Fluid jets in radio sources

An axisymmetric beam of viscous fluid moving with relativistic bulk motion is considered to account for the observed radio jets in double radio sources. The confinement of the beam is due to its viscous interaction with the ambient medium. A fluid with a magnetic viscosity, based on the finite size of the Larmor radius, gives a rapid widening of the beam at the nozzle and a steady opening angle further along the beam; the same are observed properties of radio jets. Other sources of viscosity are investigated. The viscous beam shapes can be fitted well to the contours of the observed jets in NGC 315 and NGC 6251. The scaling of the beams to galactic sizes gives a measure of the magnetic field embedded in the fluid at the nozzle. A brightness map based on a simple temperature dependence of the volume emissivity agrees well with the jet of NGC 315.