Supermassive Black Holes in the Distant Universe (Astrophysics and Space Science Library Vol.308) (2004. X, 302 p.)

Oftentimes, however, the supermassive black holes that lie at the centers of AGN are cocooned in gas and dust that absorb the emitted low­ energy X-rays and the optical and ultraviolet light, hiding the black hole from view at these wavelengths.