A new buoyancy instability in galaxy clusters due to streaming cosmic rays
A new buoyancy instability in galaxy clusters due to streaming cosmic rays
复制标题
由于宇宙射线流导致星系团中新的浮力不稳定性
DOI：
10.1093/mnras/stad1744
复制
发表时间：
2023
影响因子：
4.8
通讯作者：
Squire, Jonathan
中科院分区：
文献类型：
--
作者：
Kempski, Philipp;Quataert, Eliot;Squire, Jonathan
活动星系核（AGN）被认为可以提供防止星系团核心气体失控冷却的能量。然而，这种能量如何在整个簇内介质（ICM）中传输和热化仍不清楚。在最近的工作中，我们表明流动的宇宙射线 (CR) 会破坏稀 ICM 等离子体中的声波。在这里，我们表明，在重力存在的情况下 CR 流动也会破坏压力平衡波的稳定性。我们将这种新的不稳定性称为 CR 浮力不稳定性 (CRBI)。与没有 CR 的标准结果形成鲜明对比的是，由于 CR 流，压力平衡模式在短波长下具有高度可压缩性。最大增长率的量级为 (pc/pg)β1/2ωff，其中 pc/pg 是 CR 压力与热气体压力的比率，β 是热压力与磁压力的比率，ωffi 是自由落体频率。 CRBI 与背景热通量驱动的浮力不稳定性一起运行，即热通量驱动的浮力不稳定性 (HBI) 和磁热不稳定性 (MTI)。当热平均自由程lmfpi<气体尺度高度H时，HBI/MTI设定大尺度的增长率，而CRBI设定小尺度的增长率。相反，当lmfp∼Hand (pc/pg)β1/2≳ 1时，即使在大尺度上，CRBI增长率也超过HBI/MTI增长率。我们的结果表明，CR 驱动的不稳定性可能是在星系团中观察到的声波/弱冲击和湍流的部分原因。无线电气泡附近产生的 CR 驱动的不稳定性也可能在整个星团中重新分配 AGN 能量方面发挥重要作用。
Active Galactic Nuclei (AGN) are believed to provide the energy that prevents runaway cooling of gas in the cores of galaxy clusters. However, how this energy is transported and thermalized throughout the Intracluster Medium (ICM) remains unclear. In recent work, we showed that streaming cosmic rays (CRs) destabilize sound waves in dilute ICM plasmas. Here, we show that CR streaming in the presence of gravity also destabilizes a pressure-balanced wave. We term this new instability the CR buoyancy instability (CRBI). In stark contrast to standard results without CRs, the pressure-balanced mode is highly compressible at short wavelengths due to CR streaming. Maximal growth rates are of order (pc/pg)β1/2ωff, wherepc/pgis the ratio of CR pressure to thermal gas pressure, β is the ratio of thermal to magnetic pressure, and ωffis the free-fall frequency. The CRBI operates alongside buoyancy instabilities driven by background heat fluxes, i.e. the heat-flux-driven buoyancy instability (HBI) and the magneto-thermal instability (MTI). When the thermal mean free pathlmfpis ≪ the gas scale heightH, the HBI/MTI set the growth rate on large scales, while the CRBI sets the growth rate on small scales. Conversely, whenlmfp∼Hand (pc/pg)β1/2≳ 1, CRBI growth rates exceed HBI/MTI growth rates even on large scales. Our results suggest that CR-driven instabilities may be partially responsible for the sound waves/weak shocks and turbulence observed in galaxy clusters. CR-driven instabilities generated near radio bubbles may also play an important role redistributing AGN energy throughout clusters.