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Issue A&A
Volume 403, Number 2, May IV 2003
Page(s) 561 - 571
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030386



A&A 403, 561-571 (2003)
DOI: 10.1051/0004-6361:20030386

Dense gas in nearby galaxies

XV. Hot ammonia in NGC 253, Maffei 2 and IC 342
R. Mauersberger1, C. Henkel2, A. Weiß1, 3, A. B. Peck2, 4 and Y. Hagiwara2, 5

1  Instituto de Radioastronomía Milimétrica (IRAM), Avda. Divina Pastora 7 NC, 18012 Granada, Spain
2  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 71, 53121 Bonn, Germany
3  Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 69, 53121 Bonn, Germany
4  Smithsonian Submillimeter Array (SMA), PO Box 824, Hilo, HI 96721, USA
5  ASTRON, Westerbork Observatory, PO Box 2, 7990AA Dwingeloo, The Netherlands

(Received 22 October 2002 / Accepted 13 March 2003 )

Abstract
The detection of NH 3 inversion lines up to the $(J,K)=(6,\,6)$ level is reported toward the central regions of the nearby galaxies NGC 253, Maffei 2, and IC 342. The observed lines are up to 406 K (for $(J,K)=(6,\,6)$) and 848 K (for the $(9,\,9)$ transition) above the ground state and reveal a warm ( $T_{\rm kin}= 100 \ldots 140$ K) molecular component toward all galaxies studied. The tentatively detected (J,K)=(9,9) line is evidence for an even warmer ( >400 K) component toward IC 342. Toward NGC 253, IC 342 and Maffei 2 the global beam averaged NH 3 abundances are $1{-}2\times 10^{-8}$, while the abundance relative to warm H 2 is around 10 -7. The temperatures and NH 3 abundances are similar to values found for the Galactic central region. C-shocks produced in cloud-cloud collisions can explain kinetic temperatures and chemical abundances. In the central region of M 82, however, the NH 3 emitting gas component is comparatively cool (~30 K). It must be dense (to provide sufficient NH 3 excitation) and well shielded from dissociating photons and comprises only a small fraction of the molecular gas mass in M 82. An important molecular component, which is warm and tenuous and characterized by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions (PDRs) can explain both the high fraction of warm H 2 in M 82 and the observed chemical abundances.


Key words: galaxies: individual: NGC 253, Maffei 2, IC 342, M 82 -- galaxies: ISM -- galaxies: starburst -- galaxies: abundances -- radio lines: galaxies

Offprint request: R. Mauersberger, mauers@iram.es

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