The Physalis system: Discovery of ORC-like radio shells around a massive pair of interacting early-type galaxies with offset X-ray emission

TL;DR

This paper reports the discovery of large radio shells and offset X-ray emission around a pair of interacting galaxies, suggesting merger shocks and AGN activity as their origin, and introduces a new class of odd radio circles (ORCs).

Contribution

It presents the first identification of ORC-like radio shells around a galaxy pair with detailed multi-wavelength analysis and simulations, proposing a new formation mechanism involving merger shocks and AGN activity.

Findings

Discovery of ~145 kpc radio shells around galaxy pair

Offset X-ray emission indicating merger shocks

Simulations support merger shocks and AGN activity as origin

Abstract

We present the discovery of large radio shells around a massive pair of interacting galaxies and extended diffuse X-ray emission within the shells. The radio data were obtained with the Australian Square Kilometer Array Pathfinder (ASKAP) in two frequency bands centred at 944 MHz and 1.4 GHz, respectively, while the X-ray data are from the XMM-Newton observatory. The host galaxy pair, which consists of the early-type galaxies ESO 184-G042 and LEDA 418116, is part of a loose group at a distance of only 75 Mpc (redshift z = 0.017). The observed outer radio shells (diameter ~ 145 kpc) and ridge-like central emission of the system, ASKAP J1914-5433 (Physalis), are likely associated with merger shocks during the formation of the central galaxy (ESO 184-G042) and resemble the new class of odd radio circles (ORCs). This is supported by the brightest X-ray emission found offset from the centre of the Physalis system, instead centered at the less massive galaxy, LEDA 418116. The host galaxy pair is embedded in an irregular envelope of diffuse light, highlighting on-going interactions. We complement our combined radio and X-ray study with high-resolution simulations of the circumgalactic medium (CGM) around galaxy mergers from the Magneticum project to analyse the evolutionary state of the Physalis system. We argue that ORCs / radio shells could be produced by a combination of energy release from the central AGN and subsequent lightening up in radio emission by merger shocks traveling through the CGM of these systems.