Lyman continuum leakage in faint star-forming galaxies at redshift z=3-3.5 probed by gamma-ray bursts

Abstract

Context. The identification of the sources that reionized the Universe and their specific contribution to this process are key missing pieces of our knowledge of the early Universe. Faint star-forming galaxies may be the main contributors to the ionizing photon budget during the epoch of reionization, but their escaping photons cannot be detected directly due to inter-galactic medium opacity. Hence, it is essential to characterize the properties of faint galaxies with significant Lyman continuum (LyC) photon leakage up to z similar to 4 to define indirect indicators allowing analogs to be found at the highest redshift.Aims. Long gamma-ray bursts (LGRBs) typically explode in star-forming regions of faint, star-forming galaxies. Through LGRB afterglow spectroscopy it is possible to detect directly LyC photons. Our aim is to use LGRBs as tools to study LyC leakage from faint, star-forming galaxies at high redshift.Methods. Here we present the observations of LyC emission in the afterglow spectra of GRB 191004B at z=3.5055, together with those of the other two previously known LyC-leaking LGRB host galaxies (GRB 050908 at z=3.3467, and GRB 060607A at z=3.0749), to determine their LyC escape fraction and compare their properties.Results. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of log(N-HI/cm(-2)) = 17.2 +/- 0.15, and negligible extinction (A(V)=0.03 +/- 0.02 mag). The only metal absorption lines detected are CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Lyman-alpha (Ly alpha) emission. From its Ly alpha emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to 1 <= SFR <= 4.7 M-circle dot yr(-1). We fit the Ly alpha emission with a shell model and find parameters values consistent with the observed ones. The absolute (relative) LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of