Beam models for radio sources. III - Offset sources and single jets

Abstract

The early stages of radio source evolution are studied using a modified version of the previously developed approach to the twin beam model. A continuous source of energy and relativistic plasma lies within a cloud of confining gas in an active galactic nucleus. The nature of the 'engine' is not specified, but an initially symmetric ejection of plasma is assumed. The boundary between the relativistic plasma and the confining medium is calculated using several approximations, and the source's nondimensional reduced luminosity, its offset from the confining cloud's center, and the cloud's eccentricity are treated as the key parameters, as the other properties of the cloud can be coupled with the engine's power to give the reduced luminosity. The conclusions are as follows: sources with low energy tend to form bubbles while those with higher luminosities form jets; larger offsets increase the probability of direct jet formation; higher eccentricities provide better collimation, as do smaller offsets; even low offsets produce single jets or plasmoid streams in one direction.