Letter to the Editor

The galaxy group merger origin of the Cloverleaf odd radio circle system

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: ebulbul@mpe.mpg.de
² Hamburg Observatory, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
³ CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76 NSW 1710, Australia
⁴ School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia

Received: 8 March 2024
Accepted: 8 April 2024

Abstract

Odd radio circles (ORCs) are a newly discovered class of extended faint radio sources of unknown origin. We report the first detection of diffuse X-ray gas at the location of a low-redshift ORC (z = 0.046) known as Cloverleaf ORC. This observation was performed with the XMM-Newton X-ray telescope. The physical extent of the diffuse X-ray emission corresponds to a region of approximately 230 kpc by 160 kpc, lying perpendicular to the radio emission detected by ASKAP. The X-ray spectrum shows characteristics of thermal multiphase gas with temperatures of 1.10 ± 0.08 keV and 0.22 ± 0.01 keV and a central density of (4.9 ± 0.6)×10⁻⁴ cm⁻³, indicating that the Cloverleaf ORC resides in a low-mass galaxy group. Using X-ray observations, with hydrostatic equilibrium and isothermal assumptions, we measure the galaxy group to have a gas mass and a total mass of (7.7 ± 0.8)×10¹¹ M_⊙ and 2.6 ± 0.3 × 10¹³ M_⊙ within the overdensity radius R₅₀₀. The presence of a high-velocity subgroup identified in optical data, the orientation of the brightest cluster galaxy, the disturbed morphologies of galaxies toward the east of the Cloverleaf ORC, and the irregular morphology of the X-ray emission suggest that this system is undergoing a galaxy group merger. The radio power of the ORC could be explained by the shock reacceleration of fossil cosmic rays generated by a previous episode of black hole activity in the central active galactic nucleus.