Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

3.3 ¹³CO Observations of Galaxies

A second test of the molecular mass determinations is comparison of the CO radial distribution in a galaxy with that of the more optically thin ¹³CO line. The first ¹³CO observations in eight galaxies (Encrenaz et al 1979) were followed by additional studies in a small number of other systems (Young & Scoville 1982a, 1984, Stark & Carlson 1984, Rydbeck et al 1985). Defining the ratio of CO to ¹³CO integrated intensities to be R I (CO) / I (¹³CO), Encrenaz et al (1979) found an average value of R ~ 12, or roughly twice the mean value in the molecular annulus of our Galaxy (Solomon et al 1979, Stark 1983). Rickard & Blitz (1985) and Young & Sanders (1986) measured the CO and ¹³CO emission in the disks of several nearby luminous spiral galaxies. In six galaxies, the shapes of the azimuthally averaged distributions for CO and ¹³CO were similar. Globally, the value of R was found to range between 6 and 20 for individual galaxies, with the lower values in NGC 891. The highest value of R was found in the center of M82, where the ratio of integrated intensities was 20. Within individual galaxies, R varied by a factor of 2 on the scale of 1 to 2 kpc. Such variations could be caused by changes in the relative percentage of hot and cold clouds, such as is seen in the Milky Way between spiral arm and interarm regions (Solomon et al 1985).