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

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 Chandra X-ray data and 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 ∼ 4.2 keV) surrounded by a hot and extensive intracluster medium (ICM) at T ∼ 6.8 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 ∼55 kpc coinciding with the central BCG; and (iii) a diffuse radio source, likely a mini-halo of size ∼110 kpc around the BCG which possesses a huge ellipsoidal stellar halo of extent ∼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 Ia supernova remnant occurring in the giant stellar halo extended across the cluster core.