2012A&A...546A...8K

The star-forming nature of long γ-ray burst (GRB) host galaxies provides invaluable constraints on the progenitors of GRBs and might open a short-cut to the characteristics of typical star-forming galaxies throughout the history of the Universe. Due to the absence of near-infrared (NIR) spectroscopy, however, detailed investigations, specifically a determination of the gas-phase metallicity of gamma-ray burst hosts, was largely limited to redshifts z<1 to date. We observed the galaxy hosting GRB 080605 at z=1.64 using optical/NIR spectroscopy and high-resolution HST/WFC3 imaging in the rest-frame wavelength range between 1150 and 8700Å. These data allow us to study a z>1 GRB host in unprecedented detail and investigate the relation between GRB hosts and field galaxies. We availed of VLT/X-shooter optical/NIR spectroscopy to measure the metallicity, electron density, star-formation rate (SFR), and reddening of the host of GRB 080605. Specifically, we used different strong-line diagnostics to robustly measure the gas-phase metallicity within the interstellar medium (ISM) for the first time based on [NII] at this redshift. The host of the energetic (E_γ,iso∼2x10⁵³erg) GRB 080605 at z∼1.64 is a morphologically complex, vigorously star-forming galaxy with an Hα-derived SFR of 31⁺¹²_–6M_☉/yr. Its ISM is significantly enriched with metals. Specifically, [NII]/Hα=0.14±0.02, which yields an oxygen abundance 12+log(O/H) between 8.3 and 8.6 depending on the adopted strong-line calibrator. This corresponds to values in the range of 0.4-0.8Z_☉. For its measured stellar mass (M_*=8.0^+1.3_–1.6x10⁹M_☉) and SFR, this value is consistent with the fundamental metallicity relation defined by star-forming field galaxies. The absence of strong Lyα emission constrains the escape fraction of resonantly-scattered Lyα photons to f_esc≲0.08. Our observations provide a detailed picture of the conditions in the ISM of a highly star-forming galaxy with irregular morphology at z∼1.6. They include the first robust metallicity measurement based on [NII] for a GRB host at z>1 and directly illustrate that GRB hosts are not necessarily metal-poor, both on absolute scales as well as relative to their stellar mass and SFR. GRB hosts could thus be fair tracers of the population of ordinary star-forming galaxies at high redshift.