The Milky Way in Molecular Clouds: A New Complete CO Survey

After more than two decades of surveying the Milky Way in CO, the CfA millimeter-wave telescope and its twin instrument on Cerro Tololo in Chile have now observed at 9' resolution the entire Galactic plane over a strip 4-10 deg wide in latitude, as well as nearly all large local clouds at higher latitudes. Compared with the previous CO survey of Dame et al. (1987), the new survey has 16 times more spectra, 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle. A velocity-integrated CO map derived from the new survey agrees in detail with a map of molecular column density derived from complete and unbiased far-infrared and 21 cm surveys. This agreement suggests both that the CO(1-0) line is a reliable tracer of molecular column density, and that our inventory of Galactic molecular clouds in CO is now nearly complete. The composite CO survey will be described, and some of its many applications to studies of star formation and Galactic structure will be discussed.