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 NGC253, 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 NGC253, IC 342 and Maffei 2 the global beam averaged NH3 abundances are 1− 2 10 , while the abundance relative to warm H2 is around 10 . 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.