Herschel observations of gamma-ray burst host galaxies: implications for the topology of the dusty interstellar medium^⋆

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: pschady@mpe.mpg.de
² Physics Department, University of Calabria, 87036 Arcavacata di Rende, Italy
³ European Southern Observatory, 85748 Garching, Germany
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁵ INAF – IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 218-S9, 9000 Gent, Belgium
⁹ SUPA (Scottish Universities Physics Alliance), Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK
¹⁰ Institut de Recherche en Astrophysique et Planétologie, CNRS-Université Paul Sabatier Toulouse, 9 avenue du Colonel Roche, 31028 Toulouse, France
¹¹ Centre de Physique des Particules de Marseille, Aix Marseille Université, 168 avenue de Luminy, 13009 Marseille, France
¹² INAF – Osservatorio Astronomico di Roma, via di Frascati, 33, 00040 Monteporzio Catone, Italy
¹³ GEPI – Observatoire de Paris, CNRS UMR 8111, Univ. Paris-Diderot, 5 place Jules Jannsen, 92190 Meudon, France
¹⁴ INAF/Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate ( LC), Italy
¹⁵ School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, Wales CF24 3YB, UK

Received: 29 April 2014
Accepted: 18 August 2014

Abstract

Long-duration gamma-ray bursts (GRBs) are indisputably related to star formation, and their vast luminosity in gamma rays pin-points regions of star formation independent of galaxy mass, out to the epoch of re-ionisation. As such, GRBs provide a unique tool for studying star forming galaxies out to high-z independent of luminosity. Most of our understanding of the properties of GRB hosts (GRBHs) comes from optical and near-infrared follow-up observations, and we therefore have relatively little knowledge of the fraction of dust-enshrouded star formation that resides within GRBHs. Currently ~20% of GRBs show evidence of significant amounts of dust within their host galaxy along the GRB line of sight, and these GRBs tend to reside within redder and more massive galaxies than GRBs with optically bright afterglows. In this paper we present Herschel observations of five GRBHs with evidence of being dust-rich, targeted to understand the properties of the dust attenuation within GRBs better. Despite the sensitivity of our Herschel observations, only one galaxy in our sample was detected (GRBH 070306), for which we measure a total star formation rate of ~100 M_⊙ yr^-1, and which had a relatively high stellar mass (log[M_*] = 10.34^+0.09_-0.04. Nevertheless, when considering a larger sample of GRBHs observed with Herschel, it is clear that stellar mass is not the only factor contributing to a Herschel detection, and significant dust extinction along the GRB sightline (A_V,GRB> 1.5 mag) appears to be a considerably better tracer of GRBHs with a high dust mass. This suggests that the extinguishing dust along the GRB line of sight lies predominantly within the host galaxy ISM, and thus those GRBs with A_V,GRB> 1 mag but with no host galaxy Herschel detections are likely to have been predominantly extinguished by dust within an intervening dense cloud.