Dynamically Influenced Molecular Clouds in the Nucleus of NGC 6946: Variations in the CO Isotopic Line Ratios

We present high resolution (~5'') maps of the J = 1 - 0 transitions of ^{13}CO and C^{18}O towards the nucleus of NGC 6946, made with the Owens Valley Millimeter Array. The images are compared with existing ^{12}CO(1-0) maps to investigate localized changes in gas properties across the nucleus. As compared to ^{12}CO, both ^{13}CO and C^{18}O are more confined to the central ring of molecular gas associated with the nuclear star formation; that is, ^{12}CO is stronger relative to ^{13}CO and C^{18}O away from the nucleus and along the spiral arms. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio reaches very high values of >40. We attribute the relative ^{13}CO weakness to a rapid change in the interstellar medium from dense star forming cores in a central ring to diffuse, low density molecular gas in and behind the molecular arms. This change is abrupt, occurring in less than a beamsize (90 pc), about the size of a giant molecular cloud. Column densities determined from ^{13}CO(1-0), C^{18}O(1-0), and 1.4 mm dust continuum all indicate that the standard Galactic conversion factor, X_{CO}, overestimates the amount of molecular gas in NGC 6946 by factors of ~3-5 towards the central ring and potentially even more so in the diffuse gas away from the central starburst. We suggest that the nuclear bar acts to create coherent regions of molecular clouds with distinct and different physical conditions. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio in galactic nuclei can be a signpost of a dynamically evolving ISM.