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

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 one-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_ <~ 30 km s^-1^ and substantially reduces the mass of cooled baryons in systems with circular velocities up to v_circ_ ~ 50 km s^-1^. Above v_circ_ ~ 75 km s^-1^, 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.