Journal article · Preprint article
New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-α absorbers
University of Iceland1
CSIC2
University of Amsterdam3
University of Copenhagen4
University of Leicester5
Macquarie University6
Max Planck Institute for Extraterrestrial Physics7
European Southern Observatory8
CNRS9
National Space Institute, Technical University of Denmark10
National Institute for Astrophysics11
Agenzia Spaziale Italiana12
Istanbul University13
Clemson University14
...and 4 moreWe 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-α absorbers (DLAs) and substantial amounts of molecular hydrogen with logN(H I, H2) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and logN(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 C I and H2∗.
We confirm that H2 is detected in all C I- 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 C I 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 H2 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.
Language: | English |
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Publisher: | EDP Sciences S A |
Year: | 2019 |
Pages: | A131 |
ISSN: | 14320746 and 00046361 |
Types: | Journal article and Preprint article |
DOI: | 10.1051/0004-6361/201936250 |
ORCIDs: | 0000-0002-9389-7413 , 0000-0002-8149-8298 , 0000-0001-9058-3892 and 0000-0002-4465-8264 |