Cosmological Zoom-In Simulation of Odd Radio Circles as Merger-Driven Shocks in Galaxy Groups

TL;DR

This study tests whether major mergers in galaxy groups can produce ORCs via merger-driven shocks in the circumgalactic medium. Using a magnetohydrodynamic zoom-in simulation with on-the-fly cosmic ray treatment, the authors derive self-consistent X-ray and radio properties and compare them to observations, finding ring-like shock structures that resemble observed ORCs when viewed perpendicularly to the merger axis. The results show that the X-ray CGM properties align with observed group halos and the mock radio morphologies match the ring-like, large-extents of ORCs, but the simulated radio luminosity is far too low and the polarization too high, indicating missing fossil CR populations and possibly stronger magnetic-field amplification are needed. Overall, the work provides a prototype for ORC formation via merger shocks in galaxy groups and highlights key physics (fossil CRs, magnetic-field amplification) required to fully reproduce the observed radio properties.

Abstract

Odd Radio Circles (ORCs) are a new class of distinct radio objects that has recently been discovered. The origin of these features is yet unclear because their peculiar properties are a challenge for our current understanding of astrophysical sources for diffuse radio emission. In this work we test the feasibility of major mergers in galaxy groups as a possible formation channel for ORCs. By modeling the assembly of a massive galaxy group with a final virial mass of $M_{200}\sim 10^{13}\, \rm M_\odot$ in a magnetohydrodynamic zoom-in simulation with on-the-fly cosmic ray treatment, we are able to derive the X-ray and radio properties of the system self-consistently and compare them to observations. We show that the X-ray properties for the simulated system are agreeing with characteristics of observed galaxy groups in the regarded mass range, legitimating the comparison between the radio properties of the simulated halo and those of observed ORCs. A major merger between two galaxies in the simulation is triggering a series of strong shocks in the circumgalactic medium, which in unison are forming a ring if the line of sight is perpendicular to the merger axis. The shock is rapidly expanding in radial direction and quickly reaches the virial radius of the halo. This formation channel can hence readily explain the morphology and large extent of ORCs. However, the inferred radio luminosity of these features is lower than for observed counterparts, while the degree of polarization seems to be systematically overpredicted by the simulation. Fossil cosmic ray populations from AGN and stellar feedback might be necessary to explain the full extent of the radio properties of ORCs, since diffusive shock acceleration was the only source term for non-thermal electrons considered in this work.

Figures (12)

• Figure 1: Time evolution of the circumgalactic medium of the simulated galaxy group, showing the central region at three successive moments close to the merger event. The two columns display maps of the X-ray surface brightness and the projected energy density of cosmic ray electrons, while the two panels in each column represent two orthogonal sight lines. For the X-ray map, we assume a distance of $z=0.017$. The scale bar (the top left panel) indicates 200k. The time labels provide the elapsed time since shortly before the merger event.
• Figure 2: Radial profiles of the hot X-ray emitting gas properties in the circumgalactic medium of the merging galaxies. Left, from top to bottom: X-ray surface brightness maps shortly before and after the merger event (see \ref{['subsec:xray_CReEnergy']}). The blue- and red-shaded regions in each panel indicate the extent of the 3D cones used to compute the radial profiles of the gas number density $n(r)$ (centre column) and temperature $T(r)$ (right column) for the two merging galaxies. The cones are centred on the local potential minimum of the respective dark matter halo.
• Figure 3: Mock images of the simulated ORC at $t=73M$. Left and centre: Synchrotron flux density at four frequencies (indicated in the top-left corner of each panel). The images are smoothed with a Gaussian kernel length matching the respective resolution of the telescope, shown in the bottom of each panel. Right: Spectral index derived from the two left panels. We applied a circular mask in these images to highlight the circular shape, where its extent is indicated by the dashed line in the upper left panel.
• Figure 4: Total synchrotron power and spectrum $P_\nu$ over time $t$. Left: Time-dependent luminosity profile for varying frequencies $\nu$. The solid black line represents $\nu=944MHz$. The vertical dashed lines indicate the three time steps at which the simulated ORC was visualised in \ref{['fig:Xray_CRe']}. Right: Associated emission spectrum for four equidistant time steps. Here, the dashed lines indicate the local spectral index according to $P_\nu\propto\nu^{-\alpha}$.
• Figure 5: Polarisation fraction $\pi_\nu$, as given by \ref{['eq:polfrac']} for $\nu=944MHz$ at $t=73M$. The Stokes parameters were smoothed according to the resolution of ASKAP at the frequency indicated in the bottom left corner (see text for details). The horizontal line in the top right corner displays a size of 200k.