Thermal instability of halo gas heated by streaming cosmic rays
Thermal instability of halo gas heated by streaming cosmic rays
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DOI：
10.1093/mnras/staa385
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发表时间：
2019-08
影响因子：
4.8
通讯作者：
Philipp Kempski;E. Quataert
中科院分区：
文献类型：
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作者：
Philipp Kempski;E. Quataert
流动的宇宙射线（CR）对维里化气体的加热可能在星系晕、星系团和星系团中具有重要的能量作用。我们对流式 CR 加热的等离子体进行了线性热稳定性分析。我们分别处理有和没有背景梯度、有和没有重力的平衡。我们包括沿磁场方向的 CR 流动和扩散。热稳定性很大程度上取决于 CR 压力与气体压力的比率，这决定了模式是等压还是等容。 $\boldsymbol {k \cdot B }\ne 0$ 的模式受到 CR 扩散的强烈影响。当流动时间短于 CR 扩散时间时，热不稳定模式（$\boldsymbol {k \cdot B }\ne 0$）是以 ∝ Alfvén 速度传播的波。光电离平衡中的晕气体是热稳定的，与 CR 压力无关，而碰撞电离平衡中的气体对于物理实际参数来说是不稳定的。在重力分层等离子体中，在 CR 流存在的情况下，热过稳定模式的振荡频率可能高于浮力/自由落体频率。这可能会改变多相气体存在的临界 tcool/tff。分层 CR 加热介质的对流不稳定性标准可以写成熟悉的史瓦西形式 dseff/dz < 0，其中 seff 是涉及气体和 CR 压力的有效熵。我们讨论了我们的研究结果对星系晕、星系团和星系团的热演化和多相结构的影响。
Heating of virialized gas by streaming cosmic rays (CRs) may be energetically important in galaxy haloes, groups, and clusters. We present a linear thermal stability analysis of plasmas heated by streaming CRs. We separately treat equilibria with and without background gradients, and with and without gravity. We include both CR streaming and diffusion along the magnetic-field direction. Thermal stability depends strongly on the ratio of CR pressure to gas pressure, which determines whether modes are isobaric or isochoric. Modes with $\boldsymbol {k \cdot B }\ne 0$ are strongly affected by CR diffusion. When the streaming time is shorter than the CR diffusion time, thermally unstable modes (with $\boldsymbol {k \cdot B }\ne 0$) are waves propagating at a speed ∝ the Alfvén speed. Halo gas in photoionization equilibrium is thermally stable independent of CR pressure, while gas in collisional ionization equilibrium is unstable for physically realistic parameters. In gravitationally stratified plasmas, the oscillation frequency of thermally overstable modes can be higher in the presence of CR streaming than the buoyancy/free-fall frequency. This may modify the critical tcool/tff at which multiphase gas is present. The criterion for convective instability of a stratified, CR-heated medium can be written in the familiar Schwarzschild form dseff/dz < 0, where seff is an effective entropy involving the gas and CR pressures. We discuss the implications of our results for the thermal evolution and multiphase structure of galaxy haloes, groups, and clusters.