The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of “CO-dark” Gas in the Perseus Arm
The Structure of Dark Molecular Gas in the Galaxy. II. Physical State of “CO-dark” Gas in the Perseus Arm
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DOI：
10.3847/1538-4357/ab3a4b
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发表时间：
2019-08
期刊：
影响因子：
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通讯作者：
Michael P. Busch;R. Allen;P. Engelke;D. E. Hogg;D. Neufeld;M. Wolfire
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文献类型：
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作者：
Michael P. Busch;R. Allen;P. Engelke;D. E. Hogg;D. Neufeld;M. Wolfire
我们报告了一项新的高灵敏度（ΔTmb ~ 3 mK）调查的结果，该调查是在1665和1667 MHz下，在l = 105方向覆盖1°× 1°天空的密集9 × 9像素网格上进行的。°00,b = +2。朝向我们银河系英仙座旋臂的50度。我们将我们的Green Bank望远镜1667 MHz的OH结果与五所大学射电天文台外星系巡天在英仙座臂的速度范围内的12CO（1-0）观测结果进行了比较。在我们调查区域的81个统计上独立的点中，86%显示可检测到1667 MHz的OH发射，其中19%显示可检测到CO发射。我们利用一组弥漫分子云模型探讨了观测到的特征可能的物理条件。在这些模型的背景下，OH和CO在低于Av = 0.2的电流灵敏度限制下都消失了，但是CO的排放直到体积密度超过100-200才出现。这些结果表明，低柱密度Av和低体积密度nH的组合可以解释沿显示OH发射的视线缺乏CO发射。18厘米的OH主线，其低临界密度为n* ~ 1，在静止星系环境的很大一部分上被碰撞激发，对于足够灵敏度的观测，为漫射H2提供了光学薄的射电示踪剂。
We report the results from a new, highly sensitive (ΔTmb ∼ 3 mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 × 9 pixel grid covering a 1° × 1° patch of sky in the direction of l = 105.°00, b = +2.°50 toward the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope 1667 MHz OH results with archival 12CO(1–0) observations from the Five College Radio Astronomy Observatory Outer Galaxy Survey within the velocity range of the Perseus Arm at these galactic coordinates. Out of the 81 statistically independent pointings in our survey area, 86% show detectable OH emission at 1667 MHz, and 19% of them show detectable CO emission. We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models. In the context of these models, both OH and CO disappear at current sensitivity limits below an Av of 0.2, but the CO emission does not appear until the volume density exceeds 100–200 . These results demonstrate that a combination of low column density Av and low volume density nH can explain the lack of CO emission along sight lines exhibiting OH emission. The 18 cm OH main lines, with their low critical density of n* ∼ 1 , are collisionally excited over a large fraction of the quiescent galactic environment and, for observations of sufficient sensitivity, provide an optically thin radio tracer for diffuse H2.