Mapping the radial structure of AGN tori

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 ∼r 0 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.

Three-Dimensional Radiative Transfer Calculations of Radiation Feedback From Massive Black Holes: Outflow of Mass From the Dusty “Torus”

“…Mid-IR interferometric observations find the mid-plane density may be well fit with a power-law R −γ where γ lies within a range of approximately 0.4-1.4, with a tendency toward larger values for more luminous AGNs (Kishimoto et al 2011). This is also in agreement with the simulations presented in Hopkins et al (2012) in which γ ≈ 1.5.…”

Ultraviolet and Optical Emission Line Outflows in the Heavily Obscured Quasar SDSS J000610.67+121501.2: At the Scale of the Dusty Torus and Beyond

“…Formulae in Burtscher et al (2013) and Kishimoto et al (2011) gave the dust sublimation radii as 5.4 and 2.0 pc, respectively, smaller than the above estimation. In any case, the outflows are located externally of the torus.…”

Cosmology with AGN dust time lags–simulating the new VEILS survey

“…Therefore, the emission at wavelengths close to the (carbonaceous) sublimation temperature will be dominated by black-body-like dust grains (large, carbon) regardless of the details of the original ISM composition. This is strongly supported by surface emissivity measurements of nearby AGN in the K-band using IR interferometry (Kishimoto et al 2011b). …”