Exploring the Non-thermal Emission from Galaxy Clusters: Radio Halos, Relics, and the Missing Gamma Rays

• Applicant: Professor Dr. Christoph Pfrommer
• Subject Area: Astrophysics and Astronomy
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 444932369

The observation of extended diffuse structures of radio synchrotron emission (called 'radio halos' and 'radio relics') in galaxy clusters provides unambiguous evidence for efficient acceleration of relativistic electrons and magnetic fields that permeate these entire systems. There has been enormous progress over the last decade in understanding this emission from giant radio halos and relics, which both occur in merging clusters. However, each interpretation of the observed phenomena faces several theoretical problems or contradicts basic paradigms of plasma physics. Most puzzling, if relativistic protons (called cosmic rays) are as efficiently accelerated as relativistic electrons, then they should accumulate in clusters to produce observable diffuse gamma-ray emission by hadronically interacting with the cluster gas. It is unclear why this emission has not yet been observed. Likewise, observed correlations of radio luminosity with thermal cluster observables need to be consolidated with high-sensitivity cluster surveys in the radio that exhibit a controlled selection function in order to better quantify the frequency of occurrence and to understand the underlying plasma processes. This proposal aims at advancing the state-of-the-art in cosmological cluster simulations of non-thermal processes and comparing the results to the data from the MeerKAT Exploration of Relics, Giant Halos, and Extragalactic Radio Sources (MERGHERS) survey. We will run a series of zoom-in simulations of homogeneously selected galaxy clusters spanning a large mass range. To this end, we will use the cosmological, unstructured moving mesh code AREPO with the modern galaxy formation model FABLE, that we will further improve as part of this proposal. We will take advantage of our recent substantial advances in simulation code developments, which includes of cosmic ray hydrodynamics (in its one- and two-moment formulation), diffusive shock acceleration that depends on magnetic obliquity, and a description of the evolution of the cosmic ray electron spectrum by integrating the Fokker–Planck equation. These simulations will enable us to provide the most realistic synthetic mock radio images and spectra of these phenomena to date and by comparing to MeerKAT observations hopefully solve some of these problems. The proposal will yield substantial scientific return of the German investments in MeerKAT and also pave the road for Germany to play a leading role in the theoretical interpretation of diffuse non-thermal cluster emission in the SKA era.

• DFG Programme: Research Grants
• International Connection: South Africa
• Cooperation Partner: Kenda Knowles, Ph.D.