  Virtually all massive galaxies, including our own, host central black holes
ranging in mass from millions to billions of solar masses. The growth of these
black holes releases vast amounts of energy that powers quasars and other
weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by
the host galaxy, could halt star formation by heating and ejecting ambient gas.
A central question in galaxy evolution is the degree to which this process has
caused the decline of star formation in large elliptical galaxies, which
typically have little cold gas and few young stars, unlike spiral galaxies.
