Letter to the Editor

Spitzer IRS spectroscopy of 3CR radio galaxies and quasars: testing the unified schemes

¹ Astronomisches Institut, Ruhr-Universität, Universitätsstr. 150 / NA7, 44780 Bochum, Germany e-mail: haas@astro.rub.de
² European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching b. München, Germany
³ IPAC, California Institute of Technology (Caltech), Pasadena, CA 91125, USA
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Postfach 2024, 53010 Bonn, Germany

Received: 18 July 2005
Accepted: 11 September 2005

Abstract

With the Spitzer Space Telescope Infrared Spectrograph (IRS) we have observed seven powerful FR 2 radio galaxies and seven quasars. Both samples, the galaxies and the quasars, are comparable in isotropic 178 MHz luminosity (10 W/Hz   W/Hz) and in redshift range (0.05 z 1.5). We find for both samples similar distributions in the luminosity ratios of high- to low-excitation lines ([NeV]_{14.3~ \mu m} / [NeII]_{12.8~ \mu m}) and of high-excitation line to radio power ([NeV]_{24.3~ \mu m} / P_178\,MHz). This solves the long debate about the apparent difference of quasars and radio galaxies in favor of the orientation-dependent unified schemes. Furthermore, the luminosity ratio [OIII]_{500.7~ nm} / [OIV]_{25.9~ \mu m} of most galaxies is by a factor of ten lower than that of the quasars. This suggests that the optical emission from the central NLR is essentially absorbed (A_V  3) in the powerful FR 2 galaxies and that the [OIII]_{500.7~ nm} luminosity does not serve as isotropic tracer for testing the unified schemes.