The luminous, massive and solar metallicity galaxy hosting the Swift γ -ray burst GRB160804A at z = 0.737

Open Access

Abstract

We here present the spectroscopic follow-up observations with VLT/X-shooter of the Swift long-duration gamma-ray burst GRB160804A at z=0.737. Typically, GRBs are found in lowmass, metal-poor galaxies that constitute the sub-luminous population of star-forming galaxies. For the host galaxy of the GRB presented here, we derive a stellar mass of log (M*/M) = 9.80 ± 0.07, a roughly solar metallicity (12 + log (O/H) = 8.74 ± 0.12) based on emission line diagnostics, and an infrared luminosity of M = -21.94 mag, but find it to be dust-poor (E(B - V) < 0.05 mag). This establishes the galaxy hosting GRB160804A as one of the most luminous, massive and metal-rich GRB hosts at z < 1.5. Furthermore, the gasphase metallicity is found to be representative of the physical conditions of the gas close to the explosion site of the burst. The high metallicity of the host galaxy is also observed in absorption, where we detect several strong Fe II transitions as well as MgII and MgI. Although host galaxy absorption features are common in GRB afterglow spectra, we detect absorption from strong metal lines directly in the host continuum (at a time when the afterglow was contributing to < 15 per cent). Finally, we discuss the possibility that the geometry and state of the absorbing and emitting gas are indicative of a galactic scale outflow expelled at the final stage of two merging galaxies.© 2018 The Author(s). ; We would like to thank the anonymous referee for a constructive report provided in a timely manner. KEH and PJ acknowledge support by a Project Grant (162948-051) from The Icelandic Research Fund. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013)/ERC Grant agreement no. EGGS-278202. AUP, CCT and ZC acknowledge support from the Spanish Ministry of Economy and Competitivity under grant number AYA 2014-58381-P. AUP and CCT acknowledge support from Ramon y Cajal fellowships (RyC-2012-09975 and RyC-2012-09984). AUP acknowledges support from a grant from the BBVA foundation for researchers and cultural creators. ZC acknowledges support from the Juan de la Cierva Incorporacion fellowship IJCI-2014-21669 and from the Spanish research project AYA 2014-58381-P. This research was based on observations carried out under the programme ID 097.A-0036 (PI: J. Fynbo) with the X-shooter spectrograph mounted at the Cassegrain Very Large Telescope (VLT), Unit 2 - Kueyen, operated by the European Southern Observatory (ESO) on Cerro Paranal, Chile; and on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundacion 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 under program A32TAC_5 (PI: D'Elia).