A relativistic mixing-layer model for jets in low-luminosity radio galaxies

TL;DR

This paper develops an analytical relativistic jet model for FRI radio galaxies, describing how jets entrain material, expand, and decelerate, with application to the source 3C31 showing good agreement with observations.

Contribution

The paper introduces a new analytical model for relativistic jets in FRI galaxies that includes shear layer dynamics and entrainment, extending previous models with detailed conservation law applications.

Findings

Model successfully applied to 3C31 with consistent jet power estimates.

Predicted velocity profiles match VLA observations.

Shape of the laminar core can be tested with higher-resolution imaging.

Abstract

We present an analytical model for jets in Fanaroff & Riley Class I (FRI) radio galaxies, in which an initially laminar, relativistic flow is surrounded by a shear layer. We apply the appropriate conservation laws to constrain the jet parameters, starting the model where the radio emission is observed to brighten abruptly. We assume that the laminar flow fills the jet there and that pressure balance with the surroundings is maintained from that point outwards. Entrainment continuously injects new material into the jet and forms a shear layer, which contains material from both the environment and the laminar core. The shear layer expands rapidly with distance until finally the core disappears, and all of the material is mixed into the shear layer. Beyond this point, the shear layer expands in a cone and decelerates smoothly. We apply our model to the well-observed FRI source 3C31 and show that there is a self-consistent solution. We derive the jet power, together with the variations of mass flux and and entrainment rate with distance from the nucleus. The predicted variation of bulk velocity with distance in the outer parts of the jets is in good agreement with model fits to VLA observations. Our prediction for the shape of the laminar core can be tested with higher-resolution imaging.