PRODIGIES: Polarised Radio Observations of DIGItal Extragalactic Shocks

• Applicant: Professor Dr. Marcus Brüggen, Ph.D., since 1/2023
• Subject Area: Astrophysics and Astronomy
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 441694982

Radio relics belong to the most spectacular sources in the radio sky. These large and mostly elongated sources are found the periphery of galaxy clusters where they emit synchrotron emission, illuminating large-scale magnetic fields and cosmic-ray electrons. Moreover, they are mostly polarised, with average degrees of polarisation up to 60 %. X-ray observations show that radio relics are co-located with shock waves, suggesting that shock waves accelerate electrons from the thermal pool into the relativistic regime. Still, several unsolved questions in the understanding of radio relics exist. Mainly, the origin of the large-scale magnetic fields and the exact particle acceleration mechanism are still unknown. Theoretical studies showed that the acceleration does not only depend on the shock strength but also on the conditions of the local magnetic fields. Hence, proper knowledge of the magnetic fields is needed for the complete understanding of radio relics. The combination of low- and high-frequency radio observations is a perfect tool to derive a three-dimensional picture of the magnetic fields. The observed luminosities are a measurement of the local magnetic field strength. In particular, low-frequency observations, that are sensitive to lower energetic particles, can provide information on the strengths and variations of the downstreammagnetic fields. However, the polarisation signal, that is best measured at high frequencies, gives the orientation of the magnetic field in the plane of the sky. This is complemented by Faraday rotation measurements, that give the strength and direction of the magnetic field along the line-of-sight. Nowadays, several radio telescopes are deployed to study radio relics across all radio wavelengths and cosmological simulations are an essential tool to connect these observations and theory. In order to keep up with the advances in observations, simulations need to model a larger variety of phenomena. In particular, a net advancement in their ability to mimic complex radio emission is presently missing, and this is the motivation for this project proposal. I will use state-of-the-art cosmological simulations of magnetic fields and particle acceleration to model the diffuse emission of radio relics. The main parts of the project are:1) production of a catalogue of simulated radio relics to study the statistical properties and to help to interpret observations2) including re-acceleration to improve the modelling of cosmic-ray electrons 3) using active galaxies as sources of seed electrons4) combining simulations and observations to constrain the cosmic-ray energy budget and the acceleration efficiency5) exposing possible limitations of the observational techniques to study radio relicsHence, I will help to answer two key and unsolved questions in the understanding of radio relics: 1) What are the exact acceleration efficiencies? 2) How do relics obtain their high degree of polarisation?

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Dr. Denis Wittor, until 12/2022