Exploring the spectroscopic properties of relic radiogalaxies

Abstract

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 thousand 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 (~10,000 years ago) quenched, galaxies that have been inactive for at least one million years, and objects caught during the transition from a powerful RG to a low power FRI source.