CO Observations of the Host Galaxy of GRB000418 at z = 1.1
  We performed CO(J=2-1) observations of the host galaxy of GRB000418 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_sun/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 2sigma upper limits on the velocity
integrated CO(2-1) luminosity of L'CO < 6.9 x 10^9 K km/s/pc^2 and the
molecular gas mass of M(H2) < 5.5 x 10^9 M_sun by adopting a velocity width of
300 km/s and a CO-to-H2 conversion factor of alpha_CO = 0.8 M_sun/(K
km/s/pc^2), 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.
