Implications of Lyman continuum observations of quasars

The search for Lyman limit absorption caused by emission line regions of active nuclei provides a test of models of the central regions. The absence of such absorption has been considered by Davidson, who suggests that re-emission of a diffuse Lyman continuum by the optically thick emission line clouds partially compensates for Lyman limit absorption. Using current estimates of the sizes of the continuum source and the emission line clouds, and the gas covering factor, we show that the expected depression at the Lyman continuum is about half that expected from simple arguments concerning the covering factor. We show that observations of an object chosen to be fairly free of intervening Ly α absorption, and expected to have a relatively large covering factor from theoretical arguments, are compatible with covering factors between ∼0–25 per cent. Present Lyman limit observations provide no meaningful limit to the covering factor, but they severely constrain thermal models for the ultraviolet bump.