冷却核心星系团中的异常轮廓：过去活动星系核驱动对流的化石记录？
Abundance profiles in cooling-core clusters: a fossil record of past AGN-driven convection?
摘要：
团内等离子体铁丰度的中心峰值是冷却核心星系团的共同特征。这些丰度峰值比产生过量铁的中央最亮星系团（BCG）的恒星具有更宽的轮廓，表明富含金属的等离子体通过某些过程（如湍流扩散）被输送出BCG。与此同时，冷却核心星系团很可能通过喷流、宇宙射线泡和对流被中央活动星系核（AGN）加热。最近的AGN-driven对流模型的Reichran和Rasera预测的湍流速度分布在一个稳定状态的集群中，辐射冷却平衡加热从AGN-driven对流和热传导的组合。我们使用的速度分布作为输入到一个平流/扩散模型中的金属在集群内介质中的运输，采取铁被注入的BCG。我们比较了我们的模型的结果，XMM和钱德拉八个集群的观测。假设一个恒定的湍流水平在一个集群的生命周期，在模型中的湍流速度可以解释所观察到的丰度分布在八个集群中只有五个。然而，我们继续发展一个解析拟合的湍流速度剖面作为AGN功率的函数。然后，我们推导出每个集群的平均AGN功率（在过去的10 Gyr）所需的丰度配置文件相匹配。所需的平均值在10^43和2.10^44 erg/s之间，而目前活动星系核的功率范围要大得多，从6.10^41 erg/s（室女座）到2.10^44 erg/s（长蛇座A）。我们的研究结果表明，活动星系核驱动的对流可以解释所观察到的丰度分布，如果活动星系核的功率在一个集群的寿命之间的室女座和Hydra-A-like值，平均值在上述范围内变化。
英文摘要：
Central peaks in the iron abundance of intracluster plasma are a common feature of cooling-core galaxy clusters. These abundance peaks have a much broader profile than the stars of the central brightest cluster galaxy (BCG), which produce the excess iron, indicating that metal-enriched plasma is transported out of the BCG by some process such as turbulent diffusion. At the same time, cooling-core clusters are likely heated by central active galactic nuclei (AGNs) by means of jets, cosmic-ray bubbles, and convection. The recent AGN-driven convection model of Chandran&Rasera predicts the turbulent velocity profile in a steady-state cluster in which radiative cooling is balanced by heating from a combination of AGN-driven convection and thermal conduction. We use the velocity profiles as input into an advection/diffusion model for the transport of metals in the intracluster medium, taking the iron to be injected by the BCG. We compare the results of our model to XMM and Chandra observations of eight clusters. Assuming a constant turbulence level over a cluster's lifetime, the turbulent velocities in the model can explain the observed abundance profiles in only five of the eight clusters. However, we go on to develop an analytic fit of the turbulent velocity profile as a function of the AGN power. We then deduce for each cluster the average AGN power (during the past 10 Gyr) required to match the abundance profiles. The required average values are between 10^43 and 2.10^44 erg/s, while the present AGN powers span a much larger range from 6.10^41 (Virgo) to 2.10^44 erg/s (Hydra A). Our results suggest that AGN-driven convection can account for the observed abundance profiles if the AGN power varies over a cluster's lifetime between Virgo-like and Hydra-A-like values, with average values in the above-quoted range.
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