Large-scale Structure of Galactic CO

The CO composite maps provide a global view of the molecular clouds in Milky Way. There are many interesting features in the complete map, which may reveal some profound physical natures.

Looking at the Galactic CO map, the most prominent feature is the thin, intense ridge, lying on the two sides of Galactic center.  The l,v map shows that the high intensity of this ridge results from integration over many molecular clouds in the so-called molecular ring. The asymmetric appearance of the molecular ring indicates that it is composed of several inner spiral arms, perhaps rooted in a central bar. Most of the stronger (red to white) peaks in the l,v map correspond to giant molecuar complexes with masses in the range 105 -10Msun.  Two of the nearest complexes are those associated with the supernova remnants W44 in the Sagittarius Arm and Cas A in the Perseus Arm. 

Clouds within 1 kpc of the Sun appear in the l,v map at |v| < 20 km/s. Most of the large local clouds lying beyond the latitude integration range of l,v map also fall within this same low-velocity lane.  If we only intergrate over velocity in 2.5 kpc, the lane still remains and we can see a correlation between CO map and optical panorama, which identify the relation between CO emission and optical extinction. CO emission is related to molecular clouds, which is related to optical extinction. However, the correlation is not very precise, thus we should use far-infrared emisson as a gas tracer to revise the correlation. 

To calibrate far-infrared emission as a gas tracer, we will assume that the total gas column density in regions free of existing 21 cm survey is simply the HI column density derived from existing 21 cm surveys. Velocity-integrated intensities were converted to atomic column densities on the assumption that the 21 cm line is optically thin. Optical depth corrections are generally small except within ~2° of the plane toward the inner Galaxy, where our H2 prediction is somewhat unreliable in any case owing to large variations in dust temperature along the line of sight.  

Corrected to a constant dust temperature and with point sources removed, the map of 100 μm should be proportional to total gas column density, provided the dust temperature, the dust emissivity law, and the gas-to-dust ratio are all constant along the line of sight. The map of NHI/I100 suggests that these conditions roughly hold for most lines of sight off the Galactic plane, the value of which is fairly constant at 0.9±0.4×1020 cm-2 MJy-1 sr, with 1 σ spread of only 50%.  

The ratio of the predicted molecular column density to the observed CO intensity provides a calibration of the CO-to-H2 mass conversion factor X. Since there are significant point-to-point variations in the derived X value, and because X might be expected to vary with latitude, we determine here only a mean X at each latitudeby taking the ratio of mean Wco to mean predicted NH2 over the same observed positions.





(From ApJ,2001,547,792)