2006ApJ...650..614H

We report the first detection of CO (1⟶0) emission from a submillimeter-selected galaxy, using the Green Bank Telescope. We identify the line in the spectrum of SMM J13120+4242 as a broad emission feature at z=3.408, with ΔV_FWHM=1040±190 km/s. If the observed CO (1⟶0) line profile arises from a single object and not several merging objects, then the CO (4⟶3)/CO (1⟶0) brightness temperature ratio of ∼0.26 suggests n(H₂)>(3-10)x10²/cm3 and the presence of subthermally excited gas. The integrated line flux implies a cold molecular gas mass M(H₂)=1.6x10¹¹ M_☉, comparable to the dynamical mass estimate and 4 times larger than the H₂mass predicted from the CO (4⟶3) line, assuming a brightness temperature ratio of 1.0. While our observations confirm that this submillimeter galaxy is massive and gas-rich, they also suggest that extrapolating gas masses from J_upper≥3 transitions of CO leads to considerable uncertainties. We also report an upper limit to the mass of cold molecular gas in a second submillimeter galaxy, SMM J09431+4700, of M(H₂)≲4x10¹⁰ M_☉.