1995A&A...302..343I

Observations of the central region of the nearby spiral galaxy NGC 253 are presented in the ³P₁-³P₀ fine-structure line of atomic carbon (C I) at 492GHz and in the CO J=4-3 and J=3-2 lines at 461 and 345GHz. C I emission was mapped over a 24"x24" region, corresponding to a projected area of 290x290pc. The major axis was mapped over a length of 48" (580pc). In CO J=3-2, a region of 48"x72" (580x870pc) was sampled. In CO J=4-3, only a few central positions were observed. Both the C I and the CO J=3-2 emission are strongly concentrated towards the centre of the galaxy and have similar overall extent. The data suggest that the nucleus of NGC 253 is surrounded by a dense, clumpy torus of diameter 600pc, and a central cavity of diameter 120pc. The inner part of the torus appears to have both a higher gas density and a higher gas-to-dynamical mass ratio. Central C I column densities are of order 8x10¹⁸cm^–2, and the C/CO ratio is 0.2-0.3, depending on temperature. This ratio is higher than that of Galactic starforming clouds, but somewhat lower than that of the centre of M 82. Its explanation does not require the use of complex models; the C I and CO results, as well as other observations are consistent with relatively long line-of-sights through a clumpy medium containing both dense molecular clouds and more tenuous translucent material. The observed 492GHz C I line provides a cooling rate about 25% of that provided by the observed lower four CO transitions. However, both the higher ¹²CO transitions and the C I line at 809GHz will provide significantly higher cooling rates. Model estimates show that in the central regions of NGC 253, cooling of the dense gas component due to all C I and CO are very nearly equally important.