paper

Optical Photometric Indicators of Galaxy Cluster Relaxation

arXiv:2403.04897 · doi:10.3847/1538-4357/ad41de

Abstract

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 mm-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 per cent in our data set. Including minimal ICM information in the form of a center position increases the purity to ~60 per cent. However, all three metrics depend critically on correctly identifying the BCG, which is presently a challenge for optical red-sequence cluster finders.

arXiv comment10 pages, 6 figures. Accepted version; to appear in ApJ

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