How unusual is the cool-core radio halo cluster CL1821+643 ?

TL;DR

This study investigates the unusual presence of a large radio halo in the cool-core galaxy cluster CL1821+643, combining radio and X-ray observations to analyze its morphology and merger history.

Contribution

It provides the first detailed radio and X-ray morphological analysis of CL1821+643, revealing it as a non-relaxed cluster with a merger history, challenging typical associations between cool-cores and mini-haloes.

Findings

The radio halo has a spectral index of 1.0±0.1.

CL1821+643 is a non-relaxed, merging cluster.

The cluster experienced a non-core-disruptive merger.

Abstract

Massive galaxy clusters with cool-cores typically host diffuse radio sources called mini-haloes, whereas, those with non-cool-cores host radio haloes. We attempt to understand the unusual nature of the cool-core galaxy cluster CL1821+643 that hosts a Mpc-scale radio halo using new radio observations and morphological analysis of its intra-cluster medium. We present the Giant Metrewave Radio Telescope (GMRT) 610 MHz image of the radio halo. The spectral index, $\alpha$ defined as $S\propto \nu^{-\alpha}$, of the radio halo is $1.0\pm0.1$ over the frequency range of 323 - 610 - 1665 MHz. Archival {\it Chandra} X-ray data were used to make surface brightness and temperature maps. The morphological parameters Gini, $M_{20}$ and concentration ($C$) were calculated on X-ray surface brightness maps by including and excluding the central quasar (H1821+643) in the cluster. We find that the cluster CL1821+643, excluding the quasar, is a non-relaxed cluster as seen in the morphological parameter planes. It occupies the same region as other merging radio halo clusters in the temperature- morphology parameter plane. We conclude that this cluster has experienced a non-core-disruptive merger.