Dynamical state for 964 galaxy clusters from Chandra X-ray images

TL;DR

This study introduces new parameters and a morphology index to assess the dynamical state of 964 galaxy clusters using Chandra X-ray images, revealing a continuous distribution from disturbed to relaxed states and correlations with other cluster properties.

Contribution

The paper develops a novel morphology index delta based on adaptive parameters, improving the characterization of galaxy cluster dynamical states from X-ray images.

Findings

Dynamical parameters are continuously distributed, not bimodal.

The morphology index delta correlates with optical relaxation and other indicators.

Dynamical parameters relate to radio properties and mass estimates.

Abstract

The dynamical state of galaxy clusters describes if clusters are relaxed dynamically or in a merging process of subclusters. By using archival images from the Chandra X-ray Observatory, we derive a set of parameters to describe the dynamical state for 964 galaxy clusters. Three widely used indicators for dynamical state, the concentration index c, the centroid shift omega and the power ratio P3/P0 are calculated in the circular central region with a radius of 500 kpc. We also derive two adaptive parameters, the profile parameter kappa and the asymmetry factor alpha, in the best fitted elliptical region. The morphology index delta is then defined by combining these two adaptive parameters, which indicates the dynamical state of galaxy clusters and has good correlations to the concentration index c, the centroid shift omega, the power ratio P3/P0, and the optical relaxation factor Gamma. For a large sample of clusters, the dynamical parameters are continuously distributed from the disturbed to relaxed states with a peak in the between, rather than the bimodal distribution for the two states. We find that the newly derived morphology index delta works for the similar fundamental plane between the radio power, cluster mass and the dynamical state for clusters with diffuse radio giant-halos and mini-halos. The offset between masses estimated from the Sunyaev-Zeldovich effect and X-ray images depends on dynamical parameters. All dynamical parameters for galaxy clusters derived from the Chandra archival images are available on http://zmtt.bao.ac.cn/galaxy_clusters/dyXimages/.