Programme by Session

Whether stars could have driven the reionization of the intergalactic medium depends critically on the proportion of ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst (GRB) afterglows can be used to estimate the opacity to extreme ultraviolet (EUV) radiation along the lines-of-sight to the bursts. Assuming that long-duration GRBs trace the locations of the massive stars dominating EUV production, the average escape fraction of ionizing radiation can be calculated independently of galaxy size or luminosity. Analysis of a compilation of HI column density measures for 140 GRBs in the range 1.6 z 6.7 establishes an escape fraction at the Lyman limit of ⟨fesc⟩ ≈ 0.005, with a 98 per cent confidence upper limit of ⟨fesc⟩ ≈ 0.015. This suggests that stars provide a small contribution to the ionizing radiation budget at z 5. At higher redshifts firm conclusions are limited by the small size of the GRB sample, but any decline in average HI column density seems to be modest. No significant correlation of N(HI) is found with galaxy UV luminosity or host stellar mass. Given that many GRB hosts are low-metallicity dwarf galaxies with high specific star-formation rates, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the Universe.