The innermost dusty structure in active galactic nuclei as probed by the
  Keck interferometer
  We are now exploring the inner region of Type 1 active galactic nuclei (AGNs)
with the Keck interferometer in the near-infrared. Adding to the four targets
previously studied, we report measurements of the K-band (2.2 um) visibilities
for four more targets, namely AKN120, IC4329A, Mrk6, and the radio-loud QSO
3C273 at z=0.158. The observed visibilities are quite high for all the targets,
which we interpret as an indication of the partial resolution of the dust
sublimation region. The effective ring radii derived from the observed
visibilities scale approximately with L^1/2, where L is the AGN luminosity.
Comparing the radii with those from independent optical-infrared reverberation
measurements, these data support our previous claim that the interferometric
ring radius is either roughly equal to or slightly larger than the
reverberation radius. We interpret the ratio of these two radii for a given L
as an approximate probe of the radial distribution of the inner accreting
material. We show tentative evidence that this inner radial structure might be
closely related to the radio-loudness of the central engine. Finally, we
re-observed the brightest Seyfert 1 galaxy NGC4151. Its marginally higher
visibility at a shorter projected baseline, compared to our previous
measurements obtained one year before, further supports the partial resolution
of the inner structure. We did not detect any significant change in the implied
emission size when the K-band flux was brightened by a factor of 1.5 over a
time interval of one year.
