Carbon monoxide isotope ratios in galactic center and disks.

The authors report observations of millimeter-wavelength emission of CO and ¹³CO in six spiral galaxies, two previously unreported at ¹³CO. For each galaxy, ¹³CO emission was observed both from the center and from the disk. The ratio of integrated intensities, P(CO)/P(¹³CO), varies markedly from galactic center to galactic center, from galactic disk to galactic disk, and from point to point within a given galaxy. These variations indicate that the use of a constant P(CO)→σ(H₂) conversion to derive H₂ surface densities from extragalactic CO observations can introduce major errors.