Evaluating Tests of Virialization and Substructure Using Galaxy Clusters in the ORELSE Survey

TL;DR

This study assesses various indicators of galaxy cluster virialization using multi-wavelength data from the ORELSE survey, identifying robust proxies for dynamical state when X-ray data is limited.

Contribution

It introduces a comprehensive comparison of virialization indicators, highlighting the effectiveness of centroid offsets and brightest galaxy positions as proxies.

Findings

Centroid offsets strongly correlate with virialization state.

X-ray and optical indicators show significant offsets in disturbed clusters.

Robust virialization proxies can be identified without deep X-ray data.

Abstract

We evaluated the effectiveness of different indicators of cluster virialization using 12 large-scale structures in the ORELSE survey spanning from $0.7<z<1.3$. We located diffuse X-ray emission from 16 galaxy clusters using Chandra observations. We studied the properties of these clusters and their members, using Chandra data in conjunction with optical and near-IR imaging and spectroscopy. We measured X-ray luminosities and gas temperatures of each cluster, as well as velocity dispersions of their member galaxies. We compared these results to scaling relations derived from virialized clusters, finding significant offsets of up to 3-4$\sigma$ for some clusters, which could indicate they are disturbed or still forming. We explored if other properties of the clusters correlated with these offsets by performing a set of tests of virialization and substructure on our sample, including Dressler-Schectman tests, power ratios, analyses of the velocity distributions of galaxy populations, and centroiding differences. For comparison to a wide range of studies, we used two sets of tests: ones that did and did not use spectral energy distribution fitting to obtain rest-frame colours, stellar masses, and photometric redshifts of galaxies. Our results indicated that the difference between the stellar mass or light mean-weighted center and the X-ray center, as well as the projected offset of the most-massive/brightest cluster galaxy from other cluster centroids had the strongest correlations with scaling relation offsets, implying they are the most robust indicators of cluster virialization and can be used for this purpose when X-ray data is insufficiently deep for reliable $L_X$ and $T_X$ measurements.