Cold gas in the early Universe

Survey for neutral atomic-carbon in GRB host galaxies at 1 < z< 6 from optical afterglow spectroscopy

¹ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
e-mail: keh14@hi.is
² Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
³ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago 19, Chile
⁵ Institut d’Astrophysique de Paris, CNRS-UPMC, UMR7095, 98bis bd Arago, 75014 Paris, France
⁶ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching bei München, Germany
⁷ GEPI, Observatoire de Paris, PSL Research University, CNRS, Place Jules Janssen, 92190 Meudon, France
⁸ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW, 1670 Australia
⁹ Department of Physics & Astronomy and Leicester Institute of Space & Earth Observation, University of Leicester, University Road, Leicester, LE1 7RH UK
¹⁰ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 CopenhagenØ, Denmark
¹¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy

Received: 14 September 2018
Accepted: 24 October 2018

Abstract

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 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.