The innermost dusty structure in AGN as probed by the Keck interferometer

We are now exploring the inner region of Type 1 AGNs with the Keck interferometer in the near-infrared. Here we report further measurements of K-band (2.2 um) visibilities on 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. We interpret these as an indication of partially resolving the dust sublimation region. The effective ring radii derived from the observed visibilities approximately scale with L^1/2 where L is the AGN luminosity. Comparing the radii with those from independent optical-infrared reverberation measurements, the new data further 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 for the radial distribution of the inner accreting material. We show tentative evidence that this inner radial structure might closely be related to the radio-loudness of the central engine. Finally, we re-observed the brightest Seyfert 1 galaxy NGC4151. The marginally higher visibility at a shorter projected baseline, compared to our previous measurements obtained one year before, 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 is brightened up by a factor of 1.5 over a time interval of one year.