CI and CO in the Spiral Galaxies NGC 6946 and M 83

Details CI and CO in the Spiral Galaxies NGC 6946 and M 83 CI and CO in the Spiral Galaxies NGC 6946 and M 83

2001-04-11

arxiv.org/abs/astro-ph/0104196v2

Astronomy and Astrophysics

Astrophysics

12 pages; accepted for publication in A&A

Scientific paper

10.1051/0004-6361:20010354

Multitransition 12CO, 13CO and 492 GHz [CI] measurements of the late-type spiral galaxies NGC 6946 and M 83 (NGC 5236) show pronounced molecular gas concentrations in rapid solid-body rotation within a few hundred parsec from both nuclei. Their 12CO, 13CO and [CI] relative intensities are nearly identical. However, the very different [CII] intensities imply that the physical conditions are not. The slow decrease of 12CO intensities with increasing rotational level marks the presence of significant amounts of warm and dense molecular gas in both galaxy centers. Detailed modelling indicates that both galaxiy centers contain at least two distinct molecular components: a warm and dense component with T(kin) = 30-60 K, n(H2) = 3000-10 000 cc, and a more tenuous hotter component with T(kin) = 100-150 K, n(H2) < 1000 cc). Atomic carbon column densities exceed CO column densities by a factor of about 1.5 in NGC 6946 and about 4 in M 83. Unlike NGC 6946, M 83 contains a significant amount of molecular hydrogen associated with ionized carbon rather than CO. The centers of NGC 6946 and M 83 contain nearly identical total (atomic and molecular) gas masses of about 3 x 10**7 M(sun). Despite their prominence, the central gas concentrations in these galaxies represent only a few per cent of the stellar mass in the same volume. The peak face-on gas mass density is much higher in M 83 (120 M(sun)/pc**2) than in NGC 6946 (45 M(sun)/pc**2). The more intense starburst in M 83 is associated with a more compact and somewhat hotter PDR zone than the milder starburst in NGC 6946.

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