Massive stars formed in atomic hydrogen reservoirs: H I observations of gamma-ray burst host galaxies

¹ SUPA (Scottish Universities Physics Alliance), Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
e-mail: mm@roe.ac.uk
² Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
³ Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁶ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
⁷ CSIRO Astronomy & Space Science, Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
⁸ Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
⁹ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹⁰ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching bei München, Germany
¹¹ ISDEFE for the Herschel Science Centre (ESA-ESAC), 28692 Villanueva de la Cañada, Madrid, Spain
¹² ASI-Science Data Center, via del Politecnico snc, 00133 Rome, Italy
¹³ INAF–Osservatorio Astronomico di Roma, via di Frascati, 33, 00040 Monteporzio Catone, Italy
¹⁴ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁶ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu − CNRS − Université Paris Diderot, CE-Saclay, pt courrier 131, 91191 Gif-sur-Yvette, France
¹⁷ INAF-IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
¹⁸ Technical University of Denmark, Department of Physics, Fysikvej, Building 309, 2800 Kgs. Lyngby, Denmark
¹⁹ Physics Department, University of Calabria, via P. Bucci, 87036 Arcavacata di Rende, Italy
²⁰ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²¹ The Oskar Klein Centre, Department of Astronomy, AlbaNova, Stockholm University, 106 91 Stockholm, Sweden
²² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
²³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁴ GEPI-Observatoire de Paris Meudon. 5 place Jules Jannsen, 92195 Meudon, France
²⁵ INAF/Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy

Received: 17 May 2015
Accepted: 7 August 2015

Abstract

Long gamma-ray bursts (GRBs), among the most energetic events in the Universe, are explosions of massive and short-lived stars, so they pinpoint locations of recent star formation. However, several GRB host galaxies have recently been found to be deficient in molecular gas (H₂), believed to be the fuel of star formation. Moreover, optical spectroscopy of GRB afterglows implies that the molecular phase constitutes only a small fraction of the gas along the GRB line of sight. Here we report the first ever 21 cm line observations of GRB host galaxies, using the AustraliaTelescope Compact Array, implying high levels of atomic hydrogen (H i), which suggests that the connection between atomic gas and star formation is stronger than previously thought. In this case, it is possible that star formation is directly fuelled by atomic gas (or that the H i-to-H₂ conversion is very efficient, which rapidly exhaust molecular gas), as has been theoretically shown to be possible. This can happen in low-metallicity gas near the onset of star formation because cooling of gas (necessary for star formation) is faster than the H i-to-H₂ conversion. Indeed, large atomic gas reservoirs, together with low molecular gas masses, stellar, and dust masses are consistent with GRB hosts being preferentially galaxies which have very recently started a star formation episode after accreting metal-poor gas from the intergalactic medium. This provides a natural route for forming GRBs in low-metallicity environments. The gas inflow scenario is also consistent with the existence of the companion H I object with no optical counterpart ~19 kpc from the GRB 060505 host, and with the fact that the H I centroids of the GRB 980425 and 060505 hosts do not coincide with optical centres of these galaxies, but are located close to the GRB positions.