Selecting ultra-faint dwarf candidate progenitors in cosmological N-body
  simulations at high redshifts
  The smallest satellites of the Milky Way ceased forming stars during the
epoch of reionization and thus provide archaeological access to galaxy
formation at $z>6$. Numerical studies of these ultra-faint dwarf galaxies
(UFDs) require expensive cosmological simulations with high mass resolution
that are carried out down to $z=0$. However, if we are able to statistically
identify UFD host progenitors at high redshifts \emph{with relatively high
probabilities}, we can avoid this high computational cost. To find such
candidates, we analyze the merger trees of Milky Way type halos from the
high-resolution ${\it Caterpillar}$ suite of dark matter only simulations.
Satellite UFD hosts at $z=0 $ are identified based on four different abundance
matching (AM) techniques. All the halos at high redshifts are traced forward in
time in order to compute the probability of surviving as satellite UFDs today.
Our results show that selecting potential UFD progenitors based solely on their
mass at z=12 (8) results in a 10\% (20\%) chance of obtaining a surviving UFD
at $z=0$ in three of the AM techniques we adopted. We find that the progenitors
of surviving satellite UFDs have lower virial ratios ($\eta$), and are
preferentially located at large distances from the main MW progenitor, while
they show no correlation with concentration parameter. Halos with favorable
locations and virial ratios are $\approx 3$ times more likely to survive as
satellite UFD candidates at $z=0.$
