Abstract
We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion lines of ammonia (NH3) towards the south--western molecular lobe in M82. The relative intensities of the ammonia lines are characterized by a rotational temperature of T_rot=29+/-5 K which implies an average kinetic temperature of T_kin~60 K. A Gaussian decomposition of the observed spectra indicates increasing kinetic temperatures towards the nucleus of M82, consistent with recent findings based on CO observations. The observations imply a very low NH3 abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a decreasing NH3 abundance towards the central active regions in M82 and interpret this abundance gradient in terms of photodissociation of NH3 in PDRs. The low temperature derived here from NH3 also explains the apparent underabundance of complex molecules like CH_3OH and HNCO, which has previously been reported.