X射线冷核星系团研究进展

Developments in Studies of X-ray Cool-core Galaxy Clusters

冷核是指在一部分X射线星系团中出现的低温高亮的核状结构。冷核星系团在X射线表面亮度轮廓、温度轮廓、中心冷却时间、质量沉积率、熵轮廓等多个方面具有与非冷核星系团截然不同的观测特征。星系团冷核的发现距今已有40年,人们对冷核的认识经历了由冷流模型到稳定冷核模型的转变。但是直到今天,冷核的产生机制和维持条件仍未完全弄清。研究冷核对于理解活动星系核产能机制、星系际元素增丰过程、星系团的形成与演化、大尺度结构形成等众多领域具有非常重要的意义。

Cool cores are common structures in the center of X-ray luminous galaxy clusters. Present observations indicate that the fraction of cool-core clusters is about 50% or even larger. The dense intra-cluster medium in cool cores emits very strong X-ray radiation through bremsstrahlung, thus forms a sharply peaked X-ray surface brightness distribution, while the temperature is lower than the virial temperature. The energy loss is so fast that the cooling time of a cool core can be shorter than the age of the cluster. These phenomenons always indicate the existence of a cool core. However, so far a clear definition of a cool-core cluster is not established. Parameters such as the surface brightness concentration, the central cooling time, and the mass deposition rates all work for the low redshift clusters, but their reliabilities in high redshifts are not verified. As the gas is cooling, the mass deposition rates are not as large as predicted by the classical cooling flow theory. There is some heating mechanism that prevents the gas from cooling down to form stars, which results in fails to detect the star formation and the molec- ular clouds in optical and infrared bands. Feedback heating by central active galactic nuclei （AGN） is one of the most promising heat sources, however, the specific heating mechanism is not well understood. Other heat sources （e.g., conduction, merger shock, turbulence） may also be important. They can significantly reduce the demands on AGN heating. In this paper, we present a detailed description of the observing features of cool-core clusters, such as the X-ray surface brightness distributions, the temperature profiles, the cooling time profiles, the mass deposition rates, and the entropy profiles. We also summarize several main cool-core theoretical heating models with and without AGN feedback. Present research topics and future directions are discussed in the end.