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

TL;DR

The paper tackles how to quantify the dynamical state of strong-lensing galaxy clusters by combining X-ray morphology proxies (c, A, log(w), D) with optical indicators, using Chandra and HST data for 28 clusters. A single combined metric $M$ partitions the sample into four dynamical states: relaxed, moderately relaxed, moderately disturbed, and disturbed. The study finds a relaxation fraction of $f_{ m relaxed} = 0.43^{+0.09}_{-0.09}$ with no strong size dependence and only weak mass/redshift trends, indicating strong-lensing clusters are dynamically similar to clusters selected by other methods. These results support using strong-lensing clusters for cluster physics and cosmology while highlighting selection effects and the value of multi-proxy dynamical-state assessments.

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.

Figures (2)

• Figure 1: Adaptively smoothed X-ray images of our disturbed cluster sample using the CIAOcsmooth function. The objects with blue borders are those identified as "disturbed" and the objects with green borders are those identified as "moderately disturbed." The objects are sequenced according to their combined dynamical relaxation M parameter value (see section \ref{['sec:M']}) increasing from left to right and top to bottom.
• Figure 2: Adaptively smoothed X-ray images of our relaxed cluster sample using the CIAOcsmooth function. The objects with orange borders are those identified as "relaxed" and the objects with magenta borders are those identified as "moderately relaxed." The objects are sequenced according to their combined dynamical relaxation M parameter value (see section \ref{['sec:M']}) increasing from left to right and top to bottom.