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Title: | New constraints on the physical conditions in H<SUB>2</SUB>-bearing GRB-host damped Lyman-α absorbers | Authors: | Heintz, K. E. Bolmer, J. Ledoux, C. Noterdaeme, P. Krogager, J. -K. Fynbo, J. P. U. Jakobsson, P. COVINO, Stefano D'Elia, V. De Pasquale, M. Hartmann, D. H. Izzo, L. Japelj, J. Kann, D. A. Kaper, L. Petitjean, P. ROSSI, Andrea SALVATERRA, Ruben Schady, P. Selsing, J. Starling, R. Tanvir, N. R. Thöne, C. C. de Ugarte Postigo, A. Vergani, S. D. Watson, D. Wiersema, K. Zafar, T. |
Issue Date: | 2019 | Journal: | ASTRONOMY & ASTROPHYSICS | Number: | 629 | First Page: | A131 | Abstract: | We report the detections of molecular hydrogen (H<SUB>2</SUB>), vibrationally-excited H<SUB>2</SUB> (H<SUB>2</SUB><SUP>∗</SUP>), 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, H<SUB>2</SUB>) = 22.20 ± 0.05, 20.40 ± 0.04 (GRB 181020A) and logN(H I, H<SUB>2</SUB>) = 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<SUB>V</SUB> = 0.27 ± 0.02 mag (GRB 181020A) and [Zn/H] = -1.23 ± 0.07, [Zn/Fe] = 1.06 ± 0.08, and A<SUB>V</SUB> = 0.36 ± 0.02 mag (GRB 190114A). In addition, we examine the molecular gas content of all known H<SUB>2</SUB>-bearing GRB-DLAs and explore the physical conditions and characteristics required to simultaneously probe C I and H<SUB>2</SUB><SUP>∗</SUP>. We confirm that H<SUB>2</SUB> is detected in all C I- and H<SUB>2</SUB><SUP>∗</SUP>-bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H<SUB>2</SUB> absorbers. We find that a large molecular fraction of f<SUB>H<SUB>2</SUB></SUB> ≳ 10<SUP>-3</SUP> is required for C I to be detected. The defining characteristic for H<SUB>2</SUB><SUP>∗</SUP> to be present is less clear, though a large H<SUB>2</SUB> 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<SUP>-1</SUP> from the bulk of the neutral absorbing gas. We then derive the H<SUB>2</SUB> excitation temperatures of the molecular gas and find that they are relatively low with T<SUB>ex</SUB> ≈ 100-300 K, however, there could be evidence of warmer components populating the high-J H<SUB>2</SUB> levels in GRBs 181020A and 190114A. Finally, we demonstrate that even though the X-shooter GRB afterglow campaign has been successful in recovering several H<SUB>2</SUB>-bearing GRB-host absorbers, this sample is still hampered by a significant dust bias excluding the most dust-obscured H<SUB>2</SUB> absorbers from identification. C I and H<SUB>2</SUB><SUP>∗</SUP> 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 H<SUB>2</SUB>-rich GRB-host absorption systems. <P />Reduced spectra are also available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/629/A131">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/629/A131</A>Based on observations collected at the European Southern Observatory, Paranal, Chile, under the Stargate consortium with Program ID: 0102.D-0662. | URI: | http://hdl.handle.net/20.500.12386/28813 | URL: | https://www.aanda.org/articles/aa/abs/2019/09/aa36250-19/aa36250-19.html | ISSN: | 0004-6361 | DOI: | 10.1051/0004-6361/201936250 | Bibcode ADS: | 2019A&A...629A.131H | Fulltext: | open |
Appears in Collections: | 1.01 Articoli in rivista |
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aa36250-19.pdf | PDF editoriale | 4.34 MB | Adobe PDF | View/Open |
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