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

Abstract

We present high-resolution (~5'') maps of the J = 1–0 transitions of 13CO and C18O toward the nucleus of NGC 6946 made with the Owens Valley Millimeter Array. The images are compared with existing 12CO (1–0) maps to investigate localized changes in gas properties across the nucleus. As compared with 12CO, both 13CO and C18O are more confined to the central ring of molecular gas associated with the nuclear star formation; that is, 12CO is stronger relative to 13CO and C18O away from the nucleus and along the spiral arms. The 12CO (1–0)/13CO (1–0) line ratio reaches very high values of greater than 40. We attribute the relative 13CO weakness to a rapid change in the interstellar medium (ISM) 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 beam size (90 pc), about the size of a giant molecular cloud. Column densities determined from 13CO (1–0), C18O (1–0), and 1.4 mm dust continuum all indicate that the standard Galactic conversion factor, XCO, overestimates the amount of molecular gas in NGC 6946 by factors of ~3–5 toward 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 12CO (1–0)/13CO (1–0) line ratio in galactic nuclei can be a signpost of a dynamically evolving ISM.