Simulations of the merging cluster of galaxies Cygnus A

TL;DR

This study uses hydrodynamic simulations constrained by Chandra X-ray observations to model the pre-merger environment of the Cygnus A galaxy cluster, revealing details about its mass, temperature profiles, and merger dynamics.

Contribution

First detailed model of the Cygnus A cluster's pre-merger state, including the sub cluster CygNW, with insights into its mass ratio, temperature structure, and merger effects.

Findings

CygA has a lower baryon fraction and higher concentration than expected.

The cluster hosts a pre-merger with a 1.5:1 mass ratio.

Merger-induced heating occurs without a detectable shock, likely from compression.

Abstract

The archetype FR-II galaxy Cygnus A lies in a moderately rich cluster about to undergo a major merger. We study the pre-merger Cygnus cluster environment using smoothed particle hydrodynamics simulations constrained by 2Ms of Chandra observations of the hot intracluster medium. The observations constrain the total gravitating mass and concentration parameter, and the simulations provide the quiescent and merger-enhanced temperature profiles of the pre- and post merger of the cluster excluding the central active galactic nucleus. We present the first detailed model of the sub cluster north west of Cygnus A, named CygNW. We find a lower baryon fraction and higher concentration parameter for CygA than expected from known scaling relations in the literature. The model suggests the Cygnus cluster hosts a pre-merger with a progenitor mass ratio of about 1.5:1 at the virial radius. We notice that the intra cluster medium is heated as a result of the merger, but we find no evidence for a (pre-)merger shock in the interstitial region between both cluster haloes. We attribute the merger-induced heating to compression of the cluster outskirts. The smooth model obtained from our simulations is subtracted from the observed cluster state and shows residual temperature structure that is neither hydrostatic nor merger-heated cluster gas. We speculate that this residual heating may result from previous AGN activity over the last ~100 Myr.