Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+
Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+
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亚稳态 H3 探测银河系中心附近的热弥漫云
DOI：
10.1086/432679
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
B. McCall
中科院分区：
文献类型：
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作者：
T. Oka;T. Geballe;M. Goto;T. Usuda;B. McCall
利用H亚稳（J，K）=（3，3）能级的吸收谱线，结合沿着几条视线观测到的H和CO的其它谱线，在银河系的中心分子区（CMZ）发现了大量的高温（T ~ 250 K）、低密度（n ~ 100 cm-3）、速度色散大的气体，在中心的200个PC内。沿着我们观测到的最亮的光源，即五重天体GCS 3-2的视线，大约有四分之三的H沿着位于CMZ中，其余的H位于中间的旋臂中。CMZ中大约一半的H的速度接近-100 km s-1，表明它与半径为180 pc的膨胀分子环有关，该分子环大致形成CMZ的外边界。另一半，速度约为-50和~0 km s-1，可能更接近中心。CO在这些云中并不是很丰富。在银河系盘中的几个致密云和弥漫云中没有检测到（3，3）能级的热扩散气体，而在这些云中，先前已经报道了较大的冷H柱密度。因此，新发现的环境似乎是CMZ所独有的。在CMZ中观测到的大的H柱密度表明电离率比银河系盘中的弥漫星际介质高得多。我们发现CMZ中的H几乎完全在弥散云中，这表明CMZ中n ≥ 104 cm-3云的体积填充因子（f ≥ 0.1）被高估了至少一个数量级。
Using an absorption line from the metastable (J, K) = (3, 3) level of H together with other lines of H and CO observed along several sight lines, we have discovered a vast amount of high-temperature (T ~ 250 K) and low-density (n ~ 100 cm-3) gas with a large velocity dispersion in the central molecular zone (CMZ) of the Galaxy, i.e., within 200 pc of the center. Approximately three-fourths of the H along the line of sight to the brightest source we observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the remaining H located in intervening spiral arms. About half of the H in the CMZ has velocities near ~-100 km s-1, indicating that it is associated with the 180 pc radius expanding molecular ring, which approximately forms the outer boundary of the CMZ. The other half, with velocities of ~-50 and ~0 km s-1, is probably closer to the center. CO is not very abundant in these clouds. Hot and diffuse gas in which the (3, 3) level is populated was not detected toward several dense clouds and diffuse clouds in the Galactic disk where large column densities of colder H have been reported previously. Thus, the newly discovered environment appears to be unique to the CMZ. The large observed H column densities in the CMZ suggest an ionization rate much higher than in the diffuse interstellar medium in the Galactic disk. Our finding that the H in the CMZ is almost entirely in diffuse clouds indicates that the reported volume filling factor (f ≥ 0.1) for n ≥ 104 cm-3 clouds in the CMZ is an overestimate by at least an order of magnitude.