Quantifying the anisotropy in the infrared emission of powerful AGN

TL;DR

This study quantifies the wavelength-dependent anisotropy of infrared emission in powerful AGN, revealing that anisotropy decreases with wavelength and is less pronounced in high-luminosity radio-loud AGN, supporting clumpy torus models.

Contribution

It provides the first detailed measurement of IR anisotropy across wavelengths for a well-defined AGN sample, comparing radio-loud and radio-quiet objects.

Findings

Anisotropy decreases from 20 to 2 between 2-8 μm

Near isotropy (~1.4 ratio) at 15 μm

High-luminosity radio-loud AGN show less anisotropy

Abstract

We use restframe near- and mid-IR data of an isotropically selected sample of quasars and radio galaxies at 1.0 \leq z \leq 1.4, which have been published previously, to study the wavelength-dependent anisotropy of the IR emission. For that we build average SEDs of the quasar subsample (= type 1 AGN) and radio galaxies (= type 2 AGN) from ~1-17 {\mu}m and plot the ratio of both average samples. From 2 to 8 {\mu}m restframe wavelength 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 ~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 AGN 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.