Ultra-faint high-redshift galaxies in the Frontier Fields
  By combining cosmological simulations with Frontier Fields project lens
models we find that, in the most optimistic case, galaxies as faint as $m
\approx 33 - 34$ (AB magnitude at $1.6 \rm \mu m$) can be detected in the
Frontier Fields. Such faint galaxies are hosted by dark matter halos of mass
$\sim10^9 M_\odot$ and dominate the ionizing photon budget over currently
observed bright galaxies, thus allowing for the first time the investigation of
the dominant reionization sources. In addition, the observed number of these
galaxies can be used to constrain the role of feedback in suppressing star
formation in small halos: for example, if galaxy formation is suppressed in
halos with circular velocity $v_c < 50$ km s$^{-1}$, galaxies fainter than
$m=31$ should not be detected in the FFs.
