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 ( \mathrm { 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 Fe ii transitions as well as Mg ii and Mg i .
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 .