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 ×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 AGNs have the mid-infrared emission and L_X/L _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 < 0.8 and find that optically dull AGNs show a wide range of axis ratio, but optically active AGNs have only very round axis ratios. We conclude that hard X-rays select AGNs in host galaxies with a wide range of inclination angle, but only those AGNs 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 AGNs.