Metal-free gas supply at the edge of reionization: Late-epoch population III star formation

Abstract

While the average metallicity of the intergalactic medium rises above Z ≳ 10-3 Z 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 metal-free 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 d 2 n/dt dV ≳ 10-9 Mpc-3 yr-1 is expected at z = 6 for halos with virial temperature T vir 104 K (mass 108 M), sufficient to initiate cooling even with strong negative radiative feedback. These halos will appear as absorption systems with a typical hydrogen column density of 1020 cm-2, a sky covering factor 5 × 10-3 and a number density of 25 arcmin-2 for 5.5 ≤ z ≤ 6.5. Under the assumption of a single Population III supernova (SN) formed per metal-free halo, we expect an observed supernova rate of 2.6 × 10 -3 deg-2 yr-1 in the same redshift range. These metal-free stars and their SNe will be isolated and outside galaxies (at distances ≳150 h -1 kpc) and thus significantly less biased than the general population of 108 M halos at z 6. SN searches for metal-free explosions must thus rely on large area surveys. If metal-free stars produce very luminous SNe, like SN2006gy, then a multi-epoch survey reaching m AB = 27 at 1 μm is sufficient for detecting them at z = 6. While the Large Synoptic Survey Telescope (LSST) will not reach this depth in the z band, it will be able to detect several tens of Population III SNe in the i and r bands at z ≲ 5.5, when their observed rate is down to 3-8 × 10 -4 deg-2 yr-1. © 2009 The American Astronomical Society. All rights reserved.