为什么有些星系团不太亮？CL2015质量曲线的浓度非常低
Why are some galaxy clusters underluminous? The very low concentration of the CL2015 mass profile
摘要：
由于我们在理解样本选择效应的严重性方面取得了进展，我们对星系团多样性的了解在过去几年中一直在增加。为了了解观察到的变化的原因，我们研究了CL 2015，这是一个在X射线选择的观察样本中很容易错过的簇。它的核心切除X射线光度是低的，其质量M500，远低于平均关系的X射线选择的样品，但只有约1.5西格玛低于X射线无偏样品。我们用获得的深斯威夫特X射线数据推导出热力学分布和流体静力学质量，我们使用爱因斯坦，普朗克和SDSS数据来推导其他测量值，如综合康普顿参数，总质量和恒星质量。CL 2015的压强和电子密度分布系统地超出了通用分布的+/- 2 sigma范围;特别是电子密度分布甚至低于普朗克选择的簇。与X射线选择的样本相比，CL 2015在X射线亮度与积分压力缩放方面也有相当大的不同，但就恒星质量分数而言，它是一个正常的物体。CL 2015的流体静力学质量分布，本身或当与动力学质量一起考虑时，表明与X射线选择样品中的星团相比，该星团具有不寻常的低浓度和不寻常的稀疏性。CL 2015的不同行为是由其低浓度引起的。当考虑到浓度差异时，CL 2015的性质与比较样品一致。CL 2015可能是第一个已知的具有非常低质量浓度的星团，其高质量的X射线数据存在。与CL 2015类似的天体未能进入观测X射线选择样本，因为它们的X射线光度相对于它们的质量较低。
英文摘要：
Our knowledge of the variety of galaxy clusters has been increasing in the last few years thanks to our progress in understanding the severity of selection effects on samples. To understand the reason for the observed variety, we study CL2015, a cluster easily missed in X-ray selected observational samples. Its core-excised X-ray luminosity is low for its mass M500, well below the mean relation for an X-ray selected sample, but only ~1.5 sigma below that derived for an X-ray unbiased sample. We derived thermodynamic profiles and hydrostatic masses with the acquired deep Swift X-ray data, and we used archival Einstein, Planck, and SDSS data to derive additional measurements, such as integrated Compton parameter, total mass, and stellar mass. The pressure and the electron density profiles of CL2015 are systematically outside the +/- 2 sigma range of the universal profiles; in particular the electron density profile is even lower than the one derived from Planck-selected clusters. CL2015 also turns out to be fairly different in the X-ray luminosity versus integrated pressure scaling compared to an X-ray selected sample, but it is a normal object in terms of stellar mass fraction. CL2015's hydrostatic mass profile, by itself or when is considered together with dynamical masses, shows that the cluster has an unusual low concentration and an unusual sparsity compared to clusters in X-ray selected samples. The different behavior of CL2015 is caused by its low concentration. When concentration differences are accounted for, the properties of CL2015 become consistent with comparison samples. CL2015 is perhaps the first known cluster with a remarkably low mass concentration for which high quality X-ray data exist. Objects similar to CL2015 fail to enter observational X-ray selected samples because of their low X-ray luminosity relative to their mass.
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