Serendipitous decametre detection of ultra steep spectrum radio emission in Abell 655

TL;DR

This study presents the first low-frequency (below 30 MHz) LOFAR images of galaxy cluster Abell 655, revealing complex diffuse radio emission likely from re-energized fossil plasma and a radio halo, expanding understanding of cluster radio phenomena.

Contribution

First low-frequency LOFAR imaging of Abell 655 at decameter wavelengths, showing complex diffuse emission and proposing its origins from fossil plasma and radio halo components.

Findings

Diffuse emission extends about 700 kpc in Abell 655.

Low-frequency emission likely from re-energized fossil plasma.

Approximately 25% of Planck clusters may host detectable fossil plasma in decameter band.

Abstract

Some galaxy clusters contain non-thermal synchrotron emitting plasma permeating the intracluster medium (ICM). The spectral properties of this radio emission are not well characterized at decameter wavelengths ({\nu} < 30 MHz), primarily due to the severe corrupting effects of the ionosphere. Using a recently developed calibration strategy, we present LOFAR images below 30 MHz of the low mass galaxy cluster Abell 655, which was serendipitously detected in an observation of the bright calibrator 3C 196. We combine this observation with LOFAR data at 144 MHz, and new Band 4 Giant Metrewave Radio Telescope observations centered at 650 MHz. In the 15-30 MHz LOFAR image, diffuse emission is seen with a physical extent of about 700 kpc. We argue that the diffuse emission detected in this galaxy cluster likely has multiple origins. At higher frequencies (650 MHz), the diffuse emission resembles a radio halo, while at lower frequencies the emission seems to consist of several components and bar-like structures. It suggests that most low-frequency emission in this cluster comes from re-energized fossil plasma from old AGN outbursts, coexisting with the radio halo component. By counting the number of cluster radio detections in the decameter band, we estimate that around a quarter of the Planck clusters host re-energised fossil plasma that is detectable in the decameter band with LOFAR.