Astronomy & Astrophysics

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Issue		A&A Volume 442, Number 3, November II 2005
Page(s)		L39 - L43
Section		Letters
DOI		10.1051/0004-6361:200500185

A&A 442, L39-L43 (2005)
DOI: 10.1051/0004-6361:200500185

Letter

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

M. Haas¹, R. Siebenmorgen², B. Schulz³, E. Krügel⁴ and R. Chini¹

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 $^{\rm 26.5}$ W/Hz $\la$ $P_{\rm 178\,MHz} \la10^{\rm 29.5}$ W/Hz) and in redshift range (0.05 $\la$ z $\la$ 1.5). We find for both samples similar distributions in the luminosity ratios of high- to low-excitation lines ([NeV] $_{\rm 14.3~ \mu m}$ / [NeII] $_{\rm 12.8~ \mu m}$ ) and of high-excitation line to radio power ([NeV] $_{\rm 24.3~ \mu m}$ / $P_{\rm 178\,MHz}$ ). 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] $_{\rm 500.7~ nm}$ / [OIV] $_{\rm 25.9~ \mu m}$ 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 ( $A_{\rm V}$ $\ga$ 3) in the powerful FR 2 galaxies and that the [OIII] $_{\rm 500.7~ nm}$ luminosity does not serve as isotropic tracer for testing the unified schemes.

Key words: galaxies: active -- galaxies: nuclei -- galaxies: quasars: general -- infrared: galaxies

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