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

Abstract

We report the detections of molecular hydrogen (H 2 ), vibrationally-excited H 2 (H * 2 ), 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 log N(H i, H 2 ) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and log N(H i, H 2 ) = 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 H 2 -bearing GRB-DLAs and explore the physical conditions and characteristics required to simultaneously probe C i and H * 2 . We confirm that H 2 is detected in all C i-and H * 2 -bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H 2 absorbers. We find that a large molecular fraction of f H 2 10 −3 is required for C i to be detected. The defining characteristic for H * 2 to be present is less clear, though a large H 2 column density is an essential factor. We 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 H 2 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 H 2 levels in GRBs 181020A and 190114A. Finally, we demonstrate that even though the X-shooter GRB afterglow campaign has been successful in recovering several H 2 -bearing GRB-host absorbers, this sample is still hampered by a significant dust bias excluding the most dust-obscured H 2 absorbers from identification. C i and H * 2 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 H 2 -rich GRB-host absorption systems.

Direct Measurement of the [C i] Luminosity to Molecular Gas Mass Conversion Factor in High-redshift Star-forming Galaxies

“…In rare cases, H 2 and [C i] are detected in absorptionselected galaxies toward quasars (Srianand et al 2005;Jorgenson et al 2010) and in GRB-host absorption systems (Bolmer et al 2019;Heintz et al 2019). The GRB-selected host galaxy absorption systems arguably provide the most robust estimate of the global [C i]-to-H 2 abundance because they probe the central regions of their host galaxies, which we know for certain to be star-forming systems.…”

GRB host galaxies with strong H$_2$ absorption: CO-dark molecular gas at the peak of cosmic star formation

“…These are: GRB 080607 (Prochaska 1 Though see also the tentative detection of H 2 in the afterglow of GRB 060206 reported by Fynbo et al (2006Fynbo et al ( ). et al 2009, GRB 120815A (Krühler et al 2013), and GRB 181020A (Heintz et al 2019). All show H 2 column densities above N(H 2 ) > 10 20 cm −2 and also constitute the GRB absorbers with the largest molecular-hydrogen fractions, f H 2 (Bolmer et al 2019;Heintz et al 2019).…”

“…The first detection of H 2 in a GRB absorber was observed in the remarkable afterglow spectrum of GRB 080607 (Prochaska et al 2009) 1 . Since then, eight more H 2 -bearing GRB absorbers have been detected (Krühler et al 2013;D'Elia et al 2014;Friis et al 2015;Bolmer et al 2019;Heintz et al 2019), largely owing to the extensive VLT/X-shooter GRB afterglow legacy survey (XS-GRB; Selsing et al 2019). This GRBselected sample of star-forming galaxies provides a unique way to study the molecular gas properties of high-z galaxies in absorption.…”

Exploration of the high-redshift universe enabled by THESEUS

“…Although this becomes unfeasible at high redshift due to the Gunn-Peterson trough obscuring the Lyman-Werner bands, other absorption tracers, notably vibrationally excited H * 2 , and neutral carbon, CI, provide reliable indicators of high molecular hydrogen columns [95,96].…”