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

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 (10^26.5 W/Hz ⪉ P_178 MHz ⪉10^29.5 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 μ m} / [NeII]{12.8 μ m}) and of high-excitation line to radio power ([NeV]{24.3 μ 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 μ 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.