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New DOLoRes@TNG data confirm the anomalous nature of the radio halo hosted by a relaxed galaxy cluster

A fraction of the most massive galaxy clusters hosts diffuse radio sources named radio haloes (also giant radio haloes, or GRHs). They are extended (on 1 Mpc Scale) sources which pervade the central cluster regions in a similar way to the intracluster medium (ICM) and do not appear associated to more compact radio sources like radio galaxies. Their existence is due to synchrotron nonthermal radiation produced by ICM electrons accelerated to relativistic energies and moving in large-scale cluster magnetic fields.

Several particle acceleration mechanisms have been proposed in the past for shedding light on the origin of GRHs. In particular, mergers involving massive galaxy clusters have been suggested as a necessary ingredient for the formation of these radio sources. In fact, the huge energy of these merger events could reaccelerate mildly relativistic particles to relativistic energies and amplify the cluster magnetic fields. In the most popular theoretical model, the turbulence induced in the ICM by recent mergers seems to play a key role.

For several decades after their discovery, GRHs have always been detected in merging clusters. However, this general picture began to falter after the discovery in 2014 of a GRH in the medium-redshift cluster CL1821+643. In fact, this cluster exhibits a low ICM central temperature (a "cool core"), a typical feature found in dynamically relaxed clusters. Moreover, detailed studies of the ICM X-ray emission do not show any evidence of a major merger occurring in this cluster similar to those observed in other clusters with GRHs.

In order to clarify the puzzling anomaly of the GRH of CL1821+643, optical observations of the cluster have been proposed at the Telescopio Nazionale Galileo (TNG) in order to assess its dynamical status with the kinematical study of the cluster galaxies population. In fact, the X-ray studies cited above are not sensitive to eventual mergers along the line-of-sight, a possibility which can be probed only by analyzing the galaxy velocity distribution.

Very interestingly, the new study based on spectroscopic data acquired with the instrument DOLoRes confirms and strengthens the picture drawn by the previous studies based on X-ray data. None of the statistical tests employed to study the galaxy population in the 1D (radial velocity information), 2D (spatial information) and 3D (combined velocity and spatial information) domains is able to detect significant substructures. This means that also in the optical band there is no clear evidence of a merger process responsible for the GRH of CL1821+643.

In conclusion, while the results of the new optical study are in agreement with the existence of the central cool core, the GRH of CL1821+643 remains an open problem and brings into question our current understanding of diffuse radio sources in galaxy clusters.

Color picture (trichromy with filters g, r and i) of the central region of the cluster CL1821+643 (from archival data of the Sloan Digital Sky Survey). The bright bluish source in the center of the image is the quasar H1821+643, hosted by the dominant galaxy of the cluster.

More information in the published paper: Boschin, W., & Girardi, M., 2018, Monthly Notices of the Royal Astronomical Society, 480, 1187 .