Galaxy Properties and UV Escape Fractions During Epoch of Reionization:
  Results from the Renaissance Simulations
  Cosmic reionization is thought to be primarily fueled by the first
generations of galaxies. We examine their stellar and gaseous properties,
focusing on the star formation rates and the escape of ionizing photons, as a
function of halo mass, redshift, and environment using the full suite of the
{\it Renaissance Simulations} with an eye to provide better inputs to global
reionization simulations. This suite, carried out with the adaptive mesh
refinement code Enzo, is unprecedented in terms of their size and physical
ingredients. The simulations probe overdense, average, and underdense regions
of the universe of several hundred comoving Mpc$^3$, each yielding a sample of
over 3,000 halos in the mass range $10^7 - 10^{9.5}~\Ms$ at their final
redshifts of 15, 12.5, and 8, respectively. In the process, we simulate the
effects of radiative and supernova feedback from 5,000 to 10,000 metal-free
(Population III) stars in each simulation. We find that halos as small as
$10^7~\Ms$ are able to form stars due to metal-line cooling from earlier
enrichment by massive Population III stars. However, we find such halos do not
form stars continuously. Using our large sample, we find that the galaxy-halo
occupation fraction drops from unity at virial masses above $10^{8.5}~\Ms$ to
$\sim$50\% at $10^8 ~\Ms$ and $\sim$10\% at $10^7~\Ms$, quite independent of
redshift and region. Their average ionizing escape fraction is $\sim$5\% in the
mass range $10^8 - 10^9~\Ms$ and increases with decreasing halo mass below this
range, reaching 40--60\% at $10^7~\Ms$. Interestingly, we find that the escape
fraction varies between 10--20\% in halos with virial masses $\sim 3 \times
10^9~\Ms$. Taken together, our results confirm the importance of the smallest
galaxies as sources of ionizing radiation contributing to the reionization of
the universe.
