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تسجيل مستخدم جديدDark GRB 080325 in a Dusty Massive Galaxy at z ~ 2
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We present optical and near infrared observations of GRB 080325 classified as a Dark GRB. Near-infrared observations with Subaru/MOIRCS provided a clear detection of afterglow in Ks band, although no optical counterpart was reported. The flux ratio of rest-wavelength optical to X-ray bands of the afterglow indicates that the dust extinction along the line of sight to the afterglow is Av = 2.7 - 10 mag. This large extinction is probably the major reason for optical faintness of GRB 080325. The J - Ks color of the host galaxy, (J - Ks = 1.3 in AB magnitude), is significantly redder than those for typical GRB hosts previously identified. In addition to J and Ks bands, optical images in B, Rc, i, and z bands with Subaru/Suprime-Cam were obtained at about one year after the burst, and a photometric redshift of the host is estimated to be z_{photo} = 1.9. The host luminosity is comparable to L^{*} at z sim 2 in contrast to the sub-L^{*} property of typical GRB hosts at lower redshifts. The best-fit stellar population synthesis model for the host shows that a large dust extinction (Av = 0.8 mag) attributes to the red nature of the host and that the host galaxy is massive (M_{*} = 7.0 times 10^{10} Msun) which is one of the most massive GRB hosts previously identified. By assuming that the mass-metallicity relation for star-forming galaxies at z sim 2 is applicable for the GRB host, this large stellar mass suggests the high metallicity environment around GRB 080325, consistent with inferred large extinction.
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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.
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