Recent observations from NASA's Chandra X-Ray Observatory suggest that supermassive black holes may be spinning matter near the luminal* velocity. According to Einstein's theory of relativity, a mass spinning at such a great rate would make local spacetime itself rotate, causing surrounding matter to spiral inwards. As shown in the illustration below, black holes also emit jets of matter, due to strong magnetic fields.
If black holes can't be directly seen, then how do astronomers know how fast they are spinning? Well, it turns out that the "accretion rate", or rate at which matter crosses the event horizon, is related to the energy of these jets of matter. By judging the energy in these jets of matter, astronomers can find the accretion rate of the black hole. Several black holes observed by Chandra were found to have incredible accretion rates. One such black hole swallowed 10 Earth-masses per month and emitted 50 times as much energy in one second than our sun produces in one year! By running this data through computer simulations, astronomers have estimated the speed of entrained matter to be barely subluminal*. According to Rodrigo Nemmen of Penn State University:
"We think these monster black holes are spinning close to the limit set by Einstein's theory of relativity, which means that they can drag material around them at close to the speed of light,"Such incredible rates of spin could be the rule, instead of the exception. It is speculated that these subluminal* spin rates could be common in supermassive black holes, and could explain the jets of matter themselves.
*Luminal velocity is the speed of light. It would then follow that subluminal velocities would be those just below the speed of light.