Early Metal Enrichment of the Intergalactic Medium by Pregalactic Outflows

Abstract

We assess supernova-driven pregalactic outflows as a mechanism for distributing the product of stellar nucleosynthesis over large cosmological volumes prior to the reionization epoch. Supernova (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 ☉ at redshift z = 9 when they collapse from 2 σ 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 we show that to lift the halo gas out of the potential well, the energy injection must continue at least until blowaway occurs. If the fraction of ionizing photons that escape the dense sites of star formation into intergalactic space is greater than a few percent, pregalactic outflows will propagate into an IGM that has been prephotoionized by the same massive stars that later explode as SNe, and the expansion of the metal-enriched bubbles will be halted by the combined action of external pressure, gravity, and radiative losses. The collective explosive output of about 10,000 SNe per M ≳ 10 8 h -1 M ☉ halo at these early epochs could pollute vast regions of intergalactic space to a mean metallicity ⟨ Z ⟩ = Ω Z /Ω b ≳ 0.003 (comparable to the levels observed in the Lyα forest at z ≈ 3) without hydrodynamically perturbing the IGM much, i.e., producing large variations of the baryons relative to the dark matter. Rayleigh-Taylor instabilities between the dense shell that contains pristine swept-up material and the hot, metal-enriched, low-density bubble may contribute to the mixing and diffusion of heavy elements. The volume filling factor of the ejecta is higher than 20% if the star formation efficiency is on the order of 10%. Larger filling factors (not required by current observations) may be obtained for larger efficiencies, moderately top-heavy initial mass functions, halos for which a significant fraction of the gas is in a galactic disk and does not couple to the outflow (since matter is ejected perpendicularly to the disk), or from a population of more numerous sources—which would therefore have to originate from lower amplitude peaks. When the filling factor of the ejecta becomes significant, enriched material typically will be at a higher adiabat than expected from photoionization.