Understanding the nature of FR II optical nuclei: A new diagnostic plane for radio galaxies^*

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
² Instituto di Radioastronomia del CNR, Via P. Gobetti 101, 40129 Bologna, Italy
³ Instituto Nazionale di Astrofisica, Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Torino, Italy
⁴ SISSA/ISAS, Via Beirut 2-4, 34014 Trieste, Italy

Corresponding author: M. Chiaberge, chiab@stsci.edu

Received: 30 November 2001
Accepted: 28 June 2002

Abstract

We extend our study of the nuclei of 3CR FR II radio galaxies through HST optical images up to . In the majority of them an unresolved nucleus (central compact core, CCC) is found. We analyze their position in the plane formed by the radio and optical nuclear luminosities in relation to their optical spectral properties. The broad–lined objects (BLO) have the brightest nuclei: they are present only at optical luminosities  erg s^-1 which we suggest might represent a threshold in the radiative efficiency combined with a small range of black hole masses. About of the high and low excitation galaxies (HEG and LEG) show CCC which resemble those previously detected in FR I galaxies, in apparent contrast to the unification model. The equivalent width of the [OIII] emission line (with respect to the nuclear luminosity) reveals the nature of these nuclei, indicating that the nuclei of HEG are obscured to our line of sight and only scattered radiation is observed. This implies that the population of FR II is composed of objects with different nuclear properties, and only a fraction of them can be unified with quasars.