High-redshift galaxy outflows and the formation of dwarf galaxies

We examine the effects of galaxy outflows on the formation of dwarf galaxies in numerical simulations of the high-redshift universe. Using a smoothed particle hydrodynamic code, we conduct two detailed simulations of a (5.2 Mpc h^-1)³ comoving volume of the universe. In both simulations we implement simple, well-motivated models of galaxy identification and star formation, while our second simulation also includes a simple "blow-out" model of galaxy outflows in which supernova driven winds from newly formed disk galaxies punch out and shock the intergalactic medium while leaving the host galaxies intact. A direct comparison between these simulations suggests that there are two major mechanisms by which outflows affect dwarf formation. First, the formation of an outflow slows down the further accretion of gas onto a galaxy, causing an overall decrease of approximately 50% in the total gas mass accreted by the objects in our simulations. Additionally, our simulations uncover a significant population of ∼ 10⁹ M_⊙ objects whose formation is suppressed by the mechanism of "baryonic stripping," in which outflows from early galaxies strip the gas out of nearby overdense regions that would have otherwise later formed into dwarf galaxies. This mechanism may be important in explaining the observed discrepancy between the number of dwarf galaxies predicted and observed in the local group and provide a natural explanation for the formation of empty halos which may be required by the existence of the extremely gas-poor extragalactic high-velocity clouds.