A theoretical model of Fanaroff-Riley I jets

Abstract

A fluid model of the jets in Fanaroff-Raily I class extragalactic radio sources based on the idea that they are turbulent low Mach number supersonic flows consisting of relativistic and nonrelativistic plasma has been developed. Numerical calculations of steady jets whose physical parameters correspond to observational constraints on velocity, density, Mach number, spreading rate, relativistic particles pressure and power of FR-I jets are described. The properties of the jets, propagating through typical atmosphere of elliptical galaxy are studied. It is shown that, in spite of essential deceleration, the jets transport bulk motion kinetic energy almost without losses. The low densities of the jets thermal plasma (∼10−6cm−3) result in negligible Faraday rotation inside the jets. Mass fluxes along the jets may be used only to study the external gas entraiment but not the ‘discharge’ of central ‘prime mover’. The jets can be conventionally subdivided into a supersonic fast core providing the main energy transport and a transonic slow sheath providing the main mass transfer along the jets. Variations of radio brightness along the jets are estimated. it is concluded that an effective re-acceleration of relativistic electrons is required to explain the observed brightness distribution of FR-I jets. A correlation between the value of the radial pressure gradient in the X-ray atmosphere of the galaxy and the rate of jet darkening is expected.