2003A&A...403..561M

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 (T_kin= 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.