Deciphering the ultra-steep spectrum diffuse radio sources discovered in the cool-core cluster Abell~980

TL;DR

This study reveals complex radio and X-ray phenomena in galaxy cluster Abell 980, including ultra-steep spectrum sources, cold fronts, and a mini-halo, highlighting the cluster's dynamic history and nuclear activity.

Contribution

First detailed multi-band analysis of Abell 980 showing rare associations of USS sources with cold fronts in a cool-core cluster.

Findings

Discovery of two ultra-steep spectrum radio sources linked to cold fronts.

Identification of a central radio double and a mini-halo around the BCG.

Evidence suggesting past jet activity and buoyant radio lobes shaping the ICM.

Abstract

Clusters of galaxies are excellent laboratories for studying recurring nuclear activity in galactic nuclei since their hot gaseous medium can vastly prolong the detectability of their radio lobes via better confinement. We report here a multi-band study of the sparsely studied galaxy cluster Abell 980, based on our analysis of {\it{Chandra}} X-ray and the GMRT (150 and 325 MHz) and EVLA (1.5 GHz) radio archival data, revealing an unusually rich phenomenology. It is shown to be a quasi-relaxed cluster with a cool core ($T\sim4.2$ keV) surrounded by a hot and extensive intracluster medium (ICM) at $T\sim6.8$ keV. The radio emission shows a rich diversity, having (i) two large diffuse sources of ultra-steep spectrum (USS), extending to opposite extremities of the ICM, each associated with an X-ray brightness discontinuity (cold front), (ii) a bright radio-double of size $\sim55$ kpc coinciding with the central BCG, and (iii) a diffuse radio source, likely a mini-halo of size $\sim110$~kpc around the BCG which possesses a huge ellipsoidal stellar halo of extent $\sim 80$~kpc. The association of cold fronts with two highly aged (~ 260 Myr) USS sources in a cool-core cluster, makes it a very rare system. These USS sources are probably radio lobes from a previous episode of jet activity in the BCG, driven buoyantly towards the outskirts of the X-ray halo, thereby creating the cold fronts. A deeper radio image of this cluster may provide a rare opportunity to verify the recently proposed alternative model which explains radio mini-haloes as the aggregate radio emission from Type I supernovae occurring in the giant stellar halo extended across the cluster core.