Early Metal Enrichment by Pregalactic Outflows: II. Simulations of Blow-away

Abstract

We present results from three-dimensional numerical simulations of the dynamics of SN-driven bubbles as they propagate through and escape the grasp of subgalactic halos with masses M ~ 10^8 Msun at redshift z=9. The hydrodynamic simulations use a nested grid method to follow the evolution of explosive multi-SN events operating on the characteristic timescale of a few x 10^7 yr, the lifetime of massive stars. The results confirm that, if the star formation efficiency of subgalactic halos is close to 10%, a significant fraction of the halo gas will be lifted out of the potential well (`blow-away'), shock the intergalactic medium, and pollute it with metal-enriched material, a scenario recently advocated by Madau, Ferrara & Rees (2001). The volume filling factor of the ejecta is close to unity. Depending on the stellar distribution, we find that less than 30% of the available SN energy is converted into kinetic energy of the blown away material, the remainder being radiated away. It appears that mechanical feedback is less efficient than expected from simple energetic arguments, as off-nuclear SN explosions drive inward-propagating shocks that tend to collect and pile up cold gas in the central regions of the host halo. Low-mass galaxies at early epochs may survive multiple SN events and continue forming stars.