Reionization and the Abundance of Galactic Satellites

One of the main challenges facing standard hierarchical structure formationmodels is that the predicted abundance of galactic subhalos with circularvelocities of 10-30 km/s is an order of magnitude higher than the number ofsatellites actually observed within the Local Group. Using a simple model forthe formation and evolution of dark halos, based on the extendedPress-Schechter formalism and tested against N-body results, we show that thetheoretical predictions can be reconciled with observations if gas accretion inlow-mass halos is suppressed after the epoch of reionization. In this picture,the observed dwarf satellites correspond to the small fraction of halos thataccreted substantial amounts of gas before reionization. The photoionizationmechanism naturally explains why the discrepancy between predicted halos andobserved satellites sets in at about 30 km/s, and for reasonable choices of thereionization redshift (z_re = 5-12) the model can reproduce both the amplitudeand shape of the observed velocity function of galactic satellites. If thisexplanation is correct, then typical bright galaxy halos contain many low-massdark matter subhalos. These might be detectable through their gravitationallensing effects, through their influence on stellar disks, or as dwarfsatellites with very high mass-to-light ratios. This model also predicts adiffuse stellar component produced by large numbers of tidally disrupteddwarfs, perhaps sufficient to account for most of the Milky Way's stellar halo.Comment: 5 pages, 2 figures, Submitted to Ap