Hydrodynamic simulations of galaxy formation .2. 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) less than or similar to 30 km s(-1) and substantially reduces the mass of cooled baryons in systems with circular velocities up to v(circ) similar to 50 km s(-1). Above v(circ) similar to 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.