Broad-Line Region . The lines arising here include hydrogen and helium recombination lines, permitted and semi-forbidden lines such as C IV and [C III (most of these in the emitted UV), and complex multiplets of Fe II. The lack of other lines suggests densities in excess of 10 7 cm-3 , and some considerations suggest values as high as 10 11 . At these densities, recombination is a very efficient radiator; a typical BLR requires only 10 6 solar masses. The size scale is taken from variability (see below), at as little as a light-week. Line ratios require that the emitting regions be optically thick and heavily stratified - the best bet is again one of fairly discrete individual clouds. One face will be directly illuminated by the ionizing continuum source, with hard radiation penetrating farther into the cloud. A warm, partially ionized zone deep inside the clouds is indicated for heavy-element emission. There will be a skin all around the cloud ionized by reflected UV from other clouds. The number of BLR clouds has been estimated ( > 10
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