Can Photoionization Squelching Resolve the Substructure Crisis?

Abstract

Cold Dark Matter theory predicts that the Local Group should contain many more dwarf-sized objects than the observed number of dwarf galaxies --- the so-called sub-structure problem. We investigate whether the suppression of star formation in these small objects due to the presence of a photoionizing background can resolve the problem. We make use of results from recent hydrodynamic simulations to build a recipe for the suppression of gas infall into semi-analytic galaxy formation models, and use these to predict the luminosity function of dwarf galaxies in the Local Group. In the models without photoionization ``squelching'', we predict a large excess of faint dwarf galaxies compared with the observed number in the Local Group --- thus, the usual recipe for supernovae feedback used in semi-analytic models does not solve the sub-structure problem on its own. When we include photoionization squelching, we find good agreement with the observations. We have neglected tidal destruction, which probably further reduces the number of dwarf galaxies. We conclude that photoionizing squelching easily solves the sub-structure problem. In fact, it is likely that once this effect is taken into account, models with reduced small-scale power (e.g. Warm Dark Matter) would underproduce dwarf galaxies.