A link between black holes and new stars
Supermassive black holes (those with millions to billions of solar-masses) are thought to reside at the centers of most galaxies. These black holes must have undergone periods of intense accretion activity to grow to their large sizes, during which times they would be observed as active galactic nuclei and especially bright in X-rays. The masses of these black holes have been found to correlate closely with the cumulative mass of the stars that surround the galaxy in its bulge (and which are bound by gravitational forces). In a second discovery, it turns out that the activity of accretion for black holes and of star formation both peaked during a similar cosmic epoch, about three billion years ago, and then declined to the present day. These two results seem to suggest parallel evolutionary paths for the growth of the black holes and stellar mass, but the physical mechanisms that drive this...