Cosmic ray feedback in galaxies and galaxy clusters

Abstract

Understanding the physical mechanisms that control galaxy formation is a fundamental challenge in contemporary astrophysics. Recent advances in the field of astrophysical feedback strongly suggest that cosmic rays (CRs) may be crucially important for our understanding of cosmological galaxy formation and evolution. The appealing features of CRs are their relatively long cooling times and relatively strong dynamical coupling to the gas. In galaxies, CRs can be close to equipartition with the thermal, magnetic, and turbulent energy density in the interstellar medium, and can be dynamically very important in driving large-scale galactic winds. Similarly, CRs may provide a significant contribution to the pressure in the circumgalactic medium. In galaxy clusters, CRs may play a key role in addressing the classic cooling flow problem by facilitating efficient heating of the intracluster medium and preventing excessive star formation. Overall, the underlying physics of CR interactions with plasmas exhibit broad parallels across the entire range of scales characteristic of the interstellar, circumgalactic, and intracluster media. Here we present a review of the state-of-the-art of this field and provide a pedagogical introduction to cosmic ray plasma physics, including the physics of wave–particle interactions, acceleration processes, CR spatial and spectral transport, and important cooling processes. The field is ripe for discovery and will remain the subject of intense theoretical, computational, and observational research over the next decade with profound implications for the interpretation of the observations of stellar and supermassive black hole feedback spanning the entire width of the electromagnetic spectrum and multi-messenger data.

Deep learning cosmic ray transport from density maps of simulated, turbulent gas

“…In this paper, we concern ourselves with how CRs, on large scales 4 , transfer momentum and energy with the surrounding non-relativistic gas, which is very dependent on the highly uncertain and scale-dependent motion of the CR fluid relative to the background gas. On very large scales, we can average over many of the smaller, turbulent fluctuations in the Universe, and therefore average over the tangled magnetic field lines that guide and scatter CRs.…”

“…The fine-grained transport depends on the source and type of hydromagnetic waves that scatter and confine CRs (see e.g. [3,4] for recent reviews). If CRs scatter off compressible fast modes [5] that are created by external turbulence and cascade down to the CR gyroscale (≈0.1 AU for a GeV CR in the Milky Way), then fine-grained CR transport is believed to be diffusive and predominantly parallel to the local magnetic field.…”

“…Despite representing only a billionth of all particles in the Milky Way, CRs on a whole have as much energy as normal, non-relativistic gas [8], and it is clear from a veritable explosion of work in the last decade (e.g. [4,[9][10][11][12][13][14][15][16][17][18]) that the content of CRs in various astrophysical environments and the dynamical and thermodynamical influence of CRs on the surrounding gas sensitively depend on this transport. For example, in simulations of the Large Magellanic Cloud (LMC), a neighboring satellite galaxy of the Milky Way, if one allows CRs to stream, the galaxy remains largely intact over long periods of time as CRs easily lose pressure and escape the galaxy; however, if one replaces streaming with a small diffusivity instead, CRs build up a large pressure gradient in the galaxy and expel gas in a large-scale 'galactic wind' [19].…”

Transport in Astrophysics: VI. Ultra-High Energy Cosmic Rays

“…A Monte Carlo program, CRPropa 3.2, has been built which simulates the propagation of high-energy CRs in the Universe [11]. The CRs provide a significant contribution to the pressure in the circumgalactic medium [12]. The diffuse radio emission from galaxy clusters is a manifestation of the same cosmic-ray ion population [13].…”