A Mapping Survey of the 13 CO and 12 CO Emission in Galaxies

We present spectra of the extended 12 CO and 13 CO J = 1 → 0 emission along the major axes of 17 nearby galaxies. Spatial variations in the ratio of CO and 13 CO integrated intensities, , are found in nearly every galaxy observed. There is an overall variation in of 20%-40% from the inner 2 kpc to the disk. Roughly one-third of the survey galaxies have such gradients in detected above the 2 σ confidence level. Though some galaxies show a lower central value of , on average inside 2 kpc is 10%-30% higher than outside of 2 kpc. The average CO/ 13 CO 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 13 CO intensities). The 1 σ dispersion in between galactic nuclei of 4.2 is also quite large. The average value of outside 2 kpc is 9.8 ± 0.6 ± 1.2 with a standard deviation of 4.5. An increase in the CO/ 13 CO intensity ratio from disk to nucleus may imply that the conversion factor between CO intensity and H 2 column density, X , is lower in galactic nuclei. In addition, variations in physical conditions, most notably the gas kinetic temperature, affect both and X . Abundance variations probably do not cause the gradient in , 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 H 2 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.