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

III. Stellar masses, star formation rates, and metallicities at z >  1

¹ Sorbonne Université, CNRS, UMR7095, Institut d’Astrophysique de Paris, 75014 Paris, France
e-mail: palmerio@iap.fr
² GEPI, Observatoire de Paris, PSL University, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁴ INAF – IASF Milano, Via E. Bassini 15, 20133 Milano, Italy
⁵ Department of Physics & Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA
⁶ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁷ LERMA/LRA, École Normale Supérieure, PSL University, Observatoire de Paris, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75005 Paris, France
⁸ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
⁹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁰ Max-Planck-Institute für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
¹¹ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu - CNRS - Université Paris Diderot, CEA-Saclay, 91191 Gif-sur-Yvette, France
¹² School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Received: 3 September 2018
Accepted: 8 January 2019

Abstract

Aims. Long gamma-ray bursts (LGRB) have been proposed as promising tracers of star formation owing to their association with the core-collapse of massive stars. Nonetheless, previous studies we carried out at z <  1 support the hypothesis that the conditions necessary for the progenitor star to produce an LGRB (e.g. low metallicity), were challenging the use of LGRBs as star-formation tracers, at least at low redshift. The goal of this work is to characterise the population of host galaxies of LGRBs at 1 < z < 2, investigate the conditions in which LGRBs form at these redshifts and assess their use as tracers of star formation.

Methods. We performed a spectro-photometric analysis to determine the stellar mass, star formation rate, specific star formation rate and metallicity of the complete, unbiased host galaxy sample of the Swift/BAT6 LGRB sample at 1 < z < 2. We compared the distribution of these properties to the ones of typical star-forming galaxies from the MOSDEF and COSMOS2015 Ultra Deep surveys, within the same redshift range.

Results. We find that, similarly to z <  1, LGRBs do not directly trace star formation at 1 < z < 2, and they tend to avoid high-mass, high-metallicity host galaxies. We also find evidence for an enhanced fraction of starbursts among the LGRB host sample with respect to the star-forming population of galaxies. Nonetheless we demonstrate that the driving factor ruling the LGRB efficiency is metallicity. The LGRB host distributions can be reconciled with the ones expected from galaxy surveys by imposing a metallicity upper limit of logOH ∼ 8.55. We can determine upper limits on the fraction of super-solar metallicity LGRB host galaxies of ∼20%, 10% at z <  1, 1 <  z <  2, respectively.

Conclusions. Metallicity rules the LGRB production efficiency, which is stifled at Z ≳ 0.7 Z_⊙. Under this hypothesis we can expect LGRBs to trace star formation at z >  3, once the bulk of the star forming galaxy population are characterised by metallicities below this limit. The role played by metallicity can be explained by the conditions necessary for the progenitor star to produce an LGRB. The moderately high metallicity threshold found is in agreement with the conditions necessary to rapidly produce a fast-rotating Wolf-Rayet stars in close binary systems, and could be accommodated by single star models under chemically homogeneous mixing with very rapid rotation and weak magnetic coupling.