Cold gas in the early Universe: Survey for neutral atomic-carbon in GRB host galaxies at 1 < z < 6 from optical afterglow spectroscopy

• Published in: Astronomy and astrophysics (Berlin) 2019-01, Vol.621, p.A20
• Main Authors: Heintz, K. E., Ledoux, C., Fynbo, J. P. U., Jakobsson, P., Noterdaeme, P., Krogager, J.-K., Bolmer, J., Møller, P., Vergani, S. D., Watson, D., Zafar, T., De Cia, A., Tanvir, N. R., Malesani, D. B., Japelj, J., Covino, S., Kaper, L.
• Format: Article
• Language: English
• Subjects: Astrophysics; Physics; Sciences of the Universe
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/201834246

We present a survey for neutral atomic-carbon (C  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 log N (H  I )/cm −2 + [X/H] > 20.7 and a dust-phase iron column density of log N (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 H 2 - 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.