We present near-infrared spectroscopy of the host galaxy of dark GRB 080325 using Subaru/MOIRCS. The obtained spectrum provides a clear detection of H$alpha$ emission and marginal [NII]$lambda$6584. The host is a massive (M$_{*}sim10^{11}$M$_{odot}$)
, dusty ($A_{V}sim 1.2$) star-forming galaxy at z=1.78. The star formation rate calculated from the H$alpha$ luminosity (35.6-47.0 M$_{odot}$ yr$^{-1}$) is typical among GRB host galaxies (and star-forming galaxies generally) at z $>$1; however, the specific star formation rate is lower than normal star-forming galaxies at redshift $sim$ 1.6, in contrast to the high specific star formation rates measured for many of other GRB hosts. The metallicity of the host is estimated to be 12+log(O/H)$_{rm KK04}$$=$8.88. We emphasize that this is one of the most massive distant host galaxies for which metallcity is measured with emission-line diagnostics. The metallicity is fairly high among GRB hosts. However, this is still lower than the metallicity of normal star-forming galaxies of the same mass at z$sim$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 metalicity over a wide range of redshift and stellar mass. The low-metallicity nature of the GRB 080325 host is likely not attributable to the fundamental metallicity relation of star-forming galaxies beacuse it is a metal-poor outlier from the relation and has a low sSFR. Thus we conclude that metallicity is important to the mechanism that produced this GRB.
We present the results of the 16-cm-waveband continuum observations of four host galaxies of gamma-ray bursts (GRBs) 990705, 021211, 041006, and 051022 using the Australia Telescope Compact Array. Radio emission was not detected in any of the host ga
laxies. The 2sigma upper limits on star-formation rates derived from the radio observations of the host galaxies are 23, 45, 27, and 26 Msun/yr, respectively, which are less than about 10 times those derived from UV/optical observations, suggesting that they have no significant dust-obscured star formation. GRBs 021211 and 051022 are known as the so-called dark GRBs and our results imply that dark GRBs do not always occur in galaxies enshrouded by dust. Because large dust extinction was not observed in the afterglow of GRB021211, our result {bf suggests the possibility} that the cause of the dark GRB is the intrinsic faintness of the optical afterglow. On the other hand, by considering the high column density observed in the afterglow of GRB051022, the likely cause of the dark GRB is the dust extinction in the line of sight of the GRB.
