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arXiv 1402.6850astro-ph.GA
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从原子到分子中速云的动力学转变

A dynamical transition from atomic to molecular intermediate-velocity clouds

Tobias Röhser, Jürgen Kerp, Benjamin Winkel, Francois Boulanger, Guilaine Lagache

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13 pages, 14 figures, accepted for publication by A&A
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摘要：

在高银纬天空，远红外强度与氢柱密度密切相关，氢柱密度由原子氢和分子氢组成。超过一定的柱密度阈值，原子氢变成分子。我们分析了低层星系晕中的中速云（IVC）的气体和尘埃性质，以探索它们从原子到分子阶段的转变。通过观察，我们研究了将纯原子IVC转化为分子IVC的物理过程。将Effelsberg-Bonn HI-Survey（EBHIS）的数据与Planck卫星和IRIS的FIR波段相关联。修正黑体发射光谱拟合推断尘埃光学厚度和晶粒温度。我们去除了原子氢对FIR强度的贡献，以估计分子氢柱密度。两个IVC显示出不同的FIR特性，尽管它们在HI方面相似，例如窄的谱线和大的柱密度。一个FIR明亮IVC与H $_{2}$相关，由$^{12}$ CO $(1\rightarrow0)$发射证实;另一个IVC是FIR暗淡的，没有显示FIR过量，这表明不存在分子氢。我们建议，FIR昏暗和明亮的IVC探测原子和分子气相之间的转变。由动力学过程触发，这种转变发生在IVC下降到星系盘的过程中。最自然的驱动力是由不断增加的晕密度施加在云上的冲压压力。由于压力增加，H $_{2}$的形成时间尺度减小，从而允许在几百万年内形成大量的H $_{2}$。

英文摘要：

Towards the high galactic latitude sky, the far-infrared (FIR) intensity is tightly correlated to the total hydrogen column density which is made up of atomic (HI) and molecular hydrogen (H$_{2})$. Above a certain column density threshold, atomic hydrogen turns molecular. We analyse gas and dust properties of intermediate-velocity clouds (IVCs) in the lower galactic halo to explore their transition from the atomic to the molecular phase. Driven by observations, we investigate the physical processes that transform a purely atomic IVC into a molecular one. Data from the Effelsberg-Bonn HI-Survey (EBHIS) are correlated to FIR wavebands of the Planck satellite and IRIS. Modified black-body emission spectra are fitted to deduce dust optical depths and grain temperatures. We remove the contribution of atomic hydrogen to the FIR intensity to estimate molecular hydrogen column densities. Two IVCs show different FIR properties, despite their similarity in HI, such as narrow spectral lines and large column densities. One FIR bright IVC is associated with H$_{2}$, confirmed by $^{12}$CO $(1\rightarrow0)$ emission; the other IVC is FIR dim and shows no FIR excess, which indicates the absence of molecular hydrogen. We propose that the FIR dim and bright IVCs probe the transition between the atomic and molecular gas phase. Triggered by dynamical processes, this transition happens during the descent of IVCs onto the galactic disk. The most natural driver is ram pressure exerted onto the cloud by the increasing halo density. Because of the enhanced pressure, the formation timescale of H$_{2}$ is reduced, allowing the formation of large amounts of H$_{2}$ within a few Myr.

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