Superdiscs in radio galaxies: jet-wind interactions

Abstract

Taking a clue from their sharp-edged (strip-like) morphology observed in several cases, a new mechanism is proposed for the formation of the emission gaps seen between the radio lobes of many powerful extragalactic double radio sources. Canonical understanding of the radio gaps invokes either blocking of the backflowing lobe plasma by the denser interstellar medium (ISM) of the host galaxy, or 'squeezing' of the radio bridge in the middle through buoyancy force exerted by either the ISM or the surrounding intracluster medium (ICM). These pictures encounter difficulties in explaining situations where the sharp-edged radio gaps associated with non-cluster radio galaxies have widths running into several tens (even hundreds) of kiloparsecs. More particularly, the required dense high-pressure ISM/ICM is likely to be lacking at least in the case of high-redshift radio galaxies. We propose here that radio emission gaps in at least such cases could arise from a dynamical interaction between the powerful thermal wind outflowing from the active galactic nucleus (AGN) and the backflowing synchrotron plasma in the two radio lobes, which occurs once the rapidly advancing jets have crossed out of the wind zone into the intergalactic medium. A simple analytical scheme is presented to explore the plausibility of the sideways confinement of the thermal wind by the radio lobe pair, which would 'freeze' pancake-shaped conduits in the space, along which the hot, metal-enriched wind from the AGN can escape (roughly orthogonal to the radio axis). Some other possible consequences of this scenario are pointed out.