Science@ NASA wrote (July 27, 2007): "Deep in the heart of the Milky Way galaxy lurks an extraordinary black hole. Astronomers call it "super-massive." It has been feeding on the core of our galaxy so long, the hole has accumulated more than a million Suns of mass inside its pinprick belly."
This beautiful artist rendered picture shows a fat torus of colder gas being slowly dragged into the accretion disk of hot to super-hot gas around the equator of the spinning black hole.
This action compresses the gas, which is spinning at differential rates, causing friction. Together these effects heat up the gas more and more as it swirls in, until it is hot enough to emit x-rays, just before it is 'swallowed'.
The Chandra X-ray Observatory picks up this radiation, even in very distant galaxies. It shows that the farther and hence the younger the galaxies that we observe, the more it feeds on the gas and eventually some stars as well. Central black holes in older galaxies have 'eaten' most available 'food' and are much quieter in x-rays.
Read the SCIENCE@NASA Article.