MID-INFRARED SPECTROSCOPY OF HIGH-REDSHIFT 3CRR SOURCES

Abstract

Using the Spitzer Space Telescope, we have obtained rest frame 9-16mu spectra\nof 11 quasars and 9 radio galaxies from the 3CRR catalog at redshifts\n1.0<z<1.4. This complete flux-limited 178MHz-selected sample is unbiased with\nrespect to orientation and therefore suited to study orientation-dependent\neffects in the most powerful active galactic nuclei (AGN). The mean radio\ngalaxy spectrum shows a clear silicate absorption feature (tau_9.7mu = 1.1)\nwhereas the mean quasar spectrum shows silicates in emission. The mean radio\ngalaxy spectrum matches a dust-absorbed mean quasar spectrum in both shape and\noverall flux level. The data for individual objects conform to these results.\nThe trend of the silicate depth to increase with decreasing core fraction of\nthe radio source further supports that for this sample, orientation is the main\ndriver for the difference between radio galaxies and quasars, as predicted by\nAGN unification. However, comparing our high-z sample with lower redshift 3CRR\nobjects reveals that the absorption of the high-z radio galaxy MIR continuum is\nlower than expected from a scaled up version of lower luminosity sources, and\nwe discuss some effects that may explain these trends.\n