理想星系团核热不稳定性的增长和演化
Growth and Evolution of Thermal Instabilities in Idealized Galaxy-Cluster Cores
摘要：
热量输入大致平衡了星系团气体核心的辐射冷却，即使中心冷却时间很短，这意味着冷却触发了一个保持热平衡的反馈回路。此外，冷却时间短的核心往往具有多相结构，这表明当冷却时间< 1 Gyr时，团内介质（ICM）变得局部热不稳定。在这项工作中，我们使用2D和3D流体动力学模拟研究的凝聚在理想的星系团核心的开始。特别是，我们看看如何冷凝过程取决于冷却时间的比率，自由落体时间和引力势的几何形状。我们的结论是，ICM总是可以发展到一个状态，其中发生凝聚，如果有足够的时间，但初始时标比tcool /tff < 10是需要的热不稳定性增长足够快，影响现实的集群核心在一个时间尺度内，是相关的宇宙学结构的形成。我们发现，不稳定导致对流和扰动继续增长，而气体对流。当微扰分布的低熵尾部的时间尺度比低于tcool /tff < 3时，即使体积平均时间尺度比大得多，也会发生凝聚。在我们的模拟中，引力势的几何形状对热稳定性没有很大的影响。最后，我们发现，如果凝结是能量反馈，那么将凝结物质转化为热能的转换效率约为10^-3，就足以维持ICM中的热平衡。
英文摘要：
Heat input roughly balances radiative cooling in the gaseous cores of galaxy clusters even when the central cooling time is short, implying that cooling triggers a feedback loop that maintains thermal balance. Furthermore, cores with short cooling times tend to have multiphase structure, suggesting that the intracluster medium (ICM) becomes locally thermally unstable for cooling times < 1 Gyr. In this work, we use 2D and 3D hydrodynamic simulations to study the onset of condensation in idealized galaxy-cluster cores. In particular, we look at how the condensation process depends on the ratio of cooling time to freefall time and on the geometry of the gravitational potential. We conclude that the ICM can always evolve to a state in which condensation occurs if given enough time, but that an initial timescale ratio tcool /tff < 10 is needed for thermal instability to grow quickly enough to affect realistic cluster cores within a timescale that is relevant for cosmological structure formation. We find that instability leads to convection and that perturbations continue to grow while the gas convects. Condensation occurs when the timescale ratio in the low-entropy tail of the perturbation distribution drops below tcool /tff < 3, even if the volume-averaged timescale ratio is substantially greater. In our simulations, the geometry of the gravitational potential does not have a strong effect on thermal stability. Finally, we find that if condensation is powering feedback, a conversion efficiency of around 10^-3 for converting the condensed mass into thermal energy is sufficient to maintain thermal balance in the ICM.
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