Physical conditions in quasar broad -line regions from Hubble Space Telescope and ground based spectra

Quasars show strong, highly Doppler-broadened emission lines in the ultraviolet and visible spectral regions. These lines are of great interest since they emanate from gas close to the central supermassive black hole and this gas may be related to the gas fuelling the central engine. Many fundamental questions about the nature of the “broad-line region” (BLR) structure, and its kinematics have remained unanswered. We have used state-of-the-art photoionization models to interpret a combination of Hubble Space Telescope ultraviolet spectra and ground-based optical spectra to deduce the conditions in the BLR as a function of velocity. There has been a long-standing discrepancy between the densities deduced from studies of the variability of quasar emission lines and those inferred from standard photoionization analyses. We find that the higher metal abundances now believed to be present in most quasars lead to higher densities being predicted by photoionization models. It has been suggested that the highest-velocity gas is optically thin. We show that this presents many problems and that instead the line ratios arise naturally in an optically-thick component. This model successfully explains the line ratios and their variability. We present evidence that the highest velocity blueshifted gas shows the self-absorption expected for outflowing gas. For high-ionization lines we show that both the density and ionizing photon flux are independent of the velocity. This strongly rules out models in which the motion of this gas is gravitationally dominated but is consistent with models where the motion arises from a wind. However, for the low-ionization gas we find that both the density and the ionizing photon flux increase with velocity. The increase in ionizing photon flux is consistent with what is expected from low-ionization virialized gas illuminated by a point source. Our studies strongly support the idea that there are two different components to the BLR: a high-ionizati