Abstract
Star formation and supermassive black hole growth in galaxies appear to be self-limiting. The mechanisms for self-regulation are known as feedback. Cosmic rays, the relativistic particle component of interstellar and intergalactic plasma, are among the agents of feedback. Because cosmic rays are virtually collisionless in the plasma environments of interest, their interaction with the ambient medium is primarily mediated by large scale magnetic fields and kinetic scale plasma waves. Because kinetic scales are much smaller than global scales, this interaction is most conveniently described by fluid models. In this paper, I discuss the kinetic theory and the classical theory of cosmic ray hydrodynamics (CCRH) which follows from assuming cosmic rays interact only with self-excited waves. I generalize CCRH to generalized cosmic ray hydrodynamics, which accommodates interactions with extrinsic turbulence, present examples of cosmic ray feedback, and assess where progress is needed.
Highlights
Because cosmic rays are virtually collisionless in the plasma environments of interest, their interaction with the ambient medium is primarily mediated by large scale magnetic fields and kinetic scale plasma waves
I discuss the kinetic theory and the classical theory of cosmic ray hydrodynamics (CCRH) which follows from assuming cosmic rays interact only with self-excited waves
Cosmic rays are implicated in feedback, the mechanisms by which star formation and supermassive black hole growth regulate themselves by deposition of energy and momentum in the ambient medium
Summary
The role of cosmic rays in the dynamics and energy balance of interstellar gas, and, by implication, the aspects of galaxy evolution that are driven by the gas, such as star formation, change of chemical composition, and growth and maintenance of galactic magnetic fields, is of considerable interest. The protons of a few GeV, where most of the cosmic ray energy density resides, are essentially collisionless. Their coupling is mediated by the ambient magnetic field and by kinetic scale waves and instabilities. The first studies of cosmic ray coupling concerned transport, i.e., diffusion of cosmic rays through fluctuating magnetic fields.. Such is the level of activity in this field that in order to keep the paper of manageable length it has been necessary to omit many interesting subtopics, and references
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