A Magnetically-switched, Rotating Black Hole Model for the Production of Extragalactic Radio Jets and the Fanaroff and Riley Class Division

A model is presented in which both Fanaroff and Riley class I and II extragalactic jets are produced by magnetized accretion disk coronae in the ergospheres of rotating black holes. It employs a hybrid version of the Blandford-Payne and Blandford-Znajek magnetohydrodynamic mechanisms (similar to the Punsly-Coroniti model, with the addition of a metric-shear-driven dynamo) and a generalized form of the magnetic switch. While the jets are produced in the ergospheric accretion disk itself, the output power still is an increasing function of the black hole angular momentum. For high enough spin, the black hole triggers the magnetic switch, producing highly-relativistic, kinetic-energy-dominated jets instead of Poynting-flux-dominated ones for lower spin. The coronal mass densities needed to trigger the switch at the observed FR break power are quite small (∼ 10 g cm), implying that the source of the jet material may be either a pair plasma or very tenuous electron-proton corona, not the main accretion disk itself. The model explains the differences in morphology and Mach number between FR I and II sources and the observed trend for massive galaxies (which contain more massive black holes) to undergo the FR I/II transition at higher radio power. It also is consistent with the energy content of extended radio lobes and explains why, because of black hole spindown, the space density of FR II sources should evolve more rapidly than that of FR I sources. A specific observational test is proposed to distinguish between models like this one, in which the FR I/II division arises from processes near the black hole, and models like Bicknell’s, in which the difference is produced by processes in the host galaxy’s interstellar medium. If the present model is correct, then the ensemble average speed of parsec-scale jets in sources distinguished by their FR I morphology (not luminosity) should be distinctly slower than that for sources with FR II morphology. The model also suggests the existence of a population of high-redshift, sub-mJy FR I and II radio sources associated with spiral or pre-spiral galaxies that flared once when their black holes were formed but were never again re-kindled by mergers. Subject headings: hydrodynamics — magnetohydrodynamics: MHD — relativity — stars: rotation