Why do active galactic nuclei differ? Available to Purchase

In some galaxies, including our own, the black hole sits inconspicuously in the middle. But in others, the black hole is orbited by a thick toroidal disk of hot gas. As material in the disk spirals toward the black hole’s event horizon, the disk’s inner region becomes so agitated and hot that it radiates copiously (see the accompanying artist’s impression). Such systems are known as active galactic nuclei, of which there are two broad classes. Type 1 AGNs have broad emission lines characteristic of hot, fast-moving matter. Type 2 AGNs have narrow lines characteristic of cool, slow-moving matter. Given that a thick torus shrouds the AGN engine, the two AGN types could conceivably differ only by viewing angle: Type 1s afford a face-on view of the hot, swirling inner disk; type 2s, an obscured view. Known as AGN unification, that appealing explanation can account for some of the differences...