Abstract
Particle acceleration in relativistic jets, to very high levels of energy, occurs at the expense of the dissipation of magnetic or kinetic energy. Therefore, understanding the processes that can trigger this dissipation is key to the characterization of the energy budgets and particle acceleration mechanisms in action in active galaxies. Instabilities and entrainment are two obvious candidates to trigger dissipation. On the one hand, supersonic, relativistic flows threaded by helical fields, as expected from the standard formation models of jets in supermassive black-holes, are unstable to a series of magnetohydrodynamical instabilities, such as the Kelvin–Helmholtz, current-driven, or possibly the pressure-driven instabilities. Furthermore, in the case of expanding jets, the Rayleigh–Taylor and centrifugal instabilities may also develop. With all these destabilizing processes in action, a natural question is to ask how can some jets keep their collimated structure along hundreds of kiloparsecs. On the other hand, the interaction of the jet with stars and clouds of gas that cross the flow in their orbits around the galactic centers provides another scenario in which kinetic energy can be efficiently converted into internal energy and particles can be accelerated to non-thermal energies. In this contribution, I review the conditions under which these processes occur and their role both in jet evolution and propagation and energy dissipation.
Highlights
Relativistic jets and outflows are a common feature in radio-emitting active galaxies, associated with accretion processes onto supermassive black-holes (SMBH, [1,2,3])
Riley [8], who set a morphological division of radio galaxies between those that have outer regions brighter than their centres, Fanaroff–Riley type II (FRII) radio galaxies, and those that are brighter close to the nucleus, Fanaroff–Riley type I (FRI) radio galaxies
Jets and diffuse emissivity that can be attached to dissipation of kinetic energy
Summary
Relativistic jets and outflows are a common feature in radio-emitting active galaxies, associated with accretion processes onto supermassive black-holes (SMBH, [1,2,3]). Both the acceleration at shearing layers [51,52,53,54,55,56,57,58,59] and at the interaction site with stars or clouds (e.g., [60,61,62,63,64,65,66]) were invoked to explain high-energy (HE) and very-high-energy (VHE) from radio galaxies, at different positions along the jet In this contribution, I summarize the physical processes that represent possible ways to dissipate energy in jets, namely the development of instabilities and mass entrainment. I argue that the conversion of the initial jet kinetic energy into internal energy via shocks and turbulent dissipation at the mixing regions could favor particle acceleration As a consequence, this could explain, at least to some extent, the diffuse HE emission detected in FRI sources, because these processes are more efficient in the case of decelerating jets.
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