The fraction of ionizing radiation from massive stars that escapes to the intergalactic medium

Abstract

The part played by stars in the ionization of the intergalactic medium (IGM) remains an open question. A key issue is the proportion of the stellar ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst (GRB) afterglows can be used to determine the neutral hydrogen column-density, N H i , in their host galaxies and hence the opacity to extreme ultra-violet (EUV) radiation along the lines-of-sight to the bursts. Thus, making the reasonable assumption that longduration GRB locations are representative of the sites of massive stars that dominate EUV production, one can calculate an average escape fraction of ionizing radiation in a way that is independent of galaxy size, luminosity or underlying spectrum. Here we present a sample of N H i measures for 138 GRBs in the range 1.6 < z < 6.7 and use it to establish an average escape fraction at the Lyman limit of f esc ≈ 0.005, with a 98% confidence upper limit of f esc ≈ 0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget of the IGM at z < 5, where the bulk of the bursts lie. At higher redshifts, z > 5, firm conclusions are limited by the small size of the GRB sample (7/138), but any decline in average H i column-density seems to be modest. We also find no indication of a significant correlation of N H i with galaxy UV luminosity or host stellar mass, for the subset of events for which these are available. We discuss in some detail a number of selection effects and potential biases. Drawing on a range of evidence we argue that such effects, while not negligible, are unlikely to produce systematic errors (in either direction) of more than a factor ∼ 2 in f esc , and so would not affect the primary conclusions. Given that many GRB hosts are low metallicity, high specific star-formation rate, dwarf galaxies, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the universe.

The role of galaxies and AGN in reionizing the IGM – III. IGM–galaxy cross-correlations at z ∼ 6 from eight quasar fields with DEIMOS and MUSE

“…In practice however, a model where only galaxies brighter than M UV = −20.0 is implausible because it would require an overhelmingly high luminosity-averaged escape fraction of ≈ 1, contradicting existing z ∼ 6 measurements (Matthee et al 2018) and marking a stark departure from measurements at lower-redshift (e.g. Vanzella et al 2016;Izotov et al 2016Izotov et al , 2018Tanvir et al 2019;Fletcher et al 2019). It thus more likely that, if only the brightest objects contribute, they include at least relatively faint galaxies down to M UV ∼ −18 (−16).…”

“…Direct measurements of the ionising parameters of galaxies have proven challenging and offer a fractured picture. Spectroscopy of the afterglow of gamma ray bursts indicate extremely low escape fractions 2 % for their hosts (Tanvir et al 2019), whereas the peak separation of Lyman-α in a bright LAE (COLA1) results in an indirect measurement of f esc ∼ 15 − 30 % (Matthee et al 2018). Individual galaxies at high redshift have been found to have hard radiation spectra (e.g.…”

Cosmological Evolution of the Absorption of γ-Ray Burst X-Ray Afterglows

“…In order to further understand the absorption trend with z, we examine measurements of the HI Ly α absorption line, which is redshifted into the visible band for z > 1.6 and provides direct measurement of the host neutral hydrogen column density. Tanvir et al (2019) conducted an extensive analysis of GRB optical afterglows. We create a sub-sample from the overlap between the Swift XRT sample and the Tanvir et al (2019) sample.…”

“…Since absorber ionization and abundances could evolve with redshift, these explanations can be tested by studying redshift trends. Now that larger samples of optical afterglows are available (e.g., Tanvir et al 2019, and references therein) such correlations and trends should be revisited.…”

New Insights on Lyα and Lyman Continuum Radiative Transfer in the Greenest Peas*

“…† Hubble Fellow Vanzella et al 2016;Fletcher et al 2018). On the contrary, most galaxy samples show low LyC escape fractions (f esc,LyC ) < 10% (e.g., Leitherer et al 1995;Leitet et al 2013;Rutkowski et al 2016;Naidu et al 2018;Tanvir et al 2019). However, the recent detection of LyC radiation with f esc,LyC = 2.5 − 72% from 11 out of 11 targeted "Green Pea" (GP) galaxies (Izotov et al 2016a(Izotov et al ,b, 2018a demonstrates the existence of a population of star-forming galaxies where LyC escape is not rare, but common.…”