The nuclei of most galaxies host supermassive black holes with millions or even billions of solar-masses of material. Material in the vicinity of such black holes can accrete onto a torus of dust and gas around the black hole, and when that happens the nuclei radiate powerfully across the full spectrum. These active galactic nuclei (AGN) are among the most dramatic and interesting phenomena in extragalactic astronomy, and puzzling as well. Exactly what turns the accretion on or off is not understood, nor is how the associated processes produce the emission, generate jets of particles, or influence star formation in the galaxy.
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