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.