Periodic Reporting for period 4 - CRAGSMAN (The Impact of Cosmic Rays on Galaxy and Cluster Formation) Reporting period: 2020-04-01 to 2021-03-31 Summary of the context and overall objectives of the project Understanding the physics of galaxy formation is arguably among the greatest problems in modern astrophysics. Recent cosmological simulations have demonstrated that "feedback" by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies, to slow down star formation to the small observed rates, to move gas and metals out of galaxies into the intergalactic medium, and to balance radiative cooling of the low-entropy gas at the centers of galaxy clusters. This progress still has the caveat that "feedback" was modeled empirically and involved tuning to observed global relations, substantially weakening the predictive power of hydrodynamic simulations. More problematic, these simulations neglected cosmic rays and magnetic fields, which provide a comparable pressure support in comparison to turbulence in our Galaxy, and are known to couple dynamically and thermally to the gas. Building on our previous successes in investigating these high-energy processes, we propose a comprehensive research program for studying the impact of cosmic rays on the formation of galaxies and clusters. To this end, we will study cosmic-ray propagation in magneto-hydrodynamic turbulence and improve the modeling of the plasma physics. This will enable us to perform the first consistent magneto-hydrodynamical and cosmic-ray simulations in a cosmological framework, something that has just now become technically feasible. Through the use of an advanced numerical technique that employs a moving mesh for calculating hydrodynamics, we will achieve an unprecedented combination of accuracy, resolution and physical completeness. We complement our theoretical efforts with a focused observational program on the non-thermal emission of galaxies and clusters, taking advantage of new capabilities at radio to gamma-ray wavelengths and neutrinos. This promises important and potentially transformative changes of our understanding of galaxy formation. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far Microscopic cosmic-ray transport in galaxies and galaxy clusters. We developed a theory of Alfvén-wave regulated cosmic ray hydrodynamics that is based on a moment expansion similar to radiation hydrodynamics and should enable predictive galaxy formation simulations. The theory is fully coupled to magneto-hydrodynamics, ensures energy and momentum conservation, follows CR streaming and diffusion along magnetic field lines in the self-confinement picture and accounts for kinetic physics. In the intra-cluster medium, particles collide so rarely that they have to follow the windings of the magnetic field lines. To understand the specifics of the transport of heat and momentum and how this interacts with cosmic rays, we have implemented the equations for Braginskii magneto-hydrodynamics, employing the efficient method of super-time stepping.Exploring the impact of cosmic rays on galaxy formation. We have implemented the physics of cosmic rays in the cosmological moving-mesh code Arepo and self-consistently simulated the injection of cosmic rays at supernova shocks, following cosmic-ray propagation in the interstellar medium, in galaxies and in the large-scale structure of the Universe. We simulated the effects of cosmic rays on the formation of Milky Way-mass galaxies in a cosmological context and find that global properties such as stellar mass and star formation rate vary little between simulations with various CR transport physics, whereas structural properties such as disc sizes, circum-galactic medium densities or temperatures can be strongly affected.Galactic magnetism - formation and dynamical impact on galaxies. High redshift, massive halos are observed to have sustained, high star formation rates, which require that the amount of cold gas in the halo is continuously replenished. Supersonic, cold streams could deliver the required high accretion rates but they should break up due to hydrodynamic instabilities. We find that magnetic fields can allow streams to survive much longer and to reach the inner galaxy.Formation and evolution of galaxy clusters. We have implemented an improved model for relativistic jets from active galactic nuclei at the centres of galaxy clusters and studied magnetic amplification, cosmic-ray transport and heating of the cooling dense intra-cluster medium. The simulations explain recent observations of the Sunyaev-Zel'dovich effect and turbulently broadened X-ray lines.Non-thermal signatures of galaxies and clusters. We developed efficient methods to follow cosmic ray electron and proton spectra in three-dimensional simulations of galaxy formation. A first application to the evolution of CR electron spectra in supernova explosions reveals a number of surprising findings on the electron acceleration efficiency and questions our current paradigm of diffusive shock acceleration. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The project has delivered extremely innovative new theories, numerical algorithms and implementations. For the first time, we implemented cosmic ray physics on an unstructured mesh. We have developed a new theory and implementation of Alfvén-wave regulated cosmic ray hydrodynamics that holds the promise to enable predictive galaxy formation simulations of cosmic ray feedback. Our development and implementation of efficient code for evolving cosmic ray electron and proton spectra in a fully three-dimensional cosmological magneto-hydrodynamics simulation enables a realistic modelling of non thermal observables.We have implemented the equations for Braginskii magneto-hydrodynamics in the moving-mesh code Arepo and are studying the physics of weakly collisional plasmas in cosmological galaxy cluster simulations. We have developed efficient code for particle-in-cell simulations to study kinetic plasma physics of the cosmic ray streaming instability, which will enable to construct improved closure schemes for our new Alfvén-wave regulated cosmic ray hydrodynamics, which are based on coarse-grained kinetic plasma simulations. Performing global dynamical simulations in the context of the interstellar, circum-galactic, and intra-cluster medium with these novel sub-grid approaches and comparing the results to observations enables us to obtain a direct feedback on how these schemes perform in large-scale simulations and a fast turnaround in case there are discrepancies. If successful, this could pave the road for a new influential approach to computational astrophysics in the context of galaxy and cluster formation.We have substantiated our original hypothesis that cosmic rays can actively modify the morphologies of galaxies and drive powerful outflows. At the scales of galaxy clusters, we confirmed that cosmic-ray heating can balance the fast radiative cooling at the centres of cooling galaxy clusters, thus mitigate star formation. Thus, cosmic ray feedback could be one of the key processes that regulates and governs galaxy formation. At the same time, this novel implementation enabled us to simulate non-thermal signatures of live galaxy models at radio and gamma-ray energies, thus discovering the cause of two fundamental relations of galaxies, the far-infrared--gamma-ray and far-infrared--radio correlations. Cosmic ray driven windssimulations of the interstellar medium Galaxy simulation with cosmic rays and magnetic fields Cosmic-ray mock observables in simulated galaxies AGN jet with cosmic rays thet interacts with the magnetized intracluster plasma
