Possible evidence for a common radial structure in nearby AGN tori

TL;DR

This study proposes a model-independent method to analyze the radial structure of AGN tori using IR interferometry, revealing a potential common power-law surface brightness distribution across different objects.

Contribution

It introduces a normalized visibility analysis technique that suggests a universal radial surface brightness profile in AGN tori, independent of luminosity and distance.

Findings

Evidence for a common r^-2 surface brightness profile in AGN tori

The radial extent of the IR emission extends to about 100 times R_in

Supports a simple torus model with a density distribution between r^0 and r^-1.

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 which apparently scale with the luminosity of the central engine. Here we look at interferometric visibilities as a function of spatial scales normalized by the size of the inner torus radius 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 large range of spatial scales if different tori share a common radial structure. The currently available observations do suggest model-independently 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 r^0 and r^-1. This should be tested with further data.