Observational evidence for the AGN paradigm

Abstract

The paradigm for active galactic nuclei (AGN) comprises a massive black hole accreting material through a disk. A competing model, in which no black hole is present and the luminosity is supplied by stellar processes (“starburst” model), has been developed for radio-quiet active galaxies (i.e. Seyfert galaxies, LINERs and radio-quiet quasars). In this article, I discuss recent observational work on the nuclear structures of Seyfert galaxies and LINERs with a view to discriminating between these two models. Seyfert galaxies are, to first order, cylindrically symmetric objects. Strong evidence for compact (<pc) nuclear disks is now available through observations of broad FeKα emission, broad, double-peaked optical recombination lines and H2O megamasers. Near infrared imaging reveals the outer parts of the disks. Bi-polar structures, collimated by the nuclear disks, include radio jets and lobes, high velocity ionized gas (the narrow line region) and ambient gas photoionized by a collimated nuclear radiation source. In the best studied Seyferts, the collimation of the radio jets is better than a few degrees. Compact accretion disks and highly collimated radio ejecta are consistent with the black hole model, but are neither observed in galaxies with (only) nuclear starbursts nor expected in starburst models. It remains to be shown, however, that accretion disks and highly collimated jets are prevalent in the population of radio-quiet AGN.