A Search for CO(J=3-2) Emission from the Host Galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment
Abstract
We report on a deep search for CO(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 the rms noise level of 3.3 mK in Tmb scale at a velocity resolution of 10 km s-1. No significant emission was detected, though we find a marginal emission feature in the velocity range corresponding to the redshift of the galaxy. We derive 3 sigma upper limits on the global properties: the velocity-integrated CO(3-2) intensity of ICO(3-2) < 0.26 K km s-1 by adopting a velocity width of 67 km s-1; the H2 column density of N(H2) < 3 x 1020 cm-2; the molecular gas mass of M(H2) < 3 x 108 Msun, by assuming a CO line luminosity to H2 molecular gas mass conversion factor of XCO = 5.0 x 1020 cm-2 (K km s-1)-1; and the star formation rate of SFR < 0.1 Msun yr-1, based on the Schmidt law. The SFR is consistent with the previous results of Halpha 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.
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