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 IIextragalactic jets are produced by magnetized accretion disk coronae in theergospheres of rotating black holes. While the jets are produced in theaccretion disk itself, the output power still is an increasing function of theblack hole angular momentum. For high enough spin, the black hole triggers themagnetic switch, producing highly-relativistic, kinetic-energy-dominated jetsinstead of Poynting-flux-dominated ones for lower spin. The coronal massdensities needed to trigger the switch at the observed FR break power are quitesmall ($\sim 10^{-15} g cm^{-3}$), implying that the source of the jet materialmay be either a pair plasma or very tenuous electron-proton corona, not themain accretion disk itself. The model explains the differences in morphology and Mach number between FR Iand II sources and the observed trend for massive galaxies to undergo the FRI/II transition at higher radio power. It also is consistent with the energycontent of extended radio lobes and explains why, because of black holespindown, the space density of FR II sources should evolve more rapidly thanthat of FR I sources. If the present model is correct, then the ensemble average speed ofparsec-scale jets in sources distinguished by their FR I morphology (notluminosity) should be distinctly slower than that for sources with FR IImorphology. The model also suggests the existence of a population ofhigh-redshift, sub-mJy FR I and II radio sources associated with spiral orpre-spiral galaxies that flared once when their black holes were formed butwere never again re-kindled by mergers.Comment: 14 pages, 2 figures, final version to appear in Sept Ap