New constraints on the physical conditions in H₂-bearing GRB-host damped Lyman-α absorbers^⋆,⋆⋆

¹ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
e-mail: keh14@hi.is
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001 Santiago 19, Chile
⁴ Institut d’Astrophysique de Paris, CNRS-SU, UMR7095, 98bis bd Arago, 75014 Paris, France
⁵ Cosmic DAWN Center NBI/DTU-Space, Denmark
⁶ Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
⁷ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
⁸ Space Science Data Center, SSDC, ASI, Via del Politecnico snc, 00133 Roma, Italy
⁹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
¹⁰ Department of Astronomy and Space Sciences, Istanbul University, Beyazit, 34119 Istanbul, Turkey
¹¹ Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, USA
¹² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹⁵ INAF – IASF/Milano, Via Corti 12, 20133 Milano, Italy
¹⁶ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
¹⁷ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
¹⁸ GEPI, Observatoire de Paris, PSL University, CNRS, Place Jules Janssen, 92190 Meudon, France
¹⁹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
²⁰ Australian Astronomical Optics, Macquarie University, 105 Delhi Road, North Ryde, NSW 2113, Australia

Received: 5 July 2019
Accepted: 6 August 2019

Abstract

We report the detections of molecular hydrogen (H₂), vibrationally-excited H₂ (H₂^∗), and neutral atomic carbon (C I), an efficient tracer of molecular gas, in two new afterglow spectra of GRBs 181020A (z = 2.938) and 190114A (z = 3.376), observed with X-shooter at the Very Large Telescope (VLT). Both host-galaxy absorption systems are characterized by strong damped Lyman-α absorbers (DLAs) and substantial amounts of molecular hydrogen with logN(H I, H₂) = 22.20 ± 0.05,  20.40 ± 0.04 (GRB 181020A) and logN(H I, H₂) = 22.15 ± 0.05,  19.44 ± 0.04 (GRB 190114A). The DLA metallicites, depletion levels, and dust extinctions are within the typical regimes probed by GRBs with [Zn/H] = −1.57 ± 0.06, [Zn/Fe] = 0.67 ± 0.03, and A_V = 0.27 ± 0.02 mag (GRB 181020A) and [Zn/H] = −1.23 ± 0.07, [Zn/Fe] = 1.06 ± 0.08, and A_V = 0.36 ± 0.02 mag (GRB 190114A). In addition, we examine the molecular gas content of all known H₂-bearing GRB-DLAs and explore the physical conditions and characteristics required to simultaneously probe C I and H₂^∗. We confirm that H₂ is detected in all C I- and H₂^∗-bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H₂ absorbers. We find that a large molecular fraction of f_H2 ≳ 10⁻³ is required for C I to be detected. The defining characteristic for H₂^∗ to be present is less clear, though a large H₂ column density is an essential factor. We also find that the observed line profiles of the molecular-gas tracers are kinematically “cold”, with small velocity offsets of δv <  20 km s⁻¹ from the bulk of the neutral absorbing gas. We then derive the H₂ excitation temperatures of the molecular gas and find that they are relatively low with T_ex ≈ 100−300 K, however, there could be evidence of warmer components populating the high-J H₂ levels in GRBs 181020A and 190114A. Finally, we demonstrate that even though the X-shooter GRB afterglow campaign has been successful in recovering several H₂-bearing GRB-host absorbers, this sample is still hampered by a significant dust bias excluding the most dust-obscured H₂ absorbers from identification. C I and H₂^∗ could open a potential route to identify molecular gas even in low-metallicity or highly dust-obscured bursts, though they are only efficient tracers for the most H₂-rich GRB-host absorption systems.