2013A&A...551A..55C

From an optical spectroscopic survey of 3CR radiogalaxies (RGs) with z<0.3, we discovered three objects characterized by an extremely low level of gas excitation and a large deficit of line emission with respect to RGs of similar radio luminosity. We interpreted these objects as relic active galactic nuclei (AGN), i.e., sources observed after a large drop in their nuclear activity. We here present new spectroscopic observations for these three galaxies and for a group of ``candidate'' relics. None of the candidates can be convincingly confirmed. From the new data for the three relics, we estimate the density of the line-emitting gas. This enables us to explore the temporal evolution of the line ratios after the AGN ``death''. The characteristic timescale is the light-crossing time of the emission-line region, a few ∼10³ years, too short to correspond to a substantial population of relic RGs. Additional mechanisms of gas ionization, such as ``relic shocks'' from their past high power phase or stellar sources, should also be considered to account for the spectroscopic properties of the relic RGs. Relic RGs appear to be a mixed bag of sources in different phases of evolution, including AGN recently (∼10⁴ years ago) quenched, galaxies that have been inactive for at least ∼10⁶ years, and objects caught during the transition from a powerful RG to a low power FR I source.