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New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-alpha absorbers

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posted on 2019-08-27, 15:56 authored by KE Heintz, J Bolmer, C Ledoux, P Noterdaeme, J-K Krogager, JPU Fynbo, P Jakobsson, S Covino, V D'Elia, MD Pasquale, DH Hartmann, L Izzo, J Japelj, DA Kann, L Kaper, P Petitjean, A Rossi, R Salvaterra, P Schady, J Selsing, R Starling, NR Tanvir, CC Thöne, ADU Postigo, SD Vergani, D Watson, K Wiersema, T Zafar
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.


KEH and PJ acknowledge support by a Project Grant (162948– 051) from The Icelandic Research Fund. PN and JKK acknowledge support from the French Agence Nationale de la Recherche under contract ANR-17- CE31-0011-01 (Projet "HIH2", PI Noterdaeme) and are grateful to the European Southern Observatory for hospitality and support during a visit to the ESO headquarters in Chile. The Cosmic Dawn Center is funded by the DNRF. AdUP, CCT, DAK and LI acknowledge support from the Spanish research project AYA2017- 89384-P, and from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Insituto de Astrofísica de Andalucía (SEV-2017-0709). AdUP and CCT acknowledge support from Ramón y Cajal fellowships (RyC-2012-09975 and RyC-2012-09984). LI acknowledges support from a Juan de la Cierva Incorporación fellowship (IJCI-2016-30940).



A&A 629, A131 (2019).

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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Astronomy and Astrophysics




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