The innermost region of AGN tori: implications from the HST/NICMOS Type 1 point sources and near-IR reverberation

Abstract

Spatially resolving the innermost torus in AGN is one of the main goals of its high-spatial-resolution studies. This could be done in the near-IR observations of Type 1 AGNs where we see directly the hottest dust grains in the torus. We discuss two critical issues in such studies. Firstly, we examine the nuclear point sources in the HST/NICMOS images of nearby Type 1 AGNs, to evaluate the possible contribution from the central putative accretion disk. After a careful subtraction of host bulge flux, we show that near-IR colors of the point sources appear quite interpretable simply as a composite of a black-body-like spectrum and a relatively blue distinct component as expected for a torus and an accretion disk, respectively. Our radiative transfer models for clumpy tori also support this simple two-component interpretation. The observed near-IR colors suggest a fractional accretion disk contribution of ~25% or less at 2.2 micron. Secondly, we show that the innermost torus radii as indicated by the recent near-IR reverberation measurements are systematically smaller by a factor of ~3 than the predicted dust sublimation radius with a reasonable assumption for graphite grains of sublimation temperature 1500 K and size 0.05 micron in radius. The discrepancy might indicate a much higher sublimation temperature or a typical grain size being much larger in the innermost tori, though the former case appears to be disfavored by the observed colors of the HST point sources studied above. The near-IR interferometry with a baseline of ~100 m should be able to provide the important, independent size measurements, based on the low accretion disk contribution obtained above.