2004/01/16 by David S. Meier, Jean L. Turner
Chemistry · Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Bar (unit) #Chemistry #Galactic Center #Galaxies: Formation, Evolution, Phenomena #Galaxy #Geometry #Interstellar medium #Line (geometry) #Millimeter #Molecular cloud #Nucleus #Physics #Ring (chemistry) #Spiral galaxy #Star formation #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/382904
38 pages, 9 figures. Accepted to the Astronomical Journal
arxiv created 2004/01/16 · openalex publication_date 2004/04/01 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
We present high-resolution (∼5'') maps of the J = 1–0 transitions of 13 CO and C 18 O toward 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 with 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 greater than 40. We attribute the relative 13 CO 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 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 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 12 CO (1–0)/ 13 CO (1–0) line ratio in galactic nuclei can be a signpost of a dynamically evolving ISM.