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

New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth (1/8deg) sample the entire Galactic plane over a strip 4^deg-10^deg wide in latitude, and beamwidth or 1/4deg sample nearly all large local clouds at higher latitudes. Compared with the previous composite CO survey of Dame et al. (1987), the new survey has 16 times more spectra, up to 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle. Each of the component surveys was integrated individually using clipping or moment masking to produce composite spatial and longitude-velocity maps of the Galaxy that display nearly all of the statistically significant emission in each survey but little noise. The composite maps provide detailed information on individual molecular clouds, suggest relationships between clouds and regions widely separated on the sky, and clearly display the main structural features of the molecular Galaxy. In addition, since the gas, dust, and Population I objects associated with molecular clouds contribute to the Galactic emission in every major wavelength band, the precise kinematic information provided by the present survey will form the foundation for many large-scale Galactic studies. A map of molecular column density predicted from complete and unbiased far-infrared and 21 cm surveys of the Galaxy was used both to determine the completeness of the present survey and to extrapolate it to the entire sky at |b|<32^deg. The close agreement of the observed and predicted maps implies that only ~2% of the total CO emission at |b|<32^deg lies outside our current sampling, mainly in the regions of Chamaeleon and the Gum Nebula. Taking into account this small amount of unobserved emission, the mean molecular column density decreases from ~3×10²⁰ cm^-2 at |b|=5^deg to ~0.1×10²⁰ cm^-2 at |b|=30^deg this drop is ~6 times steeper than would be expected from a plane-parallel layer, but is consistent with recent measurements of the mean molecular column density at higher latitudes. The ratio of the predicted molecular column density map to the observed CO intensity map provides a calibration of the CO-to-H₂ mass conversion factor X≡N_H2/W_CO. Out of the Galactic plane (|b|>5^deg), X shows little systematic variation with latitude from a mean value of (1.8+/-0.3)×10²⁰ cm^-2 K^-1 km^-1 s. Given the large sky area and large quantity of CO data analyzed, we conclude that this is the most reliable measurement to date of the mean X value in the solar neighborhood.