Letter to the Editor

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^⋆

¹ INAF - Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: capetti@oato.inaf.it
² INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ School of Mathematical and Physical Sciences, University of Sussex, Falmer, Brighton BN1 9RH, UK
⁴ Department of Physics, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623, USA
⁵ SISSA-ISAS, via Bonomea 265, 34136 Trieste, Italy
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ INAF - Istituto di Radio Astronomia, via P. Gobetti 101, 40129 Bologna, Italy

Received: 29 October 2010
Accepted: 3 January 2011

Abstract

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] /Hβ 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.