The most dynamically relaxed clusters of galaxies play a special role in cosmological studies as well as astrophysical studies of the intracluster medium (ICM) and active galactic nucleus feedback. While high-spatial-resolution imaging of the morphology of the ICM has long been the gold standard for establishing a cluster’s dynamical state, such data are not available from current or planned surveys, and thus require separate, pointed follow-up observations. With optical and/or near-IR photometric imaging, and red-sequence cluster finding results from those data, expected to be ubiquitously available for clusters discovered in upcoming optical and millimeter-wavelength surveys, it is worth asking how effectively photometric data alone can identify relaxed cluster candidates, before investing in, e.g., high-resolution X-ray observations. Here we assess the ability of several simple photometric measurements, based on the redMaPPer cluster finder run on Sloan Digital Sky Survey data, to reproduce X-ray classifications of dynamical state for an X-ray selected sample of massive clusters. We find that two simple metrics contrasting the bright central galaxy (BCG) to other cluster members can identify a complete sample of relaxed clusters with a purity of ∼40% in our data set. Including minimal ICM information in the form of a center position increases the purity to ∼60%. However, all three metrics depend critically on correctly identifying the BCG, which is presently a challenge for optical red-sequence cluster finders.

中文翻译：

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星系团弛豫的光学光度指标


最动态松弛的星系团在宇宙学研究以及星系团内介质（ICM）和活跃星系核反馈的天体物理学研究中发挥着特殊作用。虽然ICM形态的高空间分辨率成像长期以来一直是建立星团动态状态的黄金标准，但目前或计划中的调查无法提供此类数据，因此需要单独的、有针对性的后续观测。通过光学和/或近红外光度成像，以及从这些数据中发现红色序列星团的结果，预计将普遍用于即将到来的光学和毫米波调查中发现的星团，值得一问的是，光度数据如何有效地单独识别在投资高分辨率 X 射线观测等项目之前，先放松候选星团。在这里，我们基于在斯隆数字巡天数据上运行的 redMaPPer 星团探测器，评估了几种简单的光度测量的能力，以重现 X 射线选定的大星团样本的动态 X 射线分类。我们发现，将明亮的中央星系 (BCG) 与其他星团成员进行对比的两个简单指标可以识别我们数据集中纯度为 ∼40% 的松弛星团的完整样本。以中心位置的形式包含最少的 ICM 信息，可将纯度提高至 ∼60%。然而，所有三个指标都严重依赖于正确识别 BCG，这目前对光学红色序列簇发现者来说是一个挑战。