New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-α absorbers
Abstract
We report the detections of molecular hydrogen (H2), vibrationally-excited H2 (H*2), and neutral atomic carbon (CI), 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 N(HI, H2) = 22.20 0.05,~20.40 0.04 (GRB\,181020A) and N(HI, H2) = 22.15 0.05,~19.44 0.04 (GRB\,190114A). The DLA metallicites, depletion levels and dust extinctions are [Zn/H] = -1.57 0.06, [Zn/Fe] = 0.67 0.03, and AV = 0.27 0.02\,mag (GRB\,181020A) and [Zn/H] = -1.23 0.07, [Zn/Fe] = 1.06 0.08, and AV = 0.36 0.02\,mag (GRB\,190114A). We then examine the molecular gas content of all known H2-bearing GRB-DLAs and explore the physical conditions and characteristics of these systems. We confirm that H2 is detected in all CI- 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 CI 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 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 the otherwise successful X-shooter GRB afterglow campaign is hampered by a significant dust bias excluding the most dust-obscured H2 absorbers from identification [Abridged].