Hydrodynamic Simulations of Galaxy Formation. II. Photoionization and the Formation of Low Mass Galaxies

Abstract

Photoionization by the high-redshift ultraviolet radiation background heats low density gas before it falls into dark matter potential wells, and it eliminates the neutral hydrogen and singly ionized helium that dominate cooling of primordial gas at temperatures of $10^4-10^5$K. We investigate the influence of photoionization on galaxy formation using high-resolution simulations with a 1-dimensional, spherically symmetric, Lagrangian hydrodynamics/gravity code. We find that the presence of a photoionizing background suppresses the formation of galaxies with circular velocities $v_{circ} \simlt 30\,$\kms\ and substantially reduces the mass of cooled baryons in systems with circular velocities up to $v_{circ} \sim 50\,$\kms. Above $v_{circ} \sim 75\,$\kms, photoionization has no significant effect. Photoionization exerts its influence primarily by heating gas before collapse; the elimination of line cooling processes is less important. We discuss the implications of these results for hierarchical theories of galaxy formation.