Probing the Ultraviolet Luminosity Function of the Earliest Galaxies with the Renaissance Simulations

In this paper, we present the first results from the Renaissance Simulations, a suite of extremely high-resolution and physics-rich AMR calculations of high-redshift galaxy formation performed on the Blue Waters supercomputer. These simulations contain hundreds of well-resolved galaxies at z∼ 25-8, and make several novel, testable predictions. Most critically, we show that the ultraviolet luminosity function of our simulated galaxies is consistent with observations of high-z galaxy populations at the bright end of the luminosity function ({M}₁₆₀₀≤slant -17), but at lower luminosities is essentially flat rather than rising steeply, as has been inferred by Schechter function fits to high-z observations, and has a clearly defined lower limit in UV luminosity. This behavior of the luminosity function is due to two factors: (i) the strong dependence of the star formation rate (SFR) on halo virial mass in our simulated galaxy population, with lower-mass halos having systematically lower SFRs and thus lower UV luminosities; and (ii) the fact that halos with virial masses below ≃ 2× {10}⁸ {M}_⊙ do not universally contain stars, with the fraction of halos containing stars dropping to zero at ≃ 7× {10}⁶ {M}_⊙ . Finally, we show that the brightest of our simulated galaxies may be visible to current and future ultra-deep space-based surveys, particularly if lensed regions are chosen for observation.