Limits on the Spatial Extent of Active Galactic Nuclei Measured with the Fine Guidance Sensors of the Hubble Space Telescope^*

The optical structures of several active galactic nuclei (AGNs) have been studied using the fine guidance sensors (FGSs) on the Hubble Space Telescope (HST). The FGSs are interferometric devices that can resolve structure on scales of 20 mas or less and hence have the potential to improve on the resolution attainable by HST's cameras. The FGSs produce interferometric fringes known as S curves, which are related to the intensity profile of the object on the sky. These have been analyzed using a simple model for the radial intensity distribution and strength of the underlying background illumination of the observed objects. Eight different observations of six different AGNs have been analyzed. No statistically significant differences from point sources are detected, but significant upper limits of order 20 mas are placed on any spatial extent. Systematic effects limiting the resolution are discussed, and some simple conclusions about the physical size and luminosity densities of the emitting regions of the AGNs implied by the data are given.