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An accretion disk is a hallmark of an active galactic nucleus. Supermassive black holes accrete gas from the surrounding galaxy mostly coming from normal mass loss from stellar processes or from infall from the intergalactic medium. When this gas eventually works its way to within the central parsec, it forms a hot, dense, thick disk which shares the rotation of the embedded black hole. The characteristic temperature of this gas mixed with dust is a few tens of thousands of degrees Kelvin which means that its thermal radiation peaks in the far ultraviolet. This peak radiation is not visible from the ground and so must be observed from space. Astronomers have seen the characteristic hallmark of accretion disks in a number of AGN. The UV emission emerges from a distance of ten to a thousand times the Schwarzschild radius of the supermassive black hole.
