New constraints on the physical conditions in H<SUB>2</SUB>-bearing GRB-host damped Lyman-α absorbers

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.