Morphology parameters: substructure identification in X-ray galaxy clusters

Abstract

In recent years multi-wavelength observations have shown the presence of substructures related to merging events in a high fraction of galaxy clusters. Clusters can be roughly grouped into two categories -- relaxed and non-relaxed -- and a proper characterisation of the dynamical state of these systems is of crucial importance both for astrophysical and cosmological studies. In this paper we investigate the use of a number of morphological parameters (Gini, $M_{20}$, Concentration, Asymmetry, Smoothness, Ellipticity and Gini of the second order moment, $G_{M}$) introduced to automatically classify clusters as relaxed or dynamically disturbed systems. We apply our method to a sample of clusters at different redshifts extracted from the {\it Chandra} archive and we investigate possible correlations between morphological parameters and other X-ray gas properties. We conclude that a combination of the adopted parameters is a very useful tool to properly characterise the X-ray cluster morphology. According to our results three parameters -- Gini, $M_{20}$ and Concentration -- are very promising for identifying cluster mergers. The Gini coefficient is a particularly powerful tool, especially at high redshift, being independent from the choice of the position of the cluster centre. We find that high Gini ($>$ 0.65), high Concentration ($>$ 1.55) and low $M_{20}$ ($<$ -2.0) values are associated with relaxed clusters, while low Gini ($<$ 0.4), low Concentration ($<$ 1.0) and high $M_{20}$ ($>$ -1.4) characterise dynamically perturbed systems. We also estimate the X-ray cluster morphological parameters in the case of {\it radio loud} clusters. In excellent agreement with previous analyses we confirm that diffuse intracluster radio sources are associated with major mergers.