Can Photoionization Squelching Resolve the Substructure Crisis?

Cold Dark Matter theory predicts that the Local Group should contain manymore dwarf-sized objects than the observed number of dwarf galaxies --- theso-called sub-structure problem. We investigate whether the suppression of starformation in these small objects due to the presence of a photoionizingbackground can resolve the problem. We make use of results from recenthydrodynamic simulations to build a recipe for the suppression of gas infallinto semi-analytic galaxy formation models, and use these to predict theluminosity function of dwarf galaxies in the Local Group. In the models withoutphotoionization ``squelching'', we predict a large excess of faint dwarfgalaxies compared with the observed number in the Local Group --- thus, theusual recipe for supernovae feedback used in semi-analytic models does notsolve the sub-structure problem on its own. When we include photoionizationsquelching, we find good agreement with the observations. We have neglectedtidal destruction, which probably further reduces the number of dwarf galaxies.We conclude that photoionizing squelching easily solves the sub-structureproblem. In fact, it is likely that once this effect is taken into account,models with reduced small-scale power (e.g. Warm Dark Matter) wouldunderproduce dwarf galaxies.Comment: LaTeX, 5 pages, 2 figures, Submitted to ApJ Letter