Active Galactic Nuclei: Unification, Blazar Variability and the Radio Galaxy/Cosmology Interface

Abstract

We first review some basic properties of the most important classes of active galactic nuclei (AGN), including quasars, blazars, Seyfert galaxies and radio galaxies. The most commonly accepted type of scheme designed to individually unify the radio-loud and radio-quiet categories of AGN is based upon three parameters: black hole mass, accretion rate, and our orientation to the accretion flow. Some recent evidence from optical microvariability of several classes of AGN points in favor of a strong unification scheme that unites both radio-loud and radio-quiet categories. An important question concerning the nature of blazars and other AGN whose jet emission appears to dominate their spectral energy distributions involves the velocities of those flows. A variety of apparently contradictory observations can be reconciled if such flows are ultrarelativistic but have an opening angle of a few degrees. Radio galaxies (RGs) were much more numerous at redshifts ~2 than they are today. Combining this fact with the realization that older RGs at such redshifts are very difficult to detect, and with cosmological simulations of the growth of structure in the universe has led us to propose that RG lobes have impacted a significant fraction of the cosmic web of baryons. These impacts may have triggered extensive star formation and perhaps even engendered new galaxies; they also probably played important roles in the spreading of magnetic fields and heavier elements into the intergalactic medium.