What activates a supermassive black hole?

Astronomy : Most, if not all, large galaxies have a supermassive black hole in their center. In many galaxies, including the Milky Way, that black hole is quiet. In some galaxies, though, material falls into the black hole, giving off intense radiation as it does so. Until now, astronomers believed that this material was the result of two galaxies either merging or passing each other closely enough to disrupt some galactic material, which would then fall into the black hole, creating an active galactic nucleus. New data from the European Southern Observatory’s Very Large Telescope and the European Space Agency’s XMM-Newton x-ray space observatory indicate that this is not actually the case for most galaxies. Viola Allevato, of Germany’s Max Planck Institute for Plasma Physics in Garching, and colleagues have examined data for more than 600 active galaxies, and they found that most active galactic nuclei are fed by processes within the galaxy itself, such as disk instabilities and starbursts, rather than by galaxy collisions or mergers.