The relative frequency of broad-lined and narrow-lined active galactic nuclei: implications for unified schemes

I discuss evidence concerning the relative occurrence of narrow-lined and broad-lined active galactic nuclei (AGN), in optical, IR, X-ray, and radio-selected samples. Both narrow-lined AGN and reddened broad-lined AGN occur more frequently at lower source powers. There is marginal evidence that narrow-lined AGN were more common in the past. Narrow-lined objects have weaker [O III] at a given radio power. These data are inconsistent with the simplest ‘unified scheme’ where a similar thick molecular torus surrounds all AGN, and with the simplest modification, that the torus geometrical thickness is a function of source power. A substantial range of geometrical thicknesses must exist at all powers. Both the typical geometrical thickness and the typical optical thickness of obscuring material probably vary with source power. This may arise naturally in accretion models, as smaller black holes are more easily smothered. I speculate that the obscuring material, rather than being an orbiting torus of cool molecular matter, may be an expelled shell of gas, only the outer parts of which are dusty.