Mid-infrared spectroscopy of high-redshift 3CRR sources
  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.
