We present a survey for neutral atomic-carbon (Câ€» I) along gamma-ray burst (GRB) sightlines, which probes the shielded neutral gas-phase in the interstellar medium (ISM) of GRB host galaxies at high redshift. We compile a sample of 29 medium-to high-resolution GRB optical afterglow spectra spanning a redshift range through most of cosmic time from 1 < z < 6. We find that seven (25%) of the GRBs entering our statistical sample have C » I detected in absorption. It is evident that there is a strong excess of cold gas in GRB hosts compared to absorbers in quasar sightlines. We investigate the dust properties of the GRB C » I absorbers and find that the amount of neutral carbon is positively correlated with the visual extinction, A V , and the strength of the 2175 Å dust extinction feature, A bump . GRBs with C » I detected in absorption are all observed above a certain threshold of logN(H » I)/cm -2 + [X/H] > 20.7 and a dust-phase iron column density of logN(Fe) dust /cm -2 > 16.2. In contrast to the SED-derived dust properties, the strength of the C » I absorption does not correlate with the depletion-derived dust properties. This indicates that the GRB C » I absorbers trace dusty systems where the dust composition is dominated by carbon-rich dust grains. The observed higher metal and dust column densities of the GRB C » I absorbers compared to H2 -and C » I-bearing quasar absorbers is mainly a consequence of how the two absorber populations are selected, but is also required in the presence of intense UV radiation fields in actively star-forming galaxies.
Bibliographical noteReproduced with permission from Astronomy & Astrophysics, Copyright 2018 ESO. First published in Astronomy and Astrophysics, 621, A20, 2019, published by EDP Sciences. The original publication is available at https://doi.org/10.1051/0004-6361/201834246. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.
- Dust, extinction
- Galaxies: high-redshift
- Galaxies: ISM
- Gamma-ray burst: general