Letter to the Editor

Possible evidence for a common radial structure in nearby AGN tori

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: mk@mpifr-bonn.mpg.de

Received: 30 September 2008
Accepted: 10 December 2008

Abstract

We present a quantitative and relatively model-independent way to assess the radial structure of nearby AGN tori. These putative tori have been studied with long-baseline infrared (IR) interferometry, but the spatial scales probed are different for different objects. They are at various distances and also have different physical sizes that apparently scale with the luminosity of the central engine. Here we look at interferometric size information, or visibilities, as a function of spatial scales normalized by the size of the inner torus radiu  R_in. This approximately eliminates luminosity and distance dependence and, thus, provides a way to uniformly view the visibilities observed for various objects and at different wavelengths. We can construct a composite visibility curve over a wide range of spatial scales if different tori share a common radial structure. The currently available observations do suggest, independent of models, a common radial surface brightness distribution in the mid-IR that is roughly of a power-law form r^-2 as a function of radius r and extends to ~100 times R_in. Taking into account the temperature decrease toward outer radii with a simple torus model, this corresponds to the radial surface density distribution of dusty material directly illuminated by the central engine roughly in the range between r⁰ and r^-1. This should be tested with further data.