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高金属度-高柱密度阻尼Lyman α系统的不存在：两相星际介质中的分子形成

On the Absence of High Metallicity-High Column Density Damped Lyman Alpha Systems: Molecule Formation in a Two-Phase Interstellar Medium

Mark R. Krumholz, Sara L. Ellison, J. Xavier Prochaska, Jason Tumlinson

备注：
5 pages, 2 figures, emulateapj format, accepted to ApJL. Minor changes to text from previous version
发表信息：
Astrophys.J.701:L12-L15,2009
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摘要：

我们认为，缺乏观察到的阻尼莱曼α（DLA）系统，同时具有高HI列密度和高金属性的结果自然从形成的分子在冷相的两相原子介质中的压力平衡。我们的结果同样适用于扩散系统中的紫外线辐射场主要是由河外背景和密集的恒星形成的，其中本地辐射场可能是数量级更高。我们指出，需要这样的辐射不敏感模型来解释使用伽马射线暴余辉观察到的DLA中不存在高柱-高金属丰度系统，因为这些系统可能会受到伽玛射线暴宿主星系中活跃恒星形成产生的强辐射场的影响。此外，我们发现，在DLAs中的最大原子气柱之间的关系设置了一个坚定的上限，在这些系统中，可以在温暖的，扩散相的质量分数。最后，我们认为，我们的结果解释了观察到的缺乏原位恒星形成DLA系统。

英文摘要：

We argue that the lack of observed damped Lyman alpha (DLA) systems that simultaneously have high HI columns densities and high metallicities results naturally from the formation of molecules in the cold phase of a two-phase atomic medium in pressure balance. Our result applies equally well in diffuse systems where the ultraviolet radiation field is dominated by the extragalactic background and in dense star-forming ones, where the local radiation field is likely to be orders of magnitude higher. We point out that such a radiation-insensitive model is required to explain the absence of high column - high metallicity systems among DLAs observed using gamma-ray burst afterglows, since these are likely subjected to strong radiation fields created by active star formation in the GRB host galaxy. Moreover, we show that the observed relationship between the maximum atomic gas column in DLAs sets a firm upper limit on the fraction of the mass in these systems that can be in the warm, diffuse phase. Finally, we argue that our result explains the observed lack of in situ star formation in DLA systems.

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