A mapping survey of the ¹³CO and ¹²CO emission in galaxies

We present spectra of the extended ¹²CO and ¹³CO J = 1 → 0 emission along the major axes of 17 nearby galaxies. Spatial variations in the ratio of CO and ¹³CO integrated intensities, ℛ, are found in nearly every galaxy observed. There is an overall variation in ℛ of 20%-40% from the inner 2 kpc to the disk. Roughly one-third of the survey galaxies have such gradients in ℛ detected above the 2 σ confidence level. Though some galaxies show a lower central value of ℛ, on average ℛ inside 2 kpc is 10%-30% higher than ℛ outside of 2 kpc. The average CO/¹³CO intensity ratio within the central 2 kpc of the survey sources is 11.6 ± 0.4 (based on the noise) ± 1.5 (based on systematic uncertainties estimated from daily variations in CO and ¹³CO intensities). The 1 σ dispersion in ℛ between galactic nuclei of 4.2 is also quite large. The average value of ℛ outside 2 kpc is 9.8 ± 0.6 ± 1.2 with a standard deviation of 4.5. An increase in the CO/¹³CO intensity ratio from disk to nucleus may imply that the conversion factor between CO intensity and H₂ column density, X, is lower in galactic nuclei. In addition, variations in physical conditions, most notably the gas kinetic temperature, affect both ℛ and X. Abundance variations probably do not cause the gradient in ℛ, though we do not rule out a decrease in effective cloud column densities in galactic nuclei possibly caused by destructive starburst superwinds. A modest rise in temperature (less than a factor of 2 or 3) from outside a 2 kpc radius toward the nucleus can easily account for the observed gradient. These results support previous work implying that X is lower in the center of the Milky Way and probably most galactic nuclei. Therefore, calculating H₂ masses using the standard Galactic X-factor, especially within the central few kiloparsecs of galaxies, overestimates the true mass by factors of a few. The standard X-factor still appears to be appropriate for galactic disks.