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The exploration of extragalactic objects with long-baseline interferometersin the near-infrared has been very limited. Here we report successfulobservations with the Keck interferometer at K-band (2.2 um) for four Type 1AGNs, namely NGC4151, Mrk231, NGC4051, and the QSO IRAS13349+2438 at z=0.108.For the latter three objects, these are the first long-baseline interferometricmeasurements in the infrared. We detect high visibilities (V^2 ~ 0.8-0.9) forall the four objects, including NGC4151 for which we confirm the high V^2 levelmeasured by Swain et al.(2003). We marginally detect a decrease of V^2 withincreasing baseline lengths for NGC4151, although over a very limited range,where the decrease and absolute V^2 are well fitted with a ring model of radius0.45+/-0.04 mas (0.039+/-0.003 pc). Strikingly, this matches independent radiusmeasurements from optical--infrared reverberations that are thought to beprobing the dust sublimation radius. We also show that the effective radius ofthe other objects, obtained from the same ring model, is either roughly equalto or slightly larger than the reverberation radius as a function of AGNluminosity. This suggests that we are indeed partially resolving the dustsublimation region. The ratio of the effective ring radius to the reverberationradius might also give us an approximate probe for the radial structure of theinner accreting material in each object. This should be scrutinized withfurther observations.

Exploring the inner region of Type 1 AGNs with the Keck interferometer