A search for CO (J=3-2) emission from the host galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment

We report on a deep search for (CO)-C-12 (J = 3-2) line emission from the host galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment (ASTE). We observed five points of the galaxy, covering the entire region. After combining all of the spectra, we obtained a global spectrum with a rms noise level of 3.3 mK in the T-mb scale at a velocity resolution of 10 km s(-1). No significant emission was detected, though we found a marginal emission feature in the velocity range corresponding to the redshift of the galaxy. We derived 3 sigma upper limits on the global properties: the velocity-integrated CO (3-2) intensity of I-CO(3-2) < 0.26 K km s(-1), adopting a velocity width of 67 km s(-1); an H-2 column density of N(H-2) < 3 x 10(20) cm(-2); a molecular gas mass of M(H-2) < 3 x 108 M(D, by assuming a CO line luminosity to H-2 molecular gas mass conversion factor of X-CO = 5.0 x 10(20) cm(-2) (K kna s(-1))(-1); and a star-formation rate of SFR < 0.1 M-circle dot yr(-1), based on the Schmidt law. The SFR is consistent with the previous results of Ha and mid-IR observations, thereby suggesting that there is no significant obscured star formation in the host galaxy of GRB 980425. This result implies that there is a variety of GRB hosts with regard to the presence of obscured star formation.