2002/04/25 by Shuro Takano, Naomasa Nakai, Kentarou Kawaguchi · 3 citations
Chemistry · Physics and Astronomy · #Spectroscopy and Laser Applications #Astrophysics and Star Formation Studies #Advanced Chemical Physics Studies
paper · pdf · doi:10.1093/pasj/54.2.195
openalex publication_date 2002/04/25 · openalex created_date 2025/10/10 · openalex updated_date 2026/06/23
Abstract 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 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 NH3 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 NH3 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.