Observations of Ammonia in External Galaxies I. NGC 253 and M 82
Observations of Ammonia in External Galaxies I. NGC 253 and M 82
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DOI：
10.1093/pasj/54.2.195
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发表时间：
2002-04
影响因子：
2.3
通讯作者：
S. Takano;N. Nakai;K. Kawaguchi
中科院分区：
文献类型：
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作者：
S. Takano;N. Nakai;K. Kawaguchi
在附近的星系NGC 253和M 82中搜索了23.7-24.1 GHz区域的氨$(J,K) = (1,1)$、$(2,2)$、$(3,3)$和$(4,4)$跃迁，以研究分子丰度与物理条件之间的关系。在 NGC 253 中清楚地检测到 $(1,1)$ 、 $(2,2)$ 和 $(3,3)$ 发射线，但在 M 82 中没有发现任何发射线。在 NGC 253 中，三条发射线的形状彼此不同。 NGC 253 中获得的旋转温度为 $\lesssim 17 \hbox{-} 50 \,{\rm K}$ ，具体取决于速度分量。主要结果有两个：（1）在NGC 253中获得了邻位丰度比，发现邻位丰度比随速度分量的不同而不同。对于检测到 $(2,2)$ 线的速度分量，该比率约为 1。另一方面，对于几乎检测不到 $(2,2)$ 线的另一个速度分量，该比率超过 6。这个高比率对应于邻位和对位之间小于 8 K 的平衡温度。 (2) NGC 253 中 ${\rm NH}_3$ 的丰度比 M 82 中的上限大一个数量级左右。这一结果为 NGC 253 和 M 82 之间已报道的分子丰度不同提供了额外的证据。此外，比较了附近分子气体丰富的星系中包括 ${\rm NH}_3$ 在内的分子丰度，发现 M 82 中的分子丰度为关于分子形成机制的系统独特性。我们讨论了导致 M 82 出现这种特性的因素。
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.