Lyman continuum leakage in faint star-forming galaxies at redshiftz= 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 ∼ 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(NH I/cm−2) = 17.2 ± 0.15, and negligible extinction (AV = 0.03 ± 0.02 mag). The only metal absorption lines detected are C IV and Si IV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Lyman-alpha (Lyα) emission. From its Lyα emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to 1 ≤ SFR ≤ 4.7 M⊙ yr−1. We fit the Lyα 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 0.35−0.11+0.10 (0.43−0.13+0.12), 0.08−0.04+0.05 (0.08−0.04+0.05) and 0.20−0.05+0.05 (0.45−0.15+0.15), respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRBs presented here have all M1600 > −19.5 mag, with the GRB 060607A host at M1600 > −16 mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore, the time involved is minimal compared to galaxy studies.

Gamma-ray bursts as probes of high-redshift Lyman-αemitters and radiative transfer models

“…This predicts that in general, the column density probed by Lyα is lower than or equal to the density observed along the line of sight, in agreement with what we find for the LAE-LGRB modeling presented here. Placing constraints on the HI anisotropy is important also for the consequences this can have on the ionizing escape fraction of galaxies (e.g., Vielfaure et al 2020). Already from this work where we find log(N OA HI (LGRB)/N HI (Lyα)) ∼ 2−3 in two of the cases, we can speculate that it is difficult to explain this high anisotropy by purely turbulent driving.…”

“…The unconstrained fitting succeeds in reproducing all Lyα profiles of the golden sample (and GRB 191004B; see Vielfaure et al 2020). However, if we compare the parameter values resulting from the fitting with those observed, we find important discrepancies for GRBs 060926 and 070110.…”

“…This suggests that LyC emission can escape through holes in the ISM even if the Lyα photons probe denser neutral gas. The only LAE-LGRB for which LyC leakage has been detected along the LGRB line of sight ( f esc (LyC) = 0.35 +0.10 −0.11 ) is GRB 191004B (Vielfaure et al 2020). However, Fig.…”

“…We also numerically integrated the flux over the line width as a comparison to control the consistency of the values and uncertainties. For the asymmetric line profile of the Lyα line, we use a skewed Gaussian parameterized as described in Vielfaure et al (2020). When lines of interest were not detected, we estimated a 3σ upper limit.…”

Exploration of the high-redshift universe enabled by THESEUS

“…Useful constraints have so far only been possible at z = 2-5, indicating an upper limit of f esc < 2% (Fig. 7; [100,101]), although note this is lower than found in the recent study of stacked LBG galaxy spectra at z ∼ 3 by [102]. However, future observations of the population of z > 5 GRBs detected by THESEUS, with both on-board spectroscopy and with 30 m class ground-based Fig.…”