The milky way in molecular clouds: A new complete CO survey

New large-scale CO surveys of the first and second Galactic quadrants and t he 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 sampl e the entire Galactic plane over a (1/8 degrees) strip 4 degrees -10 degree s wide in latitude, and beamwidth or sample nearly all large local clouds a t 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 hi gher angular resolution, and up to 10 times higher sensitivity per unit sol id angle. Each of the component surveys was integrated individually using c lipping or moment masking to produce composite spatial and longitude-veloci ty maps of the Galaxy that display nearly all of the statistically signific ant emission in each survey but little noise. The composite maps provide detailed information on individual molecular clo uds, suggest relationships between clouds and regions widely separated on t he sky, and clearly display the main structural features of the molecular G alaxy. In addition, since the gas, dust, and Population I objects associate d 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 com pleteness of the present survey and to extrapolate it to the entire sky at \b\ < 32<degrees>. The close agreement of the observed and predicted maps i mplies that only similar to2% of the total CO emission at \b\ < 32<degrees> lies outside our current sampling, mainly in the regions of Chamaeleon and the Gum Nebula. Taking into account this small amount of unobserved emissi on, the mean molecular column density decreases from similar to3 x 10(20) c m(2) at \b\ = 5 degrees to similar to0.1 x 10(20) cm(2) at \b\ = 30 degrees ; this drop is similar to6 times steeper than would be expected from a plan e-parallel layer, but is consistent with recent measurements of the mean mo lecular 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-H-2 mass conversion facto r X = NH2/W-CO. Out of the Galactic plane (\b\ > 5 degrees), X shows little systematic variation with latitude from a mean value of (1.8 +/- 0.3) x 10 (20) cm(2) K-1 km(1) s. Given the large sky area and large quantity of CO d ata analyzed, we conclude that this is the most reliable measurement to dat e of the mean X value in the solar neighborhood.