The Ly-edge paradox and the need for obscured QSOs

Abstract

Based on the most recent QSO ultraviolet spectra, the covering factor of the clouds of the Broad Line Region (BLR) is about 30%, or larger. This value would imply that in at least 30% of the QSOs our line of sight crosses one, or more, BLR clouds and, in the latter case, the UV spectrum should show a sharp Ly-edge in absorption. This Ly-edge in absorption is never observed. This paradox is solved if, as suggested by various authors, the BLR is flattened and the dusty gas in the outer parts, on the same plane, prevents the observation along the lines of sight passing through the BLR clouds. The objects observed edge-on (with respect to the flattened BLR) would be classified as obscured QSOs or, within the framework of the unified model, type 2 QSOs. The covering factor of the BLR constrains the fraction of obscured QSOs to be QSO2/QSO1 > 0.5. This lower limit is already high with respect to the number of candidate type 2 QSOs claimed so far. We discuss this constraint in relation to recent AGN surveys.