Probing the innermost dusty structure in AGN with mid-IR and near-IR
  interferometers
  With mid-IR and near-IR long-baseline interferometers, we are now mapping the
radial distribution of the dusty accreting material in AGNs at sub-pc scales.
We currently focus on Type 1 AGNs, where the innermost region is unobscured and
its intrinsic structure can be studied directly. As a first systematic study of
Type 1s, we obtained mid-/near-IR data for small samples over ~3-4 orders of
magnitudes in UV luminosity L of the central engine. Here we effectively trace
the structure by observing dust grains that are radiatively heated by the
central engine. Consistent with a naive expectation for such dust grains, the
dust sublimation radius R_in is in fact empirically known to be scaling with
L^1/2 from the near-IR reverberation measurements, and this is also supported
by our near-IR interferometry. Utilizing this empirical relationship, we
normalize the radial extent by R_in and eliminate the simple L^1/2 scaling for
a direct comparison over the samples. We then find that, in the mid-IR, the
overall size in units of R_in seems to become more compact in higher luminosity
sources. More specifically, the mid-IR brightness distribution is rather well
described by a power-law, and this power-law becomes steeper in higher
luminosity objects. The near-IR flux does not seem to be a simple inward
extrapolation of the mid-IR power-law component toward shorter wavelengths, but
it rather comes from a little distinct brightness concentration at the inner
rim region of the dust distribution. Its structure is not well constrained yet,
but there is tentative evidence that this inner near-IR-emitting structure has
a steeper radial distribution in jet-launching objects. All these should be
scrutinized with further observations.
