|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by . All rights reserved
4.2. Optically Thin CO
In a few galactic centers, the CO emission appears to be optically thin, as determined by the relatively large intensity ratios of the J = 2-1 to J = 1-0 lines (Knapp et al. 1980, Lo et al. 1980). This is curious in view of the fact that optically thin CO emission is rather uncommon in our Galaxy, and where seen is confined to relatively small regions within otherwise optically thick clouds. In 3 of the 5 galaxies studied in the J = 2-1 transition, the 2-1/1-0 intensity ratio is near unity, indicating optical thickness. However, M82 and NGC 253 show much larger ratios for their inner 13" regions, indicating optically thin emission.
The CO excitation temperature must be relatively high in order to produce the observed brightness temperatures with optically thin emission. If the warm CO is collisionally excited, then the volume-filling factor of the emitting clouds must be quite low (Knapp et al. 1980). If however, it is excited by 4.6µ radiation from the bright nuclear infrared sources (Lo et al. 1980), then there is no such constraint on the emitting region. In either case, the implied mass of H2 clouds is relatively small (< 108 M). This is still sufficient to fuel the intense star formation in the center of M82, if star formation is ~ 50% efficient, or if high-mass stars are preferentially formed.
An alternative possibility is that the molecular cloud mass is dominated by cold, optically thick clouds, which contribute only a minor fraction of the 1-0 emission and even less to the 2-1 lines (Lo et al. 1980). This is supported by the observed strengths of the 13CO, J = 1-0 lines in NGC 253 and M82 [P(12CO) / P(13CO) = 11 and 16, respectively; Encrenaz et al. 1979, Stark & Wolff 1979]. Unless 13CO is extremely overabundant relative to our Galaxy, some of the emission must be optically thick. Stark (private communication) finds T*A(12CO) / T*A(13CO) = 30 for a narrow feature at 170 km s-1 and reports P(13CO) / P(C18O) = 5 (after a 90 hour integration!). These results are consistent with the optically thin interpretation; they also indicate terrestrial isotopic abundances.