GALAXY PROPERTIES AND UV ESCAPE FRACTIONS DURING THE EPOCH OF REIONIZATION: RESULTS FROM THE RENAISSANCE SIMULATIONS

Abstract

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 Renaissance Simulations with an eye to provide better inputs to global reionization simulations. This suite probes overdense, average, and underdense regions of the universe of several hundred comoving Mpc 3, each yielding a sample of over 3000 halos in the mass range of 10 7–10 9.5 at their final redshifts of 15, 12.5, and 8, respectively. In the process, we simulate the effects of radiative and supernova feedback from 5000 to 10,000 Population III stars in each simulation. We find that halos as small as 10 7 are able to host bursty star formation due to metal-line cooling from earlier enrichment by massive Population III stars. Using our large sample, we find that the galaxy-halo occupation fraction drops from unity at virial masses above 10 8.5 to ∼50% at 10 8 and ∼10% at 10 7 , quite independent of redshift and region. Their average ionizing escape fraction is ∼5% in the mass range of 10 8–10 9 and increases with decreasing halo mass below this range, reaching 40%–60% at 10 7 . Interestingly, we find that the escape fraction varies between 10%–20% in halos with virial masses of ∼3 × 10 9 . Taken together, our results confirm the importance of the smallest galaxies as sources of ionizing radiation contributing to the reionization of the universe.