"Dark" GRB 080325 in a dusty massive galaxy at z ∼ 2

We present optical and near-infrared observations of Swift GRB 080325 classified as a "dark gamma-ray burst (GRB)." Near-infrared observations with Subaru/MOIRCS provided a clear detection of afterglow in the K_s 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 the optical faintness of GRB 080325. The J - K_s color of the host galaxy, (J - K_s = 1.3 in AB magnitude), is significantly redder than those for typical GRB hosts previously identified. In addition to J and K_s bands, optical images in B, R_c, I ′, and z ′ bands with Subaru/Suprime- Cam were obtained at about 1 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 ∼ 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 the red nature of the host is attributed to a large dust extinction (AV = 0.8 mag), and that the host galaxy is massive (M * = 7.0 × 10¹⁰ M_⊙), which makes it one of the most massive GRB hosts yet identified. By assuming that the mass-metallicity relation for star-forming galaxies at z ∼ 2 is applicable for the GRB host, this large stellar mass suggests the high-metallicity environment around GRB 080325, consistent with inferred large extinction.