The luminous, massive and solar metallicity galaxy hosting the Swift gamma-ray burst, GRB 160804A at z = 0.737

TL;DR

This paper reports the discovery of a luminous, massive galaxy with near-solar metallicity hosting a gamma-ray burst at z=0.737, challenging the typical association of GRBs with low-metallicity, low-mass galaxies.

Contribution

It provides detailed spectroscopic analysis of a high-metallicity, high-mass GRB host galaxy, expanding understanding of the diversity of GRB environments.

Findings

Host galaxy has near-solar metallicity and high mass.

Detected metal absorption lines in host continuum, indicating complex gas conditions.

Evidence suggests possible galactic outflow from merging galaxies.

Abstract

We here present the spectroscopic follow-up observations with VLT/X-shooter of the Swift long-duration gamma-ray burst GRB 160804A at z = 0.737. Typically, GRBs are found in low-mass, metal-poor galaxies which constitute the sub-luminous population of star-forming galaxies. For the host galaxy of the GRB presented here we derive a stellar mass of $\log(M_*/M_{\odot}) = 9.80\pm 0.07$, a roughly solar metallicity (12+log(O/H) = $8.74\pm 0.12$) based on emission line diagnostics, and an infrared luminosity of $M_{3.6/(1+z)} = -21.94$ mag, but find it to be dust-poor ($E(B-V) < 0.05$ mag). This establishes the galaxy hosting GRB 160804A as one of the most luminous, massive and metal-rich GRB hosts at z < 1.5. Furthermore, the gas-phase 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 FeII transitions as well as MgII and MgI. While 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%). Finally, we discuss the possibility that the geometry and state of the absorbing and emitting gas is indicative of a galactic scale outflow expelled at the final stage of two merging galaxies.