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

Abstract

Photoionization by the high-redshift ultraviolet radiation background heats\nlow density gas before it falls into dark matter potential wells, and it\neliminates the neutral hydrogen and singly ionized helium that dominate cooling\nof primordial gas at temperatures of $10^4-10^5$K. We investigate the influence\nof photoionization on galaxy formation using high-resolution simulations with a\n1-dimensional, spherically symmetric, Lagrangian hydrodynamics/gravity code. We\nfind that the presence of a photoionizing background suppresses the formation\nof galaxies with circular velocities $v_{circ} \\simlt 30\\,$\\kms\\ and\nsubstantially reduces the mass of cooled baryons in systems with circular\nvelocities up to $v_{circ} \\sim 50\\,$\\kms. Above $v_{circ} \\sim 75\\,$\\kms,\nphotoionization has no significant effect. Photoionization exerts its influence\nprimarily by heating gas before collapse; the elimination of line cooling\nprocesses is less important. We discuss the implications of these results for\nhierarchical theories of galaxy formation.\n