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Issue A&A
Volume 371, Number 1, May III 2001
Page(s) 25 - 36
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010082



A&A 371, 25-36 (2001)
DOI: 10.1051/0004-6361:20010082

The nucleus of the nearby galaxy IC 342

A. Schulz1, 2, R. Güsten3, B. Köster4, 5 and D. Krause4, 6

1  Institut f. Physik & ihre Didaktik, Universität zu Köln, Gronewaldstr. 2, 50931 Köln, Germany
2  Institut f. Astrophysik & Extraterr. Forschung, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
3  Max-Planck-Institut f. Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
4  I. Physikalisches Institut, Universität zu Köln, Universitätsstr. 17, 50937 Köln, Germany
5  Pricewaterhouse Coopers, Hohenzollernring 21-23, 50672 Köln, Germany
6  Pixelpark, Friesenplatz 25, 50672 Köln, Germany

(Received 7 July 2000 / Accepted 4 January 2001)

Abstract
To study the different components of the molecular gas in the nuclear region of the nearby spiral galaxy IC 342, $^{12}{\rm CO}$ (3-2), $^{13}{\rm CO}$ (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 ($\leq$50 K) and the temperature decreases with increasing density. While LVG calculations are contradicting our observations, we are able to explain all CO and [C II] 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 ( $n ({\rm H}_2) \approx 10^6$ cm-3) where the influence of UV photons is expected to be less important than for less dense gas seen in CO ( $n ({\rm H}_2) \approx 10^4$ cm-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 ( $T_{\rm k} \leq 30$ K), 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 $\leq$20 to 50 pc and masses of order 106 $M_{\odot}$ partly associated with H II 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.


Key words: galaxies: individual: IC 342 -- galaxies: galactic nuclei -- galaxies: spiral -- galaxies: interstellar matter -- radio lines: molecular

Offprint request: A. Schulz, p341sch@mpifr-bonn.mpg.de

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© ESO 2001


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