Molecular gas in the z = 2.8 submillimeter galaxy SMM 02399-0136

We report the detection of CO (3 --> 2) emission from the submillimeter-selected hyperluminous galaxy SMM 02399-0136. This galaxy is the brightest source detected in the recent Submillimeter Common-User Bolometer Array surveys of the submillimeter sky. The optical counterpart of the submillimeter source has been identified as a narrow-line active galactic nucleus/starburst galaxy at z = 2.8. The CO emission is unresolved, theta less than or similar to 5 ", and is coincident in redshift and position with the optical counterpart. The molecular gas mass derived from the CO observations is 8 x 10(10) h(75)(-2) M., after correcting for a lensing amplification factor of 2.5. The large CO luminosity suggests that a significant fraction of the infrared luminosity of SMM 02399-0136 arises from star formation. The high inferred star formation rate of 10(3) M. yr(-1) and the large gaseous reservoir may suggest that we are seeing the formation phase of a massive galaxy. Future CO observations of other galaxies detected in deep submillimeter surveys will test the generality of these conclusions for the bulk of the faint submillimeter population.