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Photoionization by the high-redshift ultraviolet radiation background heatslow density gas before it falls into dark matter potential wells, and iteliminates the neutral hydrogen and singly ionized helium that dominate coolingof primordial gas at temperatures of $10^4-10^5$K. We investigate the influenceof photoionization on galaxy formation using high-resolution simulations with a1-dimensional, spherically symmetric, Lagrangian hydrodynamics/gravity code. Wefind that the presence of a photoionizing background suppresses the formationof galaxies with circular velocities $v_{circ} \simlt 30\,$\kms\ andsubstantially reduces the mass of cooled baryons in systems with circularvelocities up to $v_{circ} \sim 50\,$\kms. Above $v_{circ} \sim 75\,$\kms,photoionization has no significant effect. Photoionization exerts its influenceprimarily by heating gas before collapse; the elimination of line coolingprocesses is less important. We discuss the implications of these results forhierarchical theories of galaxy formation.

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