TY - JOUR
T1 - New constraints on the physical conditions in H2-bearing GRB-host damped Lyman-α absorbers
AU - Heintz, K. E.
AU - Bolmer, J.
AU - Ledoux, C.
AU - Noterdaeme, P.
AU - Krogager, J-K
AU - Fynbo, J. P. U.
AU - Jakobsson, P.
AU - Covino, S.
AU - D'Elia, V
AU - De Pasquale, M.
AU - Hartmann, D. H.
AU - Izzo, L.
AU - Japelj, J.
AU - Kann, D. A.
AU - Kaper, L.
AU - Petitjean, P.
AU - Rossi, A.
AU - Salvaterra, R.
AU - Schady, P.
AU - Selsing, J.
AU - Starling, R.
AU - Tanvir, N. R.
AU - Thone, C. C.
AU - de Ugarte Postigo, A.
AU - Vergani, S. D.
AU - Watson, D.
AU - WierSema, K.
AU - Zafar, T.
PY - 2019
Y1 - 2019
N2 - We 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.
AB - We 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.
U2 - 10.1051/0004-6361/201936250
DO - 10.1051/0004-6361/201936250
M3 - Journal article
VL - 629
JO - Astronomy & Astrophysics
JF - Astronomy & Astrophysics
SN - 0004-6361
M1 - A131
ER -