Mapping the radial structure of AGN tori^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mk@mpifr-bonn.mpg.de
² Physics Department, University of California, Santa Barbara, CA 93106, USA
³ Observatoire de la Côte d Azur, Département FIZEAU, Boulevard de l’Observatoire, BP 4229, 06304 Nice Cedex 4, France

Received: 30 May 2011
Accepted: 22 September 2011

Abstract

We present mid-IR interferometric observations of six type 1 AGNs at multiple baseline lengths ranging from 27 m to 130 m, reaching high angular resolutions up to λ/B ~ 0.02 arcsec. For two of the targets, we have simultaneous near-IR interferometric measurements as well, taken within a week. We find that all the objects are partially resolved at long baselines in these IR wavelengths. The multiple-baseline data directly probe the radial distribution of the material on sub-pc scales. We show that for our sample, which is small but spans over  ~2.5 orders of magnitudes in the UV/optical luminosity L of the central engine, the radial distribution clearly and systematically changes with luminosity.

The brightness distribution at a given mid-IR wavelength seems to be rather well described by a power law, which makes a simple Gaussian or ring size estimation quite inadequate. In this case, a half-light radius R_1/2 can be used as a representative size. We show that the higher luminosity objects become more compact in normalized half-light radii R_1/2/R_in in the mid-IR, where R_in is the dust sublimation radius empirically given by the L^1/2 fit of the near-IR reverberation radii. This means that, contrary to previous studies, the physical mid-IR emission size (e.g. in pc) is not proportional to L^1/2, but increases with L much more slowly. With our current datasets, we find that R_1/2 ∝ L^{0.21 ± 0.05} at 8.5 μm, and R_1/2 nearly constant at 13 μm.

The derived size information also seems to correlate with the properties of the total flux spectrum, in particular the smaller R_1/2/R_in objects having bluer mid-IR spectral shape. We use a power-law temperature/density gradient model as a reference, and infer that the radial surface density distribution of the heated dust grains at a radius r changes from a steep  ~r^-1 structure in high luminosity objects to a shallower  ~r⁰ structure in those of lower luminosity. The inward dust temperature distribution does not seem to smoothly reach the sublimation temperature – on the innermost scale of  ~R_in, a relatively low temperature core seems to co-exist with a slightly distinct brightness concentration emitting roughly at the sublimation temperature.