The Star Formation Rate and Metallicity of the Host Galaxy of the Dark GRB 080325 at z=1.78

Abstract

We present near-infrared spectroscopy of the host galaxy of the dark gamma-ray burst (GRB) 080325 using Subaru/Multi-Object Infrared Camera and Spectrograph. The obtained spectrum provides a clear detection of Hα emission and marginal [Nii]λ6584. The host is a massive (M_* ∼ 10¹¹ M_⊙), dusty ({A}_V∼ 1.2) star-forming galaxy at z = 1.78. The extinction-corrected star formation rate (SFR) calculated from the Hα luminosity (35.6-47.0 M_⊙ yr^-1) is typical among GRB host galaxies (and star-forming galaxies generally) at z \gt 1; however, the specific SFR is lower than for normal star-forming galaxies at redshift ∼1.6, in contrast to the high specific SFR measured for many of other GRB hosts. The metallicity of the host is estimated to be 12 + log(O/H)_KK04 = 8.88. We emphasize that this is one of the most massive host galaxies at z \gt 1 for which metallicity is measured with emission-line diagnostics. The metallicity is fairly high among GRB hosts, however, this is still lower than the metallicity of normal star-forming galaxies of the same mass at z ∼ 1.6. The metallicity offset from normal star-forming galaxies is close to a typical value of other GRB hosts and indicates that GRB host galaxies are uniformly biased toward low metallicity over a wide range of redshifts and stellar masses. The low-metallicity nature of the GRB 080325 host likely cannot be attributed to the fundamental metallicity relation of star-forming galaxies because it is a metal-poor outlier from the relation and has a low specific star formation rate. Thus, we conclude that metallicity is important to the mechanism that produced this GRB.