Why X-Ray-selected Active Galactic Nuclei Appear Optically Dull

We investigate why half of X-ray-selected active galactic nuclei (AGNs) in deep surveys lack signs of accretion in their optical spectra. The majority of these "optically dull" AGNs are no more than 66 times fainter than their host galaxies in rest-frame R band; as such, AGN lines are unlikely to be overwhelmed by stellar continuum in at least half the sample. We find that optically dull AGNs have the mid-infrared emission and L sub(X)/L sub(IR) ratios characteristic of local Seyfert galaxies, suggesting that the cause of optical dullness is not missing UV-optical continua. We compare the morphologies of 22 optically dull and 9 optically active AGNs at 0.5 < z