Abstract
Using the Spitzer Space Telescope, we have obtained rest frame 9-16mu spectra of 11 quasars and 9 radio galaxies from the 3CRR catalog at redshifts 1.0<z<1.4. This complete flux-limited 178MHz-selected sample is unbiased with respect to orientation and therefore suited to study orientation-dependent effects in the most powerful active galactic nuclei (AGN). The mean radio galaxy spectrum shows a clear silicate absorption feature (tau_9.7mu = 1.1) whereas the mean quasar spectrum shows silicates in emission. The mean radio galaxy spectrum matches a dust-absorbed mean quasar spectrum in both shape and overall flux level. The data for individual objects conform to these results. The trend of the silicate depth to increase with decreasing core fraction of the radio source further supports that for this sample, orientation is the main driver for the difference between radio galaxies and quasars, as predicted by AGN unification. However, comparing our high-z sample with lower redshift 3CRR objects reveals that the absorption of the high-z radio galaxy MIR continuum is lower than expected from a scaled up version of lower luminosity sources, and we discuss some effects that may explain these trends.
Highlights
There is strong evidence that many luminous active galactic nuclei (AGNs) are surrounded by dust in a torus-like geometry leading to orientation-dependent obscuration of the nuclear region (Antonucci & Miller 1985; Barthel 1989; Antonucci 1993; Urry & Padovani 1995)
From the MIR point of view, this strongly supports the idea that the quasars and radio galaxies in our sample are intrinsically identical with the former being essentially unobscured while the latter suffer from extinction
The 3CRR sample at 1 < z < 1.4 represents the most radio-luminous steep-spectrum quasars and double-lobed radio galaxies for which the rest-frame MIR 9–16 μm wavelength range is accessible with the Spitzer Infrared Spectrograph (IRS)
Summary
There is strong evidence that many luminous active galactic nuclei (AGNs) are surrounded by dust in a torus-like geometry leading to orientation-dependent obscuration of the nuclear region (Antonucci & Miller 1985; Barthel 1989; Antonucci 1993; Urry & Padovani 1995). A large number of z < 1 3CR objects have been studied in the MIR and FIR with ISO (e.g., van Bemmel et al 2000; Meisenheimer et al 2001; and as compiled by Siebenmorgen et al 2004 and by Haas et al 2004) and with Spitzer (e.g., Shi et al 2005; Haas et al 2005; Ogle et al 2006; Cleary et al 2007) While such studies were vital to establish the unification picture at 0.5 < z < 1.0, some of this work indicated that on average quasars seem to be a factor of a few more luminous in the infrared than radio galaxies when normalized by their total radio luminosity. It was argued that non-thermal contributions of a beamed radio core increase
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