2011ApJ...738...33H

We performed CO (J = 2-1) observations of the host galaxy of GRB 000418 at z = 1.1181 with the Plateau de Bure Interferometer. Previous studies show that the host galaxy has properties similar to those of an ultraluminous infrared galaxy (ULIRG). The star formation rate (SFR) of the host galaxy as derived from submillimeter and radio continuum emission is a few 100 M _☉/yr, which is an order of magnitude greater than the SFR derived from optical line emission. The large discrepancy between the SFRs derived from different observing wavelengths indicates the presence of a bulk of dust-obscured star formation and molecular gas that is enough to sustain the intense star formation. We failed to detect CO emission and derived 2σ upper limits on the velocity-integrated CO (2-1) luminosity of L'_CO< 6.9 x10⁹ K km/s pc² and the molecular gas mass of M_H2 < 5.5 \time 10⁹ M_☉ by adopting a velocity width of 300 km/s and a CO-to-H₂ conversion factor of α_CO = 0.8 M_☉(K km/s pc²)^–1, which are standard values for ULIRGs. The lower limit on the ratio of far-infrared luminosity to CO luminosity, a measure of the star formation efficiency, is higher compared to that of other gamma-ray burst hosts and other galaxy populations, which is consistent with active star formation taking place in this galaxy.