Astronomy and Astrophysics, volume 403, 561-571 (2003/5-4)
Dense gas in nearby galaxies. XV. Hot ammonia in NGC 253, Maffei 2 and IC 342.
MAUERSBERGER R., HENKEL C., WEISS A., PECK A.B. and HAGIWARA Y.
Abstract (from CDS):
The detection of NH3 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 406K (for (J,K)=(6,6)) and 848K (for the (9,9) transition) above the ground state and reveal a warm (Tkin= 100 ...140K) molecular component toward all galaxies studied. The tentatively detected (J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC 342. Toward NGC 253, IC 342 and Maffei 2 the global beam averaged NH3 abundances are 1-2x10–8, 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 M82, however, the NH3 emitting gas component is comparatively cool (∼30K). 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 M82. 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 M82 and the observed chemical abundances.
galaxies: individual: NGC 253, Maffei 2, IC 342, M 82 - galaxies: ISM - galaxies: starburst - galaxies: abundances - radio lines: galaxies
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