arXiv 1504.022152015-04astro-ph.GA

冷堆芯循环：簇堆芯中的冷气和AGN射流反馈

Cool core cycles: Cold gas and AGN jet feedback in cluster cores

Deovrat Prasad, Prateek Sharma, Arif Babul

备注：: 22 pages, 15 figures; ApJ accepted version (figures downgraded to smaller size, as required for arxiv submission)
DOI：: 10.1088/0004-637X/811/2/108

摘要：

使用球形几何中的高分辨率3-D和2-D（轴对称）流体动力学模拟，我们研究了由反馈驱动的双极活动星系核（AGN）喷流加热的冷星系团核心的演化。活动星系核的反馈需要冷气体的凝结和随之而来的增强的吸积来平衡辐射冷却和合理的效率，并与观测到的冷核特性相匹配。与纯冷却流相比，小至$5 \times 10^{-5}$的反馈效率（机械光度$\approx ε\dot{M}_{\rm acc} c^2$ ;其中$\dot{M}_{\rm acc}$是1 kpc时的质量吸积速率）足以将冷却/吸积速率降低$\sim 10$。这个值比之前考虑的值要小，并且与喷流效率和只有一小部分1 kpc的气体被吸积到超大质量黑洞（SMBH）上的事实相一致。我们发现在我们所有的模拟与冷模式反馈的滞后循环：{ \em冷凝}的冷气体时，冷却时间的比例的自由落体时间（$t_{\rm cool}/t_{\rm ff}$）是$\lesssim 10$导致吸积速率的突然增强;大的吸积速率导致强烈的喷流和热ICM的{ \em过热}，使得$t_{\rm cool}/t_{\rm ff} > 10$ ;冷气体的进一步冷凝被抑制，并且吸积速率下降，导致核心的缓慢冷却和冷气体的冷凝，重新开始循环。因此，对于相同的核心熵值或$t_{\rm cool}/t_{\rm ff}$，核心特性，如喷流功率、吸积率，会有一个扩展。对于更高的效率和更低的质量晕，观察到更少的循环次数，因为核心过热到更长的冷却时间。三维模拟结果表明，形成了几kpc规模，旋转支持，大规模（$\sim 10^{11} M_\odot$）冷气体环。（摘要删节）

英文摘要：

Using high-resolution 3-D and 2-D (axisymmetric) hydrodynamic simulations in spherical geometry, we study the evolution of cool cluster cores heated by feedback-driven bipolar active galactic nuclei (AGN) jets. Condensation of cold gas, and the consequent enhanced accretion, is required for AGN feedback to balance radiative cooling with reasonable efficiencies, and to match the observed cool core properties. A feedback efficiency (mechanical luminosity $\approx ε\dot{M}_{\rm acc} c^2$; where $\dot{M}_{\rm acc}$ is the mass accretion rate at 1 kpc) as small as $5 \times 10^{-5}$ is sufficient to reduce the cooling/accretion rate by $\sim 10$ compared to a pure cooling flow. This value is smaller compared to the ones considered earlier, and is consistent with the jet efficiency and the fact that only a small fraction of gas at 1 kpc is accreted on to the supermassive black hole (SMBH). We find hysteresis cycles in all our simulations with cold mode feedback: {\em condensation} of cold gas when the ratio of the cooling-time to the free-fall time ($t_{\rm cool}/t_{\rm ff}$) is $\lesssim 10$ leads to a sudden enhancement in the accretion rate; a large accretion rate causes strong jets and {\em overheating} of the hot ICM such that $t_{\rm cool}/t_{\rm ff} > 10$; further condensation of cold gas is suppressed and the accretion rate falls, leading to slow cooling of the core and condensation of cold gas, restarting the cycle. Therefore, there is a spread in core properties, such as the jet power, accretion rate, for the same value of core entropy or $t_{\rm cool}/t_{\rm ff}$. A fewer number of cycles are observed for higher efficiencies and for lower mass halos because the core is overheated to a longer cooling time. The 3-D simulations show the formation of a few-kpc scale, rotationally-supported, massive ($\sim 10^{11} M_\odot$) cold gas torus. (abstract abridged)

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