星系晕中的气体凝聚
Gas Condensation in the Galactic Halo
摘要：
使用自适应网格细化（AMR）的垂直分层热晕气体的流体动力学模拟，我们研究的条件下，云可以形成和冷凝出的热晕介质潜在的燃料恒星形成的气体盘。我们发现晕云不是由线性等压扰动发展而来的。这是冷却时间长于Brunt-Vaisala时间的状态，证实了先前的线性分析。我们扩展到非线性制度的分析，考虑轻度或强烈的非线性扰动与overdensities高达100，也改变了初始高度，云的大小，和金属的气体。在这里，结果取决于冷却时间与将云加速到声速所需时间的比率（类似于动力学时间）。如果这个比值超过一个临界值，云就会加速而不进一步冷却，并被开尔文-亥姆霍兹和/或瑞利-泰勒不稳定性破坏。如果它小于临界值，云在破裂之前冷却并凝结。超密度为10-20的吸积气体预计是边缘不稳定的;冷却分数将取决于金属丰度，传入云的大小以及到星系的距离。冷气流中局部增强的高密度有更大的冷却可能性。我们的研究结果的演变云播种冷吸积，几乎没有解决目前的宇宙学流体动力学模拟和星系晕中检测到的吸收线系统的影响。
英文摘要：
Using adaptive mesh refinement (AMR) hydrodynamic simulations of vertically stratified hot halo gas, we examine the conditions under which clouds can form and condense out of the hot halo medium to potentially fuel star formation in the gaseous disk. We find that halo clouds do not develop from linear isobaric perturbations. This is a regime where the cooling time is longer than the Brunt-Vaisala time, confirming previous linear analysis. We extend the analysis into the nonlinear regime by considering mildly or strongly nonlinear perturbations with overdensities up to 100, also varying the initial height, the cloud size, and the metallicity of the gas. Here, the result depends on the ratio of cooling time to the time required to accelerate the cloud to the sound speed (similar to the dynamical time). If the ratio exceeds a critical value near unity, the cloud is accelerated without further cooling and gets disrupted by Kelvin-Helmholtz and/or Rayleigh-Taylor instabilities. If it is less than the critical value, the cloud cools and condenses before disruption. Accreting gas with overdensities of 10-20 is expected to be marginally unstable; the cooling fraction will depend on the metallicity, the size of the incoming cloud, and the distance to the galaxy. Locally enhanced overdensities within cold streams have a higher likelihood of cooling out. Our results have implications on the evolution of clouds seeded by cold accretion that are barely resolved in current cosmological hydrodynamic simulations and absorption line systems detected in galaxy halos.
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