2001A&A...371...25S

To study the different components of the molecular gas in the nuclear region of the nearby spiral galaxy IC 342, ¹²CO (3-2), ¹³CO (3-2) and (2-1), as well as HCN(1-0) through (4-3) observations are presented and analyzed in conjunction with a variety of line and continuum emission data of the literature. We find several giant molecular clouds embedded in a medium with lower column and volume density. The gas shows strong density and temperature gradients with a very clumpy substructure; the lower-density gas is rather warm (≤50K) and the temperature decreases with increasing density. While LVG calculations are contradicting our observations, we are able to explain all CO and [CII] line data by applying a model of photon dominated regions (PDR); for the gas seen in these lines we find no evidence for an additional dominant heating mechanism. On the other hand, for our HCN observations tracing the dense gas clumps (n(H₂)≃10⁶cm^–3) where the influence of UV photons is expected to be less important than for less dense gas seen in CO (n(H₂)≃10⁴cm^–3), an LVG model approach should be applicable and, in fact, yields results which fit well the data and support the outlined scenario. For this gas component which is rather cool (T_k≤30K), heating processes involving photoelectrons should be unimportant and other mechanisms like turbulent energy dissipation or cosmic rays should be considered. The structure of the nucleus of IC 342 with several giant molecular clouds with sizes of ≤20 to 50pc and masses of order 10⁶M_☉ partly associated with HII regions and embedded into a lower-density interstellar medium shows striking similarities in terms of cloud distribution and their physical behaviour with our Galactic Centre region.