Volume 403, Number 2, May IV 2003
|Page(s)||561 - 571|
|Published online||06 May 2003|
Dense gas in nearby galaxies
XV. Hot ammonia in NGC 253, Maffei 2 and IC 342
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
Corresponding author: R. Mauersberger, email@example.com
Accepted: 13 March 2003
The detection of NH3 inversion lines up to the 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 ) and 848 K (for the transition) above the ground state and reveal a warm ( K) molecular component toward all galaxies studied. The tentatively detected 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 NH3 abundances are , while the abundance relative to warm H2 is around 10-7. The temperatures and NH3 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 NH3 emitting gas component is comparatively cool (~30 K). It must be dense (to provide sufficient NH3 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 H2 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
© ESO, 2003
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