Dense gas in nearby galaxies

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 ($T_{\rm kin}= 100 \ldots 140$ 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 $1{-}2\times 10^{-8}$, 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 M 82, however, the NH3 emitting gas component is comparatively cool (~30 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 H2 in M 82 and the observed chemical abundances