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

We report the detections of molecular hydrogen (H2), vibrationally-excited H2 (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, H2 ) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and log N(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 (GRB181020A) and [Zn/H]=−1.23±0.07, [Zn/Fe]=1.06±0.08, and A_V =0.36±0.02 mag (GRB190114A). 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 H∗2 . We confirm that H2 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 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 H∗2 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 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 H2-rich GRB-host absorption systems.