Discovery of a spiral-host episodic radio-galaxy

TL;DR

This paper reports the discovery of a rare spiral-host episodic radio galaxy with relic lobes showing evidence of re-acceleration, providing insights into galaxy and cluster evolution through accretion and shocks.

Contribution

It presents the first detailed observation of a spiral-host large radio galaxy with triple-double episodic lobes, suggesting new mechanisms for relic re-acceleration beyond cluster mergers.

Findings

Relic lobes show spectral flattening and high polarization.

Re-acceleration likely caused by accretion shocks, not cluster mergers.

Host galaxy exhibits recent star formation despite being optically red.

Abstract

We report the discovery of a unique radio galaxy at z=0.137, which could possibly be the second spiral-host large radio galaxy and also the second triple-double episodic radio galaxy. The host galaxy shows signs of recent star formation in the UV but is optically red and is the brightest galaxy of a possible cluster. The outer relic radio lobes of this galaxy, separated by ~1 Mpc, show evidence of spectral flattening and a high fraction of linear polarisation. We interpret that these relic lobes have experienced re-acceleration of particles and compression of the magnetic field due to shocks in the cluster outskirts. From the morphology of the relics and galaxy distribution, we argue that re-acceleration is unlikely to be due to a cluster-cluster merger and suggest the possibility of accretion shocks. The source was identified from SDSS, GALEX, NVSS and FIRST survey data but we also present follow up optical observations with the Lulin telescope and 325 MHz low frequency radio observations with the GMRT. We briefly discuss the scientific potential of this example in understanding the evolution of galaxies and clusters by accretion, mergers, star formation, and AGN feedback.