Cosmic dance in the Shapley Concentration Core

I. A study of the radio emission of the BCGs and tailed radio galaxies

¹ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
e-mail: gabriella.di.gennaro@cfa.harvard.edu
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF-Astrophysics and Space Science Observatory Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ Naval Research Laboratory, 4555 Overlook Avenue SW, Code 7213, Washington, DC 20375, USA
⁷ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁸ INAF – IASF Milano, Via Bassini 15, 20133 Milano, Italy
⁹ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy

Received: 8 February 2018
Accepted: 11 July 2018

Abstract

Context. The Shapley Concentration (⟨z⟩ ≈ 0.048) covers several degrees in the southern hemisphere, and includes galaxy clusters in advanced evolutionary stages, groups of clusters in the very early stages of merger, fairly massive clusters with ongoing accretion activity, and smaller groups located in filaments in the regions between the main clusters.

Aims. With the goal to investigate the role of cluster mergers and accretion on the radio galaxy population, we performed a multi-wavelength study of the brightest cluster galaxies (BCGs) and of the galaxies showing extended radio emission in the cluster complexes of Abell 3528 and Abell 3558. In total, our study is based on a sample of 12 galaxies.

Methods. We observed the clusters with the Giant Metrewave Radio Telescope (GMRT) at 235, 325, and 610 MHz, and with the Very Large Array (VLA) at 8.46 GHz. We complemented our study with the TIFR GMRT Sky Survey (TGSS) at 150 MHz, the Sydney University Molonglo Sky Survey (SUMSS) at 843 MHz, and the Australia Telescope Compact array (ATCA) at 1380, 1400, 2380, and 4790 MHz data. Finally, optical imaging with the VLT Survey Telescope (VST) is also available for the host galaxies as well as the mid-infrared coverage with the Wide-Field Infrared Survey Explorer (WISE).

Results. We found significant differences in the properties of the radio emission of the BCGs in the two cluster complexes. The BCGs in the A 3528 complex and in A 3556, which are relaxed cool-core objects, are powerful active radio galaxies. They also present hints of restarted activity. On the contrary, the BCGs in A 3558 and A 3562, which are well-known merging systems, are very faint, or quiet, in the radio band. The optical and infrared properties of the galaxies, on the other hand, are fairly similar in the two complexes, showing all passive red galaxies.

Conclusions. Our study shows remarkable differences in the radio properties of the BGCs, which we relate to the different dynamical state of the host cluster. On the contrary, the lack of changes between such different environments in the optical band suggests that the dynamical state of galaxy clusters does not affect the optical counterparts of the radio galaxies, at least over the lifetime of the radio emission.