Observing cold gas in submillimeter galaxies:: Detection of CO (1→0) emission in SMM J13120+4242 with the Green Bank Telescope

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 Delta V-FWHM = 1040 +/- 190 km s(-1). 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 similar to 0.26 suggests n(H-2) > (3-10) x 10(2) cm(-3) and the presence of subthermally excited gas. The integrated line flux implies a cold molecular gas mass M(H-2) = 1.6 x 10(11) M-circle dot, comparable to the dynamical mass estimate and 4 times larger than the H-2 mass 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(H-2) less than or similar to 4 x 10(10) M-circle dot.