Spectral Index of the Filaments in the Abell 523 Radio Halo

Valentina Vacca,Ettore Carretti,Gabriele Giovannini,Federica Govoni,Richard A Perley,Luigina Feretti,Francesca Loi,Matteo Murgia

doi:10.3390/galaxies9040112

Abstract

The galaxy cluster Abell 523 hosts a radio halo characterized by the presence of two filaments transversely located with respect to the cluster merger axis. In this paper, we present a spectral index image of these filaments between 1.410 and 1.782 GHz obtained with Jansky Very Large Array observations. We find a steepening of the spectral index of the filaments at frequencies ≳1.4 GHz and an indication that bright patches are characterized by flat spectral indices. Our results are consistent with a scenario of highly-efficient turbulence induced by merger phenomena.

Highlights

Galaxy clusters characterized by major merger events have been observed to host diffuse synchrotron sources classified as radio halos and radio relics according to their properties and location with respect to the cluster center
Different spectral behaviour are expected for the two classes of sources: diffuse emission sustained by turbulence is expected to be characterized by a complex spectral index distribution with a spectral steepening at high frequencies, while a spectrum spatial gradient is expected in case of radio emission associated with merger shocks with flatter spectra along the outer edge of the shock
We present detailed spectral index images between 1.410 GHz and 1.782 GHz to study the spectral behaviour of the filaments in this frequency range

Summary

Introduction

Galaxy clusters characterized by major merger events have been observed to host diffuse synchrotron sources classified as radio halos and radio relics according to their properties and location with respect to the cluster center. They are faint with an average radio brightness ∼0.1 μJy/arcsec, extended up to maximum projected linear size ∼Mpc, and easier to detect and study at lower frequencies due to their steep spectra (α 1–1.5, see e.g., [1]), where α is related to the flux density Sν at the observing frequency ν via Sν ∝ ν−α). Current data are in agreement with these predictions concerning radio relics but suggest a diversity of spectral behaviours for radio halos (see e.g., [1]) A detailed knowledge of their spectral properties over a wide frequency range is precious to investigate the properties and understand the origin these sources

Methods

Results

Conclusion