Optical/X-ray/radio view of Abell 1213: A galaxy cluster with anomalous diffuse radio emission

¹ Fundación Galileo Galilei – INAF (Telescopio Nazionale Galileo), Rambla José Ana Fernández Perez 7, 38712 Breña Baja (La Palma), Canary Islands, Spain
e-mail: boschin@tng.iac.es
² Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
³ Departamento de Astrofísica, Univ. de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
⁴ Dipartimento di Fisica dell’Università degli Studi di Trieste – Sezione di Astronomia, Via Tiepolo 11, 34143 Trieste, Italy
⁵ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁶ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁷ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, 20133 Milano, Italy
⁸ Dipartimento di Fisica, Università degli Studi di Milano, Via G. Celoria 16, 20133 Milano, Italy
⁹ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹⁰ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹¹ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy

Received: 3 January 2023
Accepted: 27 February 2023

Abstract

Context. Abell 1213, a low-richness galaxy system, is known to host an anomalous radio halo detected in data of the Very Large Array (VLA). It is an outlier with regard to the relation between the radio halo power and the X-ray luminosity of the parent clusters.

Aims. Our aim is to analyze the cluster in the optical, X-ray, and radio bands to characterize the environment of its diffuse radio emission and to shed new light on its nature.

Methods. We used optical data from the Sloan Digital Sky Survey to study the internal dynamics of the cluster. We also analyzed archival XMM-Newton X-ray data to unveil the properties of its hot intracluster medium. Finally, we used recent data from the LOw Frequency ARray (LOFAR) at 144 MHz, together with VLA data at 1.4 GHz, to study the spectral behavior of the diffuse radio source.

Results. Both our optical and X-ray analysis reveal that this low-mass cluster exhibits disturbed dynamics. In fact, it is composed of several galaxy groups in the peripheral regions and, in particular, in the core, where we find evidence of substructures oriented in the NE–SW direction, with hints of a merger nearly along the line of sight. The analysis of the X-ray emission adds further evidence that the cluster is in an unrelaxed dynamical state. At radio wavelengths, the LOFAR data show that the diffuse emission is ∼510 kpc in size. Moreover, there are hints of low-surface-brightness emission permeating the cluster center.

Conclusions. The environment of the diffuse radio emission is not what we would expect for a classical halo. The spectral index map of the radio source is compatible with a relic interpretation, possibly due to a merger in the N–S or NE–SW directions, in agreement with the substructures detected through the optical analysis. The fragmented, diffuse radio emissions at the cluster center could be attributed to the surface brightness peaks of a faint central radio halo.