Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of the Swift/BAT6 complete sample of bright LGRBs

II. Star formation rates and metallicities at z < 1^⋆,⋆⋆

¹ INAF–Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11 34131 Trieste Italy
e-mail: japelj@oats.inaf.it
² Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, 1000 Ljubljana, Slovenia
³ GEPI–Observatoire de Paris Meudon. 5 place Jules Jannsen, 92195 Meudon, France
⁴ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁵ Institut d’Astrophysique de Paris, Université Paris 6-CNRS, UMR7095, 98bis boulevard Arago, 75014 Paris, France
⁶ INAF–IASF Milano, via E. Bassini 15, 20133 Milano, Italy
⁷ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
⁹ Unidad Asociada Observatori Astronómic (IFCA – Universitat de Valéncia), Valencia, Spain
¹⁰ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹¹ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne, Observatoire de Sauverny, 1290 Versoix, Switzerland
¹² Department of Astronomy, Yale University, 260 Whitney Avenue, New Haven, CT 06511, USA
¹³ Departamento de Astrofísica y Ciencias de la Atmósfera, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁴ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁵ Laboratoire AIM, IRFU/Service d’Astrophysique – CEA/DSM – CNRS – Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
¹⁶ Faculty of Sciences, University of Nova Gorica, Vipavska cesta 11c, 5270 Ajdovščina, Slovenia
¹⁷ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁸ NASA Postdoctoral Program Fellow, Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 15 February 2016
Accepted: 4 April 2016

Abstract

Aims. Long gamma-ray bursts (LGRBs) are associated with the deaths of massive stars and might therefore be a potentially powerful tool for tracing cosmic star formation. However, especially at low redshifts (z< 1.5) LGRBs seem to prefer particular types of environment. Our aim is to study the host galaxies of a complete sample of bright LGRBs to investigate the effect of the environment on GRB formation.

Methods. We studied host galaxy spectra of the Swift/BAT6 complete sample of 14 z< 1 bright LGRBs. We used the detected nebular emission lines to measure the dust extinction, star formation rate (SFR), and nebular metallicity (Z) of the hosts and supplemented the data set with previously measured stellar masses M_⋆. The distributions of the obtained properties and their interrelations (e.g. mass-metallicity and SFR-M_⋆ relations) are compared to samples of field star-forming galaxies.

Results. We find that LGRB hosts at z< 1 have on average lower SFRs than if they were direct star formation tracers. By directly comparing metallicity distributions of LGRB hosts and star-forming galaxies, we find a good match between the two populations up to 12 +log ${}^{\mathrm{\right(}}\frac{\mathrm{O}}{\mathrm{H}}^{\mathrm{\left)}}$~8.4−8.5, after which the paucity of metal-rich LGRB hosts becomes apparent. The LGRB host galaxies of our complete sample are consistent with the mass-metallicity relation at similar mean redshift and stellar masses. The cutoff against high metallicities (and high masses) can explain the low SFR values of LGRB hosts. We find a hint of an increased incidence of starburst galaxies in the Swift/BAT6 z< 1 sample with respect to that of a field star-forming population. Given that the SFRs are low on average, the latter is ascribed to low stellar masses. Nevertheless, the limits on the completeness and metallicity availability of current surveys, coupled with the limited number of LGRB host galaxies, prevents us from investigating more quantitatively whether the starburst incidence is such as expected after taking into account the high-metallicity aversion of LGRB host galaxies.