Optically thick radiatively driven shocks near QSOs

The dynamics of gas surrounding a QSO and acted on by radiation pressure is studied. Ionization equilibrium is used to derive a condition for the gas to become optically thick in the continuum. It is shown that once this happens, there exists a very simple flow in which a shock moves away from the QSO with constant speed. The gross features of this flow are very insensitive to the details of the input physics. It is suggested that such a flow can explain the emission lines and some of the absorption lines in the spectra of QSOs. (author)