各向异性粘性对星系团活动星系核泡演化的影响
Effects of Anisotropic Viscosity on the Evolution of Active Galactic Nuclei Bubbles in Galaxy Clusters
摘要：
活动星系核喷流与星系团内介质的相互作用是制约超大质量黑洞对星系团内介质的反馈和供给的关键因素。关于活动星系核反馈的许多认识是从纯流体动力学模型中获得的，然而，这种近似是否适用于磁化的、弱碰撞的ICM需要严格地检验。例如，在流体动力学模拟中，活动星系核吹出的气泡很容易被流体不稳定性破坏，这使得很难解释观测到的气泡（如英仙座西北部的幽灵气泡）的一致性。为了研究磁张力和粘性在现实条件下是否可以保持气泡的完整性，我们进行了第一次Braginskii磁流体动力学模拟射流膨胀气泡在介质中与纠缠的磁场。我们发现，单独的磁张力是不够的，以防止气泡变形，由于大的速度剪切在早期阶段的演变。尽管在缠结磁场中未抑制的各向异性粘度可以具有与各向同性粘度类似的效果，但是当压力各向异性受到微观等离子体不稳定性的限制时，粘度的水平基本上受到限制，从而不能防止气泡变形，如在无粘性情况下。我们的研究结果表明，Braginskii粘度是不太可能的主要机制，抑制AGN气泡的流体不稳定性，它仍然是一个具有挑战性的任务，重现所观察到的平滑和连贯的气泡。由于ICM的动力学影响和加热严重依赖于气泡的形态，我们的研究突出了“微观物理”对AGN反馈过程的宏观性质的基本作用。
英文摘要：
The interaction between jets from active galactic nuclei (AGNs) and the intracluster medium (ICM) provides key constraints on the feeding and feedback of supermassive black holes. Much understanding about AGN feedback is gained from purely hydrodynamic models; however, whether such an approximation is adequate for the magnetized, weakly collisional ICM needs to be critically examined. For example, AGN-blown bubbles in hydrodynamic simulations are easily disrupted by fluid instabilities, making it difficult to explain the coherence of observed bubbles such as the northwest ghost bubble in Perseus. In order to investigate whether magnetic tension and viscosity in realistic conditions could preserve the bubble integrity, we performed the first Braginskii-magnetohydrodynamic simulations of jet-inflated bubbles in a medium with tangled magnetic field. We find that magnetic tension alone is insufficient to prevent bubble deformation due to large velocity shear at early stage of the evolution. Although unsuppressed anisotropic viscosity in tangled magnetic field can have similar effects as isotropic viscosity, when the pressure anisotropy is bounded by microscopic plasma instabilities, the level of viscosity is substantially limited, thereby failing to prevent bubble deformation as in the inviscid case. Our results suggest that Braginskii viscosity is unlikely to be the primary mechanism for suppressing the fluid instabilities for AGN bubbles, and it remains a challenging task to reproduce smooth and coherent bubbles as observed. Because the dynamical influence and heating of the ICM critically depend on the bubble morphology, our study highlights the fundamental role of "microphysics" on the macroscopic properties of AGN feedback processes.
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