Dense gas in nearby galaxies - XV. Hot ammonia in NGC 253, Maffei 2 and IC 342

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 406 K (for (J,K) = (6,6)) and 848 K (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 (>400 K) component toward IC 342. Toward NGC 253, IC 342 and Maffei 2 the global beam averaged NH3 abundances are 1-2 x 10(-8), while the abundance relative to warm H-2 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 (similar to30 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 H-2 in M 82 and the observed chemical abundances.