The Redshift and Metallicity of the Host Galaxy of Dark GRB 080325 at
  z=1.78
  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.
