Lower Metal Enrichment of Virialized Gas in Minihalos

We differentiate between the metal enrichment of the gas in virialized minihalos and that of the intergalactic medium at high redshift, pertinent to cosmological reionization, with the initial expectation that gas in the high density regions within formed dark matter halos may be more robust thus resistant to mixing with lower density intergalactic medium. Using detailed hydrodynamic simulations of gas clouds in minihalos subject to destructive processes associated with the encompassing intergalactic shocks carrying metal-enriched gas, we find, as an example, that, for realistic shocks of velocities of 10− 100km/s, more than (90%, 65%) of the high density gas with ρ ≥ 500ρb inside a minihalo virialized at z = 10 of mass (10, 10)M⊙ remains at a metallicity lower than 3% of that of the intergalactic medium by redshift z = 6. It may be expected that the high density gas in minihalos becomes fuel for subsequent star formation, when they are incorporated into larger halos where efficient atomic cooling can induce gas condensation hence star formation. Since minihalos virialize at high redshift when the universe is not expected to have been significantly reionized, the implication is that gas in virialized minihalos may provide an abundant reservoir of primordial gas to possibly allow for the formation of Population-III metal-free stars to extend to much lower redshift than otherwise expected based on the enrichment of intergalactic medium. Subject headings: supernovae: general — galaxies: formation — intergalactic medium — cosmology: theory — early universe