Every galaxy is thought to harbor a supermassive black hole in the center, or nucleus, of the galaxy, and in active galaxies this black hole is fed by infalling matter. This “central engine” is typically surrounded by dusty molecular gas in a doughnut configuration, which hides the black hole and the infalling material from our view along certain viewing directions. The picture of a central engine plus obscuring doughnut is thought to apply to all accreting supermassive black holes, explaining the apparent variety of active galaxies from the very brightest quasars to the lower-luminosity radio galaxies under a single “unified scheme.”

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