The Chandra Strong Lens Sample: Measuring the Dynamical States and Relaxation Fraction of a Sample of 28 Strong Lensing Selected Galaxy Clusters

TL;DR

This study assesses the dynamical states of 28 strong lensing galaxy clusters using X-ray and optical data, classifies their relaxation status, and compares these fractions with other cluster samples to understand selection biases.

Contribution

Introduces a combined morphological parameter to classify cluster dynamical states and compares relaxation fractions across different selection methods.

Findings

43% of clusters are relaxed, consistent with other samples.

No significant correlation between dynamical state and cluster size.

Weak dependence of dynamical state on mass and redshift.

Abstract

We present the results of our dynamical state proxy measurements performed on 28 strong lensing galaxy clusters from the Sloan Giant Arcs Survey (SGAS). Using Chandra ACIS-I/S X-ray data supplemented with HST WFC3 imaging, we measure four morphological parameters: the concentration parameter (c), asymmetry parameter (A), centroid shift (log(w)), and the X-ray-BCG centroid separation (D [kpc]). Our goals are to (A) provide a robust classification of the dynamical state of the clusters in this strong lensing selected sample to enable studies that test various problems in cluster astrophysics and observational cosmology; (B) identify correlations, biases, or disagreements between different measurement proxies and cluster properties; and (C) measure the relaxation fraction (the fraction of clusters classified as relaxed based on X-ray morphology) and compare it to relaxation fractions from cluster samples selected using other methods. We combine the four morphological parameters into a single metric, the combined parameter M, which effectively separates the cluster sample into four dynamical state categories: relaxed; moderately relaxed; moderately disturbed; and disturbed. We find no significant trend in a cluster's dynamical state with its size, and only a weak, statistically limited dependence on mass and redshift. Based on our classification system, we find that $43\%^{+9}_{-9}$ of the clusters are relaxed, which is consistent with relaxation fractions measured for other cluster samples selected on mass-observables. This implies a strong lensing selected sample of clusters is on average dynamically similar to clusters selected via different methods.