Why X-ray–selected Agn Appear Optically Dull

We investigate why half of X-ray–selected active galactic nuclei (AGN) in deep surveys lack signs of accretion in their optical spectra. The majority of these “optically–dull” AGN are no more than ∼ 6 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 AGN have the mid–infrared emission and Lx/LIR ratios characteristic of local Seyferts, 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 AGN at 0.5 < z < 0.8, and find that optically–dull AGN show a wide range of axis ratio, but optically–active AGN have only very round axis ratios. We conclude that hard X-rays select AGN in host galaxies with a wide range of inclination angle, but only those AGN in the most face-on or spheroidal host galaxies show optical emission lines. Thus, extranuclear dust in the host galaxy plays an important role in hiding the emission lines of optically–dull AGN. [This manuscript uses low–resolution figures. A full–resolution manuscript can be obtained at http://satchmo.as.arizona.edu/~jrigby/optdull.ps] Subject headings: galaxies: active—X-rays: galaxies—infrared: galaxies