The detection of NH _ { 3 } 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 \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 NH _ { 3 } abundances are 1 - 2 10 ^ { -8 } , while the abundance relative to warm H _ { 2 } is around 10 ^ { -7 } .
The temperatures and NH _ { 3 } 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 NH _ { 3 } emitting gas component is comparatively cool ( \sim 30 K ) .
It must be dense ( to provide sufficient NH _ { 3 } 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 .