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

¹ GEPI, Observatoire de Paris, PSL University, CNRS, Place Jules Janssen, 92190 Meudon, France
e-mail: jean-baptiste.vielfaure@obspm.fr
² Institut d’Astrophysique de Paris, UMR 7095, CNRS-SU, 98 bis boulevard Arago, 75014 Paris, France
³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁴ Cosmic Dawn Center (DAWN), Denmark
⁵ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁶ Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁷ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
⁸ DTU Space, National Space Institute, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
⁹ Department of Physics & Astronomy and Leicester Institute of Space & Earth Observation, University of Leicester, University Road, Leicester LE1 7RH, UK
¹⁰ ASI-Space Science Data Center, Via del Politecnico snc, 00133 Rome, Italy
¹¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
¹² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹³ INAF – IASF/Milano, Via Corti 12, 20133 Milano, Italy
¹⁴ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
¹⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁶ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁷ Université Paris Diderot, AIM, Sorbonne Paris Cité, CEA, CNRS, 91191 Gif-sur-Yvette, France
¹⁸ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, LC, Italy
¹⁹ Department of Astronomy and Space Sciences, Istanbul University, Beyazıt 34119, Istanbul, Turkey
²⁰ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, USA
²¹ Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
²² Astronomy, Physics and Statistics Institute of Sciences (APSIS), 725 21st Street NW, Washington, DC 20052, USA
²³ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
²⁴ Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands
²⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy

Received: 1 May 2020
Accepted: 14 June 2020

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(N_H I/cm⁻²) = 17.2 ± 0.15, and negligible extinction (A_V = 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.