Overpressured cocoons in extragalactic radio sources

Abstract

We show 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 quantitites, as we demonstrate for the radio galaxy Cygnus A. We suggest that overpressured cocoons in high-redshift radio galaxies engulf and compress circumgalactic clouds, driving them over the Jeans limit and triggering star formation. Since overpressured cocoons have a much larger ratio of width to length than is indiciated by the widths of observed radio lobes, they can trigger star formation at a much higher rate than can either the direct impact of the jet or entertainment in the radio lobe. We propose that this process leads to the observed alignments of optical continuum emission with radio source axes.e jet or entrainment in the radio lobe. We propose that this process leads to the observed alighments of optical continuum emission with radio source axes.