A COMPARISON OF CO (J=1-]0) AND CO (J=2-]1) EMISSION IN THE MILKY-WAY MOLECULAR RING

We have carried put a CO (J = 2 --> 1) survey of the Scutum Arm region of the Milky Way molecular ring. Our goals are to compare CO (J = 2 --> 1) maps of individual Galactic clouds with the large-scale CO (J = 2 --> 1) emission from the Galactic plane, and to predict the CO (J = 2 --> 1) appearance of a Galactic cloud ensemble in an external galaxy. The angular resolution and spatial coverage of our survey are compatible with the existing CO (J = 1 --> 0) survey of this region by Sanders et al., which we use for comparison. We identify 34 molecular clouds in our map region; their relationships between size and line width and between virial mass and luminosity are consistent with the relationships seen in CO (J = 1 --> 0) emission. However, we note that previous studies have shown considerable variation in these relationships; we attribute much of this variation to differences in their cloud definition algorithm and statistical method. We find that the median ratio of integrated emission in the two lowest CO transitions for the clouds in our sample is I(2 --> 1)/I(1 --> 0) = 0.69, implying that the typical emitting region in the line of sight contains cold gas that is not actively star-forming. Our conclusion that the molecular ring emission is not dominated by star-forming regions is consistent with other large-scale studies of the Milky Way. Our distributions of cloud size and temperature also imply that relatively massive molecular clouds that lack star formation, such as Maddalena's cloud, are not rare in the inner Galaxy.