{+0.49}$, a high-mass slope of
$-1.88_{-0.49}^{+0.43}$ and a break mass of $0.57_{-0.08}^{+0.12}$ M$_{\odot}$,
compared to $-1.3$, $-2.3$ and 0.5 M$_{\odot}$ for a Kroupa IMF. For a
log-normal IMF model we obtain values of $0.33_{-0.16}^{+0.15}$ M$_{\odot}$ for
the location parameter and of $0.68_{-0.12}^{+0.17}$ for $\sigma$ (0.22
M$_{\odot}$ and 0.57 for the Chabrier system IMF). All three parametrizations
produce similar agreement with the data. Our results agree with previous
analysis of shallower optical HST data. However analysis of similar optical
data of other dwarfs finds IMFs significantly more bottom-light than in the
Milky Way. These results suggest two, non mutually exclusive, possibilities:
that the discrepancy of the dwarf galaxies IMF with respect to the Milky Way
is, at least partly, an artifact of using a single power law model, and that
there is real variance in the IMF at low masses between the currently studied
nearby dwarfs, with Com Ber being similar to the Milky Way, but other dwarfs
differing significantly.
