CI and CO in the spiral galaxies NGC 6946 and M 83

We present J = 2-1, J = 3-2, J = 4-3 (CO)-C-12 and 492 GHz [CI] maps as well as J = 2-1 and J = 3-2 (CO)-C-13 measurements of the late type spiral galaxies NGC 6946 and M 83 (NGC 5236). Both galaxies contain a pronounced molecular gas concentration in rapid solid-body rotation within a few hundred parsec from their nucleus. NGC 6946 and M 83 have nearly identical relative intensities in the (CO)-C-12, (CO)-C-13 and [CI] transitions, but very different [CII] intensities, illustrating the need for caution in interpreting CO observations alone. The slow decrease of velocity-integrated (CO)-C-12 intensities with increasing rotational level implies the presence of significant amounts of warm and dense molecular gas in both galaxy centers. Detailed modelling of the observed line ratios indicates that the molecular medium in both galaxies consists of at least two separate components. These are a warm and dense component (T-kin = 30-60 K, n(H-2) = 3000-10 000 cm(-3)) and a much more tenuous hot component (T-kin = 100-150 K, n(H-2) less than or equal to 1000 cm(-3)). Total 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 10(7) M-circle dot. 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-circle dot pc(-2)) than in NGC 6946 (45 M-circle dot 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.