A Far Ultraviolet Spectroscopic Explorer Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo
A Far Ultraviolet Spectroscopic Explorer Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo
复制标题
银河晕中速云中分子氢的远紫外光谱探测器调查
DOI：
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发表时间：
2002
期刊：
影响因子：
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通讯作者：
K. Sembach
中科院分区：
文献类型：
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作者：
P. Richter;B. Wakker;B. Savage;K. Sembach
远紫外光谱探测器（FUSE）的数据用于研究银河系下晕中速云（IVCs）的分子氢（H2）含量。我们分析了56个（主要是河外）背景源的星际吸收，以研究61个IVC组分在H柱密度≥1019 cm-2时Lyman和Werner波段的H2吸收。对于具有良好信噪比（S/N）（~9 /分辨率单元或更高）的数据，在~1014 ~ ~1017 cm-2的柱密度范围内，有14例IVC气体中的H2被令人信服地检测到。我们发现在较低信噪比的FUSE光谱中，IVCs中还有17种可能的H2检测。氢分子分数f在10-6和10-3之间变化，这意味着稠密的，大部分是中性气相，可能与这些云中的冷中性介质（CNM）有关。如果H2保持在形成-解离平衡状态，这些云中的CNM可以被表征为致密（D ~ 0.1 pc）的细丝，其体积密度约为nH ~ 30 cm-3。IVC气体中H2的检出率相对较高，表明这些云中的CNM普遍存在。可能存在具有更高分子分数的更密集的区域，但由于它们的体积小，在吸收中很难检测到它们。
Far Ultraviolet Spectroscopic Explorer (FUSE) data are used to investigate the molecular hydrogen (H2) content of intermediate-velocity clouds (IVCs) in the lower halo of the Milky Way. We analyze interstellar absorption toward 56 (mostly extragalactic) background sources to study H2 absorption in the Lyman and Werner bands in 61 IVC components at H I column densities ≥1019 cm-2. For data with good signal-to-noise ratio (S/N) (~9 per resolution element and higher), H2 in IVC gas is convincingly detected in 14 cases at column densities varying between ~1014 and ~1017 cm-2. We find an additional 17 possible H2 detections in IVCs in FUSE spectra with lower S/N. The molecular hydrogen fractions, f, vary between 10-6 and 10-3, implying a dense, mostly neutral gas phase that is probably related to the cold neutral medium (CNM) in these clouds. If the H2 stays in formation-dissociation equilibrium, the CNM in these clouds can be characterized by compact (D ~ 0.1 pc) filaments with volume densities on the order of nH ~ 30 cm-3. The relatively high detection rate of H2 in IVC gas implies that the CNM in these clouds is ubiquitous. More dense regions with much higher molecular fractions may exist, but it would be difficult to detect them in absorption because of their small size.