Odd Radio Circles (ORCs): probing radio features at group scales with simulations

Abstract:

Extragalactic radio astronomy has proven to be immensely useful in the study of high energy particle physics by constraining the acceleration of cosmic ray electrons and their subsequent synchrotron emission. Investigations of galaxy clusters have shown to be particularly fruitful, since such massive environments grant favourable conditions for efficient particle acceleration via merger driven shocks. Yet, a new class of radio objects discovered only a few years ago imposes the question if also merger shocks in less massive systems can accelerate CR electrons to GeV energies. Specifically, a growing number of observations is pointing towards the existence of ringlike features around small galaxy groups  and even Milky Way-like objects, accordingly titled Odd Radio Circles (ORCs). Recent works have proposed that these could be interpreted as radio analogs to the merger shocks observed in more massive systems. However, ORCs are appearing in a colder and less magnetized medium than the radio features in galaxy clusters, challenging particle acceleration  mechanisms in more constraining environments. Now, by interpreting multiple observed ORCs at different stages as an evolutional time sequence and comparing them to simulations of such systems, one can infer underlying properties of these unique features, while simultaneously testing the limits of diffusive shock acceleration (DSA). To this end, I will present the first of its kind magnetohydrodynamical simulation of a galaxy group, utilising on-the-fly treatment of spectral cosmic rays. I will study the emerging radio features and discuss both similarities and differences to the observed counterparts. Finally, I will review the relative importance of DSA in the formation of ORCs compared to galactic sources of high energy particles, such as supernovae and AGN.

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