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

Spatially resolving the innermost region of the putative torus-like structure in an active galactic nucleus (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. One is the possible contribution from the central putative accretion disk (the near-IR part of the big blue bump emission), which should be taken into account for the torus measurements. The other is the expected size of the inner boundary of the torus, essential for the feasibility of spatially resolving the region. Firstly, we examine the nuclear near-IR point sources in the HST/NICMOS images of nearby Type 1 AGNs, to evaluate the accretion disk contribution. After the subtraction of the host bulge flux through two-dimensional decompositions, 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 in the near-IR, respectively. The near-IR colors of our radiative transfer models for clumpy tori also support this simple two-component interpretation. The observed near-IR colors for the available sample suggest a fractional accretion disk contribution of ∼25% or less at 2.2 μm. 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 μm 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. Alternatively, the central engine radiation might be significantly anisotropic. The near-IR interferometry with a baseline of ∼100 m should be able to provide the important, independent size measurements for the innermost torus region, based on the low fractional contribution from the accretion disk obtained above.