Early Metal Enrichment of the Intergalactic Medium by Pregalactic Outflows

Abstract

We assess supernova (SN)-driven pregalactic outflows as a mechanism for distributing the product of stellar nucleosynthesis over large cosmological volumes prior to the reionization epoch. SN ejecta will escape the grasp of halos with virial temperatures T_vir>10^{4.3} K (corresponding to masses M>10^8 h^{-1} M_sun at redshift z=9 when they collapse from 2-sigma fluctuations) if rapid cooling can take place, and a significant fraction of their baryonic mass is converted into stars over a dynamical timescale. We study the evolution of SN-driven bubbles as they blow out from subgalactic halos and propagate into the intergalactic medium (IGM), and show that to lift the halo gas out of the potential well the energy injection must continue at least until blow-away occurs. The collective explosive output of about ten thousands SNe per halo at these early epochs could pollute vast regions of intergalactic space to a mean metallicity =Omega_Z/Omega_b>0.003 (comparable to the levels observed in the Lyman-alpha forest at z=3) without much perturbing the IGM hydrodynamically, i.e. producing large variations of the baryons relative to the dark matter. The volume filling factor of the ejecta is higher than 20% if the star formation efficiency is of order 10%. When the filling factor of the ejecta becomes significant, enriched material will typically be at a higher adiabat than expected from photoionization.