Absorption from clouds in QSO emission-line regions

The absorption properties of the constituent clouds of QSO emission-line regions are discussed. An ensemble of large (relative to the continuum source) radiation bounded clouds can already be ruled out. Model calculations show that in this case there should be complete absorption at the C I edge (1100 A), and this has not been found in any QSO spectrum. Another possibility is that the clouds are small compared with the continuum source, and many lie along a line of sight. Spectrum synthesis shows that, if the covering factor is small, their absorption has little visible effect on the QSO spectrum. (author)