2013A&A...552A..34T

To study the abundance and temperature of ammonia in the center of a nearby galaxy M51 and to compare them with those in other nearby galaxies, we observed its (J,K)=(1,1), (2,2), (3,3), and (4,4) inversion transitions at the wavelength of 1.3cm. The observations were carried out with the Effelsberg 100-m radio telescope. The (1,1), (2,2), and (3,3) transitions are clearly detected, but the (4,4) transition is barely detected. The rotational temperature obtained from the (1,1) and (2,2) transitions of para-ammonia is 25±2K, which is similar to those of M82 and the Large Magellanic Cloud (N 159 W), but significantly lower than those of IC342 and NGC1068 among the nearby galaxies. The column density of ammonia is (8.1±2.4)x10¹³cm², and the abundance relative to H₂ is ∼4.5x10^–9. The abundance in M51 is about a factor of 5 lower than those of NGC253 and IC342, but about one order of magnitude higher than those of M82 and the Large Magellanic Cloud. The addition of the data of M51 further supports the exceptionally low abundances of ammonia reported previously in these two galaxies. For understanding the abundance of ammonia in M51 and other nearby galaxies, their temperatures were compared. As a result, we found that the galaxies with low temperature tend to have low abundance of ammonia. In addition, the photodissociation rate of ammonia was compared to those of related molecules detected in the nearby galaxies to discuss the effect of photodissociation. We found that the low abundance of ammonia in some galaxies cannot be explained only by the effect of photodissociation.