The Ero Host Galaxy of Grb 020127: Implications for the Metallicity of Grb Progenitors

We present optical and near-IR observations of the host galaxy of GRB 020127, for which we measure R Ks 1⁄4 6:2mag. This is only the second GRB host to date classified as an ERO. The spectral energy distribution (SED) is typical of a dusty starburst galaxy, with a redshift z 1:9, a luminosity L 5L , and an inferred stellar mass of M 10 10 M , two orders of magnitude moremassive than typical GRB hosts. A comparison to the z 2 massmetallicity (M-Z ) relation suggests that the host metallicity is about 0:5 1Z . This result shows that at least some GRBs occur in massive, metal-enriched galaxies, and that the proposed low-metallicity bias of GRB progenitors is not as severe as previously claimed. Instead, we propose that the blue colors and sub-L luminosities of most GRB hosts reflect their young starburst populations. This explanation also accounts for the prevalence of low-redshift GRBs in low-mass galaxies, since star formation activity shifts from highto low-mass galaxies as a function of decreasing redshift (‘‘downsizing’’). Thus, the low-metallicity bias claimed for z P 0:2 GRB hosts is likely a secondary effect, which reflects theM -Z relation, and consequently GRBs and their hosts may serve as a reliable tracer of cosmic star formation activity. Subject headinggs: galaxies: starburst — gamma rays: bursts