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 1⁄4 3:408, with VFWHM 1⁄4 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) > (3Y10) ; 10 cm 3 and the presence of subthermally excited gas. The integrated line flux implies a cold molecular gas mass M (H2) 1⁄4 1:6 ; 1011 M , comparable to the dynamical mass estimate and 4 times larger than the H2 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 Jupper 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)P 4 ; 10 M . Subject headinggs: galaxies: evolution — galaxies: formation — galaxies: high-redshift — galaxies: ISM — infrared: galaxies — radio lines: galaxies