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Astron. Astrophys. 326, 59-68 (1997)

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Dense gas in nearby galaxies

X. H [FORMULA] CO and CH [FORMULA] OH: molecular abundances and physical conditions

S. Hüttemeister 1, 2, R. Mauersberger 3, 4 and C. Henkel 3

1 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
4 Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA

Received 7 November 1996 / Accepted 24 April 1997

Abstract

Multilevel observations of formaldehyde ([FORMULA]) and methanol ([FORMULA]) toward the nearby spiral galaxies NGC 253, Maffei 2, IC342, M 82 and NGC 6946 are presented. [FORMULA] was detected in all galaxies (tentatively in NGC 6946). [FORMULA] was detected in all objects with the notable exception of M 82.

H2 CO line intensity ratios point out differences in gas density both between galaxies and within the central regions of individual objects. Model calculations show that the bulk of the gas emitting [FORMULA] in NGC 253 is at a density of [FORMULA] [FORMULA], while the [FORMULA] lines in M 82 and IC 342 trace two different, spatially separated gas components with densities of [FORMULA] [FORMULA] and [FORMULA] [FORMULA]. The south-western molecular hotspot in M 82 and the center of IC 342 are the regions with the highest density.

Methanol is subthermally excited in all galaxies, with the lowest excitation temperatures found in IC 342. The [FORMULA] abundance in NGC 253 and the non-starburst nuclei of IC 342 and Maffei 2 are comparable. A map of the [FORMULA] lines in NGC 253 shows that [FORMULA] traces clumpy structures better than other molecules requiring high gas density to be excited. [FORMULA] toward M 82 is at least an order of magnitude less abundant than in otherwise comparable galaxies. This confirms the existence of global chemical differences, and thus very large scale variations in the state of the molecular gas phase, even between galaxies commonly classified as starburst nuclei.

Key words: ISM: molecules – galaxies: ISM – galaxies: nuclei – galaxies: starburst – radio lines: galaxies

Send offprint requests to: S. Hüttemeister, RAIUB

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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