Insights on the origin of ORCs from cosmological simulations by Klaus Dolag et al. on Wednesday 30 November

We investigate shock structures driven by merger events in high-resolution

simulations that result in a galaxy with a virial mass M ~ 1e12 Msol. We find

that the sizes and morphologies of the internal shocks resemble remarkably well

those of the newly-detected class of odd radio circles (ORCs). This would

highlight a so-far overlooked mechanism to form radio rings, shells and even

more complex structures around elliptical galaxies. Mach numbers of M = 2-3 for

such internal shocks are in agreement with the spectral indices of the observed

ORCs. We estimate that ~5 percent of galaxies could undergo merger events which

occasionally lead to such prominent structures within the galactic halo during

their lifetime, explaining the low number of observed ORCs. At the time when

the shock structures are matching the physical sizes of the observed ORCs, the

central galaxies are typically classified as early-type galaxies, with no

ongoing star formation, in agreement with observational findings. Although the

energy released by such mergers could potentially power the observed radio

luminosity already in Milky-Way-like halos, our predicted luminosity from a

simple, direct shock acceleration model is much smaller than the observed one.

Considering the estimated number of candidates from our cosmological

simulations and the higher observed energies, we suggest that the proposed

scenario is more likely for halo masses around 1e13 Msol in agreement with the

observed stellar masses of the galaxies at the center of ORCs. Such shocks

might be detectable with next generation X-ray instruments like the Line

Emission Mapper (LEM).

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2208.15003v2