Overpressured cocoons in extragalactic radio sources

It is shown that the cocoons of shocked gas which surround powerful double radio sources can have significantly higher pressures than the surrounding intergalactic medium. The pressures can be high enough to confine the jets in these sources, obviating the need for magnetic confinement. The cocoon pressure and the age of a radio source may be estimated from observable quantities, as demonstrated here for the radio galaxy Cygnus A. It is suggested that overpressured cocoons in high-redshift radio galaxies engulf and compress circumgalactic clouds, driving them over the Jeans limit and triggering star formation. It is proposed that this process leads to the observed alignments of optical continuum emission with radio source axes.