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Title: New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-alpha absorbers
Authors: Heintz, KE
Bolmer, J
Ledoux, C
Noterdaeme, P
Krogager, J-K
Fynbo, JPU
Jakobsson, P
Covino, S
D'Elia, V
Pasquale, MD
Hartmann, DH
Izzo, L
Japelj, J
Kann, DA
Kaper, L
Petitjean, P
Rossi, A
Salvaterra, R
Schady, P
Selsing, J
Starling, R
Tanvir, NR
Thöne, CC
Postigo, ADU
Vergani, SD
Watson, D
Wiersema, K
Zafar, T
First Published: 2019
Publisher: EDP Sciences for European Southern Observatory (ESO)
Citation: Astronomy and Astrophysics, 2019, In Press
Abstract: We report the detections of molecular hydrogen (H2), vibrationally-excited H2 (H∗ 2 ), and neutral atomic carbon (Ci), 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 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 Ci 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 fH2 & 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 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 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.
DOI Link: TBA 10.1051/0004-6361/201936250
eISSN: 1432-0746
Links: TBA
Embargo on file until: 1-Jan-10000
Version: Post-print
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © 2019, EDP Sciences for European Southern Observatory (ESO). Deposited with reference to the publisher’s open access archiving policy. (
Description: The file associated with this record is under embargo until publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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