  Radiation, winds and jets from the active nucleus of a massive galaxy can
interact with its interstellar medium leading to ejection or heating of the
gas. This can terminate star formation in the galaxy and stifle accretion onto
the black hole. Such Active Galactic Nucleus (AGN) feedback can account for the
observed proportionality between central black hole and host galaxy mass.
Direct observational evidence for the radiative or quasar mode of feedback,
which occurs when the AGN is very luminous, has been difficult to obtain but is
accumulating from a few exceptional objects. Feedback from the kinetic or radio
mode, which uses the mechanical energy of radio-emitting jets often seen when
the AGN is operating at a lower level, is common in massive elliptical
galaxies. This mode is well observed directly through X-ray observations of the
central galaxies of cool core clusters in the form of bubbles in the hot
surrounding medium. The energy flow, which is roughly continuous, heats the hot
intracluster gas and reduces radiative cooling and subsequent star formation by
an order of magnitude. Feedback appears to maintain a long-lived
heating/cooling balance. Powerful, jetted radio outbursts may represent a
further mode of energy feedback which affect the cores of groups and
subclusters. New telescopes and instruments from the radio to X-ray bands will
come into operation over the next few years and lead to a rapid expansion in
observational data on all modes of AGN feedback.
