We report the first detection of CO ( 1 \rightarrow 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 \pm 190 \textrm { km s } ^ { -1 } .
If the observed CO ( 1 \rightarrow 0 ) line profile arises from a single object and not several merging objects , then the CO ( 4 \rightarrow 3 ) /CO ( 1 \rightarrow 0 ) brightness temperature ratio of \sim 0.26 suggests n ( \textrm { H } _ { 2 } ) > 3 - 10 \times 10 ^ { 2 } \textrm { cm } ^ { -3 } and the presence of sub-thermally excited gas .
The integrated line flux implies a cold molecular gas mass M ( \textrm { H } _ { 2 } ) = 1.6 \times 10 ^ { 11 } M _ { \sun } , comparable to the dynamical mass estimate and 4 times larger than the \textrm { H } _ { 2 } mass predicted from the CO ( 4 \rightarrow 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 } \geq 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 ( \textrm { H } _ { 2 } ) \lesssim 4 \times 10 ^ { 10 } M _ { \sun } .