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 (110) 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 V sub(FWHM) = 1040 c 190 km s super(-1). If the observed CO (110) line profile arises from a single object and not several merging objects, then the CO (413)/CO (110) brightness temperature ratio of 6 0.26 suggests n(H sub(2)) > (3-10) x 10 super(2) cm super(-3) and the presence of subthermally excited gas. The integrated line flux implies a cold molecular gas mass M(H sub(2)) = 1.6 x 10 super(11) M sub( ), comparable to the dynamical mass estimate and 4 times larger than the H sub(2) mass predicted from the CO (413) 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 sub(upper) . 53 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 sub(2)) 4 x 10 super(10) M sub( ).