Metal-Free Gas Supply at the Edge of Reionization: Late-Epoch Population III Star Formation

While the average metallicity of the intergalactic medium rises above Z & 10Z⊙ by the end of the reionization, pockets of metal-free gas can still exist at later times. We quantify the presence of a long tail in the formation rate of metalfree halos during late stages of reionization (redshift z ≈ 6), which might offer the best window to detect Population III stars. Using cosmological simulations for the growth of dark matter halos, coupled with analytical recipes for the metal enrichment of their interstellar medium, we show that pockets of metal-free gas exist at z ≈ 6 even under the assumption of high efficiency in metal pollution via winds. A comoving metal-free halo formation rate dn/dt dV & 10 Mpc yr is expected at z = 6 for halos with virial temperature Tvir ≈ 10 4 K (mass ∼ 10 M⊙), sufficient to initiate cooling even with strong negative radiative feedback. These halos will appear as absorption systems with a typical hydrogen