An out-of-plane CO (J = 2-1) survey of the milky way. I. The data

We present extensive CO (J = 2-1) survey data of the first quadrant of the Galaxy taken with the same angular resolution as the Columbia CO (J = 1-0) survey. The CO (J = 2-1) data were compared with the CO (J = 1-0) data of Cohen Dame, & Thaddeus. The distribution of CO (J = 2-1) emission mimics that of CO (J = 1-0) emission with the CO (J = 2-1)/CO (J = 1-0) intensity ratio ( ≡ R_2-1/1-0) of 0.66 averaged over the observed area This value is typical of nearby giant molecular clouds and of gas disks of spiral galaxies and is consistent with the CO emission arising from subthermally excited, optically thick gas. Molecular gas in the inner part of the Galaxy tends to exhibit a higher value of R_2-1/1-0 than in the solar neighborhood.