Astronomy and Astrophysics, volume 551A, 55-55 (2013/3-1)
Exploring the spectroscopic properties of relic radiogalaxies.
CAPETTI A., ROBINSON A., BALDI R.D., BUTTIGLIONE S., AXON D.J., CELOTTI A. and CHIABERGE M.
Abstract (from CDS):
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.
galaxies: active - galaxies: jets - galaxies: elliptical and lenticular, cD - galaxies: nuclei
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