The beautiful mess in Abell 2255

TL;DR

This paper presents detailed LOFAR radio observations of Abell 2255, revealing complex filamentary structures in its radio halo, and explores the connection between thermal X-ray and non-thermal radio emissions in this merging galaxy cluster.

Contribution

It provides new high-resolution radio images of Abell 2255's radio halo and analyzes the spectral and thermal properties, highlighting the complex structure and possible seed particle origins.

Findings

Discovery of bright, filamentary structures in the radio halo.

Complex spectral index distribution across the halo.

Overall correlation between X-ray and radio emissions.

Abstract

We present LOFAR observations of one of the most spectacular objects in the radio sky: Abell 2255. This is a nearby ($z = 0.0806$) merging galaxy cluster hosting one of the first radio halos ever detected in the intra-cluster medium (ICM). The deep LOFAR images at 144 MHz of the central $\sim10$ Mpc$^2$ region show a plethora of emission on different scales, from tens of kpc to above Mpc sizes. In this work, we focus on the innermost region of the cluster. Among the numerous interesting features observed, we discover remarkable bright and filamentary structures embedded in the radio halo. We incorporate archival WSRT 1.2 GHz data to study the spectral properties of the diffuse synchrotron emission and find a very complex spectral index distribution in the halo spanning a wide range of values. We combine the radio data with Chandra observations to investigate the connection between the thermal and non-thermal components by quantitatively comparing the radio and X-ray surface brightness and the spectral index of the radio emission with the thermodynamical quantities of the ICM. Despite the multitude of structures observed in the radio halo, we find that the X-ray and radio emission are overall well correlated. The fact that the steepest spectrum emission is located in the cluster center and traces regions with high entropy possibly suggests the presence of seed particles injected by radio galaxies that are spread in the ICM by turbulence generating the extended radio halo.