2001ApJ...554L.143W

We report the detection of the (J, K)=(1, 1), (2, 2), and (3, 3) inversion lines of ammonia (NH₃) toward the southwestern 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 toward 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 toward the central active regions in M82 and interpret this abundance gradient in terms of photodissociation of NH₃ in photodissociation regions. The low temperature derived here from NH₃ also explains the apparent underabundance of complex molecules like CH₃OH and HNCO, which has previously been reported.