Observations of Ammonia in External Galaxies I. NGC 253 and M 82

The ammonia
$(J,K) = (1,1)$
,
$(2,2)$
,
$(3,3)$
, and
$(4,4)$
transitions in the 23.7–24.1 GHz region were searched for in nearby galaxies, NGC 253 and M 82, to study the relation between the molecular abundances and physical conditions. The
$(1,1)$
,
$(2,2)$
, and
$(3,3)$
emission lines were clearly detected in NGC 253, but no lines were found in M 82. In NGC 253 the shapes of the three emission lines were different from one another. The rotational temperatures obtained in NGC 253 were
$\lesssim 17 \hbox{-} 50 \,{\rm K}$
, depending on the velocity components. There are two main results: (1) The ortho-to-para abundance ratio was obtained in NGC 253 and found to be different depending on the velocity components. The ratio was about 1 for the velocity component where the
$(2,2)$
line is detected. On the other hand, the ratio was more than 6 for another velocity component where the
$(2,2)$
line is barely detected. This high ratio corresponds to an equilibration temperature of less than 8 K between ortho and para. (2) The abundance of
${\rm NH}_3$
in NGC 253 was found to be about an order of magnitude larger than the upper limit in M 82. This result is an additional evidence for the different molecular abundances which were already reported between NGC 253 and M 82. In addition, the molecular abundances including
${\rm NH}_3$
in nearby galaxies with rich molecular gas were compared, and we found that the molecular abundance in M 82 is systematically peculiar concerning the formation mechanisms of molecules. We discussed factors which cause such a peculiarity in M 82.