A Role for Turbulence in Circumgalactic Precipitation
A Role for Turbulence in Circumgalactic Precipitation
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DOI：
10.3847/1538-4357/aae8e2
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发表时间：
2018-03
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通讯作者：
G. Voit
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作者：
G. Voit
在任何半径范围内，弥漫在大质量星系中的热环境介质的冷却时间tcool都不会比自由落体时间tff的10倍低太多。理论模型已经解释了这一发现，假设当tcool/tff> 10时，冷云开始从周围介质中凝结出来，并引发强烈的黑洞反馈响应，但他们还没有提供一个简单的解释临界tcool/tff比。本文探索了一个将临界比与重力波振荡的湍流驱动联系起来的冷凝的启发式模型。在线性区域，重力内波在热平衡介质中是热不稳定的。因此，在tcool/tff = 1的平衡介质中，浮力振荡会增长，直到饱和，而不会在取决于tcool/tff的振幅下凝聚。然而，在一个10 tcool/tff 20的介质中，速度色散大约是星系恒星速度色散一半的湍流可以驱动这些振荡凝结。有趣的是，这确实是在多相星系团核心之间观察到的气相速度分散。因此，环境气体冷凝的临界tcool/tff比和该气体中的湍流水平可能由冷凝、反馈和湍流之间的耦合确定。如果反馈能量中进入湍流的部分是次主导的，这样的系统可以收敛到一个调节良好的平衡状态。
The cooling time, tcool, of the hot ambient medium pervading a massive galaxy does not drop much below 10 times the freefall time tff at any radius. Theoretical models have accounted for this finding by hypothesizing that cold clouds start to condense out of the ambient medium when tcool/tff ≲ 10 and fuel a strong black hole feedback response, but they have not yet provided a simple explanation for the critical tcool/tff ratio. This paper explores a heuristic model for condensation linking the critical ratio to turbulent driving of gravity wave oscillations. In the linear regime, internal gravity waves are thermally unstable in a thermally balanced medium. Buoyancy oscillations in a balanced medium with tcool/tff ≫ 1 therefore grow until they saturate without condensing at an amplitude depending on tcool/tff. However, in a medium with 10 ≲ tcool/tff ≲ 20, turbulence with a velocity dispersion roughly half the galaxy’s stellar velocity dispersion can drive those oscillations into condensation. Intriguingly, this is indeed the gas-phase velocity dispersion observed among multiphase galaxy cluster cores. It is therefore possible that both the critical tcool/tff ratio for condensation of ambient gas and the level of turbulence in that gas are determined by coupling between condensation, feedback, and turbulence. Such a system can converge to a well-regulated equilibrium state, if the fraction of feedback energy going into turbulence is subdominant.