On the Origin of Diffuse Radio Emission in Abell 85 -- Insights from new GMRT Observations

TL;DR

This paper presents new GMRT radio observations and X-ray analysis of Abell 85, revealing the detailed structure of a radio phoenix and confirming the presence of a bow shock, thus providing insights into the origin of diffuse radio emission in galaxy clusters.

Contribution

The study provides the first high-resolution radio map of the radio phoenix in Abell 85 and confirms the association with a bow shock through combined radio and X-ray analysis.

Findings

Revealed the complex filamentary structure of the radio phoenix.

Confirmed the presence of a bow shock at the radio phoenix location.

Found consistency between X-ray and radio Mach number estimates.

Abstract

Extended, steep, and ultra-steep spectrum radio emission in a galaxy cluster is usually associated with recent mergers. Simulations show that radio phoenixes are aged radio galaxy lobes whose emission reactivates when a low Mach shock compresses it. A85 hosts a textbook example of a radio phoenix at about 320 kpc southwest of the cluster center. We present a new high resolution 325 MHz GMRT radio map illustrating this radio phoenix's complex and filamentary structure. The full extent of the radio structure is revealed for the first time from these radio images of A85. Using archival \textit{Chandra} X-ray observations, we applied an automated 2-D shock finder to the X-ray surface brightness and Adaptive Circular Binning (ACB) temperature maps which confirmed a bow shock at the location of the radio phoenix. We also compared the Mach number from the X-ray data with the radio-derived Mach number in the same region using multi-frequency radio observations and find that they are consistent within the 1$\sigma$ error level.