(Abridged) We examine the radial entropy distribution and its scaling using 31 nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey (REXCESS). The entropy profiles are robustly measured at least out to R_1000 in all systems and out to R_500 in 13 systems. Compared to theoretical expectations, the observed distributions show a radial and mass-dependent excess entropy that is greater and extends to larger radii in lower mass systems. At R_500, the mass dependence and entropy excess are both negligible within the uncertainties. Mirroring this behaviour, the scaling of gas entropy is shallower than self-similar in the inner regions, but steepens with radius, becoming consistent with self-similar at R_500. The dispersion in scaled entropy in the inner regions is linked to the presence of cool cores and dynamical activity; at larger radii the dispersion decreases by a factor of two and the dichotomy between subsamples disappears. Parameterising the profiles with a power law plus constant model, there are two peaks in central entropy K_0; however, we cannot distinguish between a bimodal or a left-skewed distribution. The outer slopes are correlated with system temperature; their distribution is unimodal with a median value of 0.98. Renormalising the dimensionless entropy profiles by the gas mass fraction profile f_gas(< R), leads to a remarkable reduction in the scatter, implying that gas mass fraction variations with radius and mass are the cause of the observed entropy properties. We discuss a tentative scenario to explain the behaviour of the entropy and gas mass fraction in the REXCESS sample, in which extra heating and merger mixing maintains an elevated central entropy level in the majority of the population, and a smaller fraction of systems develops a cool core.
The X-ray Properties of Optically Selected Clusters of Galaxies
本文介绍了红序星系团巡天（RCS）中发现的9个中等红移（0.16 < z < 0.42）星系团的Chandra和Suzaku X射线观测结果。表面亮度分布拟合β模型，气体质量的确定，R2500内提取的综合光谱，和X射线温度和光度推断。通过将该样品与我们先前对九个高红移（0.6 < z < 1.0）光学选择团簇的X射线研究进行比较，测试了从自相似演化中预期的Lx-Tx关系。我们发现，光学选择的集群是系统的亮度低于X射线选择集群相似的X射线温度在中等和高Z。我们无法用这些数据来约束Lx-Tx关系的演变，但发现它与没有演变相一致，在相对较大的不确定性内。为了研究选择效应，我们将我们的样品的X射线性质与代表性的X射线选择的REXCESS样品中的簇的性质进行比较，所述簇也在R2500内确定。我们发现，虽然RCS集群X射线属性跨越了其他方法选择的大质量集群的整个范围内，其平均X射线属性是最相似的动态扰动X射线选择集群。这种相似性表明，真正的集群分布可能包含一个更高的比例比通常在X射线选定的调查中检测到的干扰对象。
Diversity of Ionized Gas Structures in Nearby Metal-poor Dwarf Galaxies
University of Tsukuba（筑波大学）；Tsukuba Institute for Advanced Research (TIAR), University of Tsukuba（筑波大学先进研究院）；Tomonaga Center for the History of the Universe (TCHoU), Faculty of Pure and Applied Sciences, University of Tsukuba（筑波大学理学院朝永振一郎宇宙历史中心）；The University of Tokyo（东京大学）；Stockholm University（斯德哥尔摩大学）；Universidad Nacional Autónoma de México, Instituto de Astronomía（墨西哥国立自治大学天文学研究所）；Nagoya University（名古屋大学）；Waseda University（早稻田大学）
