Observing Cold Gas in Submillimeter Galaxies: Detection of CO(1-0) Emission in SMM J13120+4242 with the Green Bank Telescope
Abstract
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(VFWHM)=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 x 102 cm-3 and the presence of sub-thermally excited gas. The integrated line flux implies a cold molecular gas mass M(H2)=1.6 x 1011 Msun, 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)< 4 x 1010 Msun.
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