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Dame, T.M., 1983: Molecular Clouds and Galactic Spiral Structure. Ph.D. thesis. Columbia University.
Galactic CO line emission at 115 GHz has been surveyed in the region 12° < ℓ < 60° and -1° < b < 1° in order to study the distribution of molecular clouds in the inner galaxy; an inner strip 0.5° wide has been sampled every beamwidth (0.125°), the rest every two beamwidths.
Comparison of the survey with similar HI data reveals a detailed correlation with the most intense 21-cm features, implying that the CO and HI trace the same galactic features and have the same large-scale kinematics. To each of the classical 21-cm (HI) spiral arms of the inner galaxy there corresponds a CO molecular arm which is generally more clearly defined and of higher contrast.
The higher contrast of the CO spiral features is found to be partially due to the lower velocity dispersion of the CO (4.5 km/sec). The remaining difference in contrast cannot be attributed to streaming motions since both species have essentially the same large-scale kinematics. It is concluded that the arm-interarm contrast for the galactic CO emission is more than twice that of the 21-cm emission.
A simple model is developed in which all of the CO emission from the inner galaxy arises from spiral arms. The modeling results suggest that molecular clouds are essentially transient objects, existing for 15 to 40 million years after their formation in a spiral arm, and are largely confined to spiral features about 300 pc wide.
A CO survey of the Perseus spiral arm in the range ℓ = 104° to 128° is used to determine a mass spectrum for molecular clouds. The slope of the differential log N - log M curve is found to be -1.45±0.08, implying that the galactic CO emission and molecular mass are dominated by the largest complexes.
A variety of methods are employed to estimate distances and masses for the largest clouds detected by the inner-galaxy survey and a catalogue is compiled. The catalogued clouds, the largest of which have masses of several 106 M☉ and linear dimensions in excess of 100 pc, are found to be excellent spiral-arm tracers. One of the nearest of the clouds, that associated with the supernova remnant W44, is fully mapped in both CO and 13CO and is discussed in detail.
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