Multi-frequency studies of radio relics in the galaxy clusters A4038, A1664 and A786

TL;DR

This study analyzes radio relics in three galaxy clusters across multiple frequencies, revealing their spectral properties, morphologies, and possible origins, with models suggesting they are remnants of radio lobes compressed by shocks or lurking galaxies.

Contribution

It provides detailed multi-frequency observations and spectral modeling of radio relics, challenging previous shock-origin interpretations for the A786 relic.

Findings

A4038 relic shows ultra-steep spectrum emission extending 210 x 80 kpc.

A1664 relic's spectrum fits a model of radio lobes lurking for ~4x10^7 years.

A786 relic is likely a lurking radio galaxy, not a shock site.

Abstract

We present a multi-frequency study of radio relics associated with the galaxy clusters A4038, A1664 and A786. Radio images, integrated spectra, spectral index maps and fits to the integrated spectra in the framework of the adiabatic compression model are presented. Images of the relic in A4038 at 150, 240 and 606 MHz with the Giant Metrewave Radio Telescope have revealed extended ultra-steep spectrum (\alpha ~ -1.8 to -2.7) emission of an extent 210 x 80 kpc^2. The model of passively evolving radio lobes compressed by a shock best fits the integrated spectrum. The relic with a circular morphology at the outskirts of the cluster A1664 has an integrated spectral index of -1.10+/-0.06 and is best fit by the model of radio lobes lurking for ~ 4x10^7 yr. The relic near A786 has a curved spectrum and is best fit by a model of radio lobes lurking for ~ 3x10^7 yr. At 4.7 GHz, a compact radio source, possibly the progenitor of the A786 relic, is detected near the center of the radio relic. The A786 radio relic is thus likely a lurking radio galaxy rather than a site of cosmological shock as has been considered in earlier studies.