QUANTIFYING THE ANISOTROPY IN THE INFRARED EMISSION OF POWERFUL ACTIVE GALACTIC NUCLEI

We use rest-frame near- and mid-IR data of an isotropically selected sample of quasars and radio galaxies at 1.0 <= z <= 1.4, which have been published previously by Leipski et al., to study the wavelength-dependent anisotropy of the IR emission. For that we build average spectral energy distributions of the quasar subsample (=type 1 AGNs) and radio galaxies (=type 2 AGNs) from similar to 1 to 17 mu m and plot the ratio of both average samples. From 2 to 8 mu m rest-frame wavelengths the ratio gradually decreases from 20 to 2 with values around 3 in the 10 mu m silicate feature. Longward of 12 mu m, the ratio decreases further and shows some high degree of isotropy at 15 mu m (ratio similar to 1.4). The results are consistent with upper limits derived from the X-ray/mid-IR correlation of local Seyfert galaxies. We find that the anisotropy in our high-luminosity radio-loud sample is smaller than in radio-quiet lower-luminosity active galactic nuclei (AGNs) which may be interpreted in the framework of a receding torus model with luminosity-dependent obscuration properties. It is also shown that the relatively small degree of anisotropy is consistent with clumpy torus models.