Unified schemes for active galaxies: a clue from the missing Fanaroff-Riley type I quasar population

Abstract

We link the lack of FR I type structure among quasars to the void of radio loud quasars below a critical disk luminosity of ~10^46 erg/sec in the PG sample. We argue that the opening angle of the obscuring torus in radio loud quasars depends on the power of the central engine, approaching the jet's beaming angle near the FR I/FR II break. Consequently, low power radio quasars would either be classified as radio galaxies (FR I) or strongly core-boosted (BL Lac) object, depending on the aspect angle, and no conspicuous transitional population would be expected for FR I sources. A closing torus with decreasing power would not only obscure the optical nucleus for most aspect angles but would also enhance the entrainment of the cool torus material into the jet stream, causing obscuration along the jet's periphery, as well as the jet's deceleration to form a FR I source. Above a critical luminosity, the wider torus allows for FR II type jets and visibility of the nuclear optical emission, characteristic of radio loud quasars. Apparently, at the same engine power the torus opening in radio weak quasars and Seyferts is substantially wider than in radio loud quasars, probably because of different dynamics or feeding mechanisms in disk and elliptical galaxies. This provides a clue for the radio-loud/radio-quiet dichotomy of quasars if the the jet/torus interaction leads to injection of relativistic e^+/- pairs via p-p collisions. Strong jet/torus interaction may lead to a substantial injection of secondary pairs and collimation in radio loud quasars, while weak interaction in radio weak quasars leads neither to pair injection nor to good collimation.