Models have suggested that the broad emission lines are formed in relatively dense clouds with half a light year of a black hole, with the broadening of the lines produced by the Doppler effect, but it is unknown as to whether the motions of the gas are caused by turbulence, rotation around the black hole, expansion, or contraction. Quasars and other active galaxies emit jets that extend far beyond the limits of the parent galaxy. Observations show that these jets originate within 3-30 light years of the parent quasar or galactic nucleus. Matter in the black hole's accretion disk is continually being depleted by falling into the black hole or being blown out from the galaxy in the form of jets. A quasar can only continue to radiate as long as there is gas available to replenish the accretion disk.
Extra matter is needed.
One possibility as to where this matter comes from is that very dense star clusters form near the centers of galaxies. These stars might then supply the fuel, either through gas that is lost during the normal course of stellar evolution by means of stellar winds and supernovae explosions, or becuase the tidal forces exerted by the black hole are strong enough to tear the stars apart. It has been observed that very bright quasars were much more common a few billion years ago than they are now. An explanation for newer quasars being darker is simply that they have used up all the possible fuel sources available to them. Many of the newer quasars have been found in galaxies that have recently been involved with a collison with another galaxy, showing that there is now no other source of fuel available to it, other than the energy given out by the black hole after two galaxies collide.