The exploration of extragalactic objects with long-baseline interferometers in the near-infrared has been very limited .
Here we report successful observations with the Keck interferometer at K-band ( 2.2 \mu m ) for four Type 1 AGNs , namely NGC4151 , Mrk231 , NGC4051 , and the QSO IRAS13349+2438 at z =0.108 .
For the latter three objects , these are the first long-baseline interferometric measurements in the infrared .
We detect high visibilities ( V ^ { 2 } \sim 0.8 - 0.9 ) for all the four objects , including NGC4151 for which we confirm the high V ^ { 2 } level measured by Swain et al .
( 2003 ) .
We marginally detect a decrease of V ^ { 2 } with increasing 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 radius 0.45 \pm 0.04 mas ( 0.039 \pm 0.003 pc ) .
Strikingly , this matches independent radius measurements from optical–infrared reverberations that are thought to be probing the dust sublimation radius .
We also show that the effective radius of the other objects , obtained from the same ring model , is either roughly equal to or slightly larger than the reverberation radius as a function of AGN luminosity .
This suggests that we are indeed partially resolving the dust sublimation region .
The ratio of the effective ring radius to the reverberation radius might also give us an approximate probe for the radial structure of the inner accreting material in each object .
This should be scrutinized with further observations .