Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of theSwift/BAT6 complete sample of LGRBs
Aims. Long gamma-ray bursts (LGRBs) are associated with massive stars and are therefore linked to star formation. However, the conditions needed for the progenitor stars to produce LGRBs can affect the relation between the LGRB rate and star formation. By using the power of a complete LGRB sample, our long-term aim is to understand whether such a bias exists and, if it does, what its origin is. Methods. To reach our goal we use the Swift/BAT6 complete sample of LGRBs. In this first paper, we build the spectral energy distribution (SED) of the 14 z < 1 host galaxies of the BAT6 LGRB sample and determine their stellar masses (M_*) from SED fitting. To investigate the presence of a bias in the LGRB-star formation relation we compare the stellar mass distribution of the LGRB host galaxies (i) with star-forming galaxies observed in deep surveys (UltraVISTA) within the same redshift limit; (ii) with semi-analytical models of the z < 1 star-forming galaxy population; and (iii) with dedicated numerical simulations of LGRB hosts having different metallicity thresholds for the progenitor star environment. Results. We find that at z < 1, LGRBs tend to avoid massive galaxies and are very powerful for selecting a population of faint low-mass star-forming galaxies, partly below the completeness limits of galaxy surveys. The stellar mass distribution of the hosts is not consistent with that of the UltraVISTA star-forming galaxies weighted by their star formation rate (SFR). This implies that, at least at z < 1, LGRBs are not unbiased tracers of star formation. To make the two distributions consistent, a much steeper faint end of the mass function would be required or a very shallow SFR-mass relation for the low-mass galaxy population. The comparison with the GRB host galaxy simulations indicates that, to reproduce the stellar mass distribution, a metallicity threshold of the order of Z_(th) = 0.3−0.5 Z_⊙ is necessary to form a LGRB. Models without a metallicity threshold or with an extreme threshold of Z_(th) = 0.1 Z_⊙ are excluded at z < 1. Under a very basic assumption, we estimate that the LGRB rate can directly trace the SFR starting from z ~ 4 and above. Conclusions. GRB hosts at z < 1 have lower luminosities and stellar masses than expected if LGRBs were unbiased star formation tracers. The use of the Swift/BAT6 complete sample keeps this result from being affected by possible biases that could have influenced past results based on incomplete samples. The preference for low metallicities (Z ≲ 0.5 Z_⊙) inferred by the comparison with the simulations can be a consequence of the particular conditions needed for the progenitor star to produce a GRB.
© 2015 ESO. Received 17 September 2014. Accepted 4 June 2015. Published online 14 September 2015. We thank the referee for the constructive suggestions for improving the paper. We acknowledge Maria Angela Campisi for making her simulations available, Olivier Ilbert and the COSMOS team for helping with the Ultra VISTA data and sharing their catalogues, Daniele Malesani and Sandra Savaglio for useful discussions. S.D.V. and E.L.F. acknowledge the UnivEarthS Labex programme at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). A.F.S. acknowledges support from grants AYA2010-22111- C03-02 and AYA2013-48623-C2-2 from the Spanish Ministerio de Economia y Competitividad, and grants Prometeo 2009/064 and PrometeoII 2014/060 from the Generalitat Valenciana. This work is partially based on observations with: the ESO Telescopes at Paranal Observatory under programme ID 092.D-0305, PI: S.D. Vergani and ID 092.A-0231, PI: T. Krühler; the Gemini Observatory, acquired through the Gemini Science Archive, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovaçáo (Brazil) and Ministerio de Ciencia, Tecnologá e Innovación Productiva (Argentina).; GROND (http:// www.mpe.mpg.de/~jcg/GROND/); Gran Telescopio Canarias (GTC), instaled in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias in the island of La Palma; the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias; the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555; the SDSS (Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the US Department of Energy Office of Science. The SDSS-III web site is http://www. sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University); the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA; the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28- 1). Part of the research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2013-2016) under grant agreement number 312430 (OPTICON). This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of the http://www.mpe.mpg.de/~jcg/grbgen.html page and of the GHostS database (www.grbhosts.org), which is partly funded by Spitzer/NASA grant RSA Agreement No. 1287913.
Published - aa25013-14.pdf