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 _ { \odot } yr ^ { -1 } , 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 2 \sigma upper limits on the velocity integrated CO ( 2–1 ) luminosity of L ^ { \prime } _ { CO } < 6.9 \times 10 ^ { 9 } K km s ^ { -1 } pc ^ { 2 } and the molecular gas mass of M _ { H _ { 2 } } < 5.5 \times 10 ^ { 9 } M _ { \odot } by adopting a velocity width of 300 km s ^ { -1 } and a CO-to-H _ { 2 } conversion factor of \alpha _ { CO } = 0.8 M _ { \odot } ( K km s ^ { -1 } pc ^ { 2 } ) ^ { -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 .