  Using cosmological simulations that incorporate gas dynamics and
gravitational forces, we investigate the influence of photoionization by a UV
radiation background on the formation of galaxies. In our highest resolution
simulations, we find that photoionization has essentially no effect on the
baryonic mass function of galaxies at $z=2$, down to our resolution limit of
$5e9 M_\sun$. We do, however, find a strong interplay between the mass
resolution of a simulation and the microphysics included in the computation of
heating and cooling rates. At low resolution, a photoionizing background can
appear to suppress the formation of even relatively massive galaxies. However,
when the same initial conditions are evolved with a factor of eight better mass
resolution, this effect disappears. Our results demonstrate the need for care
in interpreting the results of cosmological simulations that incorporate
hydrodynamics and radiation physics. For example, we conclude that a simulation
with limited resolution may yield more realistic results if it ignores some
relevant physical processes, such as photoionization. At higher resolution, the
simulated population of massive galaxies is insensitive to the treatment of
photoionization and star formation, but it does depend significantly on the
amplitude of the initial density fluctuations. By $z=2$, an $\Omega=1$ cold
dark matter model normalized to produce the observed masses of present-day
clusters has already formed galaxies with baryon masses exceeding $1e11
M_\sun$.

Astrophysics