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

Abstract

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 a third of the survey galaxies have such gradients in R detected above the 2-sigma 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. 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. Also 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 towards 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 kpc of galaxies, overestimates the true mass by factors of a few. The standard X-factor still appears to be appropriate for galactic disks.