Radio spectral study of the cluster of galaxies Abell 2255

TL;DR

This study uses multi-wavelength radio observations of galaxy cluster Abell 2255 to analyze the spectral properties of its halo, relics, and radio galaxy, revealing new steep-spectrum emission regions and insights into cluster merger processes.

Contribution

First high-sensitivity 2 m wavelength image of Abell 2255 revealing a new Mpc-scale emission region, and detailed spectral index analysis of halo, relics, and radio galaxy.

Findings

Spectral index varies across the halo and relics, indicating different physical conditions.

A new steep-spectrum Mpc-size emission region was detected between the halo and NW relic.

The radio galaxy's spectrum steepens along its tail due to energy losses.

Abstract

Spectral index studies of halos, relics, and radio galaxies provide useful information on their origin and connection with merger processes. We present WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm, 85 cm, and 2 m. The spectral index images allowed us to study the integrated spectrum of halo and relic and to investigate the physical properties of the Beaver head-tail radio galaxy belonging to the cluster. In the radio halo, the spectral index is steeper at the center and flatter at the locations of the radio filaments, clearly detected at 25 cm. In the relics, the spectral index flattens, moving away from the cluster center. For the Beaver radio galaxy, the spectrum severely steepens from the head towards the end of the tail, because of the energy losses suffered by the relativistic particles. In the 2 m map, which is the first high-sensitivity image presented in the literature at such a long wavelength, a new Mpc-size emission region is detected between the known radio halo and the NW relic. Not detecting this feature in the more sensitive 85 cm observations implies that it must have a very steep spectrum (alpha <= -2.6). The observational properties of the radio halo suggest that either we are looking at a superposition of different structures (filaments in the foreground plus real halo in the background) seen in projection across the cluster center or that the halo is intrinsically peculiar. The newly detected extended region to the NW of the halo could be considered as an asymmetric extension of the halo itself. However, since radio halos are known in the literature as structures showing a regular morphology, the new feature could represent the first example of steep Mpc-size diffuse structures (MDS), detected around clusters at very low frequencies.