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-alpha absorbers (DLAs) and substantial amounts of molecular hydrogen with logN(HI, H2) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and logN(HI, 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 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). 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 CI- 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 fH2 ≳ 10-3 is required for CI 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 Tex ≈ 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 H-2 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.
Original language | English |
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Article number | A131 |
Number of pages | 21 |
Journal | Astronomy and Astrophysics |
Volume | 629 |
DOIs | |
Publication status | Published - 16 Sept 2019 |
Bibliographical note
Reproduced with permission from Astronomy & Astrophysics, Copyright 2019 ESO. First published in Astronomy and Astrophysics, 629, A131, 2019, published by EDP Sciences. The original publication is available at https://doi.org/10.1051/0004-6361/201936250. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.Keywords
- galaxies: ISM
- galaxies: high-redshift
- ISM: molecules
- dust, extinction
- gamma-ray burst: general
- gamma-ray burst: individual: 181020A and 190114A