Photoionization, Numerical Resolution, and Galaxy Formation

Using cosmological simulations that incorporate gas dynamics andgravitational forces, we investigate the influence of photoionization by a UVradiation background on the formation of galaxies. In our highest resolutionsimulations, we find that photoionization has essentially no effect on thebaryonic 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 massresolution of a simulation and the microphysics included in the computation ofheating and cooling rates. At low resolution, a photoionizing background canappear to suppress the formation of even relatively massive galaxies. However,when the same initial conditions are evolved with a factor of eight better massresolution, this effect disappears. Our results demonstrate the need for carein interpreting the results of cosmological simulations that incorporatehydrodynamics and radiation physics. For example, we conclude that a simulationwith limited resolution may yield more realistic results if it ignores somerelevant physical processes, such as photoionization. At higher resolution, thesimulated population of massive galaxies is insensitive to the treatment ofphotoionization and star formation, but it does depend significantly on theamplitude of the initial density fluctuations. By $z=2$, an $\Omega=1$ colddark matter model normalized to produce the observed masses of present-dayclusters has already formed galaxies with baryon masses exceeding $1e11M_\sun$.