The Host Galaxies of Gamma-Ray Bursts Ii: A Mass-Metallicity Relation for Long-Duration Gamma-Ray Burst Host Galaxies

Abstract

We present a statistically robust mass-metallicity relation for long-duration gamma-ray burst (LGRB) host galaxies at z < 1. By comparing the LGRB host mass-metallicity relation to samples representative of the general star-forming galaxy population, we conclude that LGRBs occur in host galaxies with lower metallicities than the general population, and that this trend extends to z ∼ 1, with an average offset of −0.42 ± 0.18 from the M-Z relation for star-forming galaxies. Our sample in this work includes new spectroscopic data for 6 LGRB host galaxies obtained at the Keck and Magellan telecopes, as well as 2 new host galaxies from the literature. Combined with data from our previous work, this yields a total sample 6 LGRB host galaxies at z < 0.3 and 10 host galaxies at 0.3 < z < 1. We have determined a number of interstellar medium properties for our host galaxies using optical emission-line diagnostics, including metallicity, ionization parameter, young stellar population age, and star formation rate. Across our full sample of 16 LGRB hosts we find an average metallicity of log(O/H) + 12 = 8.4 ± 0.3. Notably, we also measure a comparatively high metallicity of log(O/H) + 12 = 8.83 ± 0.1 for the z = 0.296 host galaxy of GRB 050826. We also determine stellar masses (M⋆) for our LGRB host galaxy sample, finding a mean stellar mass of log(M⋆/M⊙) = 9.25+0.19−0.23.