An optical spectroscopic survey of the 3CR sample of radio galaxies with
  z<0.3. IV. Discovery of the new spectroscopic class of relic radio galaxies
  From an optical spectroscopic survey of 3CR radio galaxies with z<0.3, we
discovered a new spectroscopic class of powerful radio-loud AGN. The defining
characteristics of these galaxies are that compared with radio galaxies of
similar radio luminosity they have: a [O III]\Hb ratio of ~0.5, indicative of
an extremely low level of gas excitation; a large deficit of [O III] emission
and radio core power. We interpret these objects as relic AGN, i.e. sources
that experienced a large drop in their level of nuclear activity, causing a
decrease in their nuclear and line luminosity. This class opens a novel
approach to investigating lifetimes and duty cycles of AGN.
