Dense gas in nearby galaxies: XV. Hot ammonia in NGC253, Maffei2 and IC342

Authors:
R. Mauersberger,
C. Henkel,
A. Weiss,
A.B. Peck,
Y. Hagiwara

Comments: 11 pages, 3 Figures, 5 Tables

The detection of NH3 inversion lines up to the (J,K)=(6,6) level is reported
toward the central regions of the nearby galaxies NGC253, Maffei2, and IC342.
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 (T_kin= 100 – 140 K)
molecular component toward all galaxies studied. The tentatively detected
(J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC342.
Toward NGC253, IC342 and Maffei2 the global beam averaged NH3 abundances are
1-2 10^-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 H_2 in M82 and the observed
chemical abundances.

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