Shock formation of the broad emission-line regions in QSOs and active galactic nuclei

Abstract

We present a model for the formation of the BLR of QSOs and AGNs which accounts simultaneously for the formation, structure and velocities of the clouds. These emerge as a natural consequence of the interaction of optically thin supersonic flows with the dense stellar cluster which is assumed to exist in the immediate circumquasar environment. Low density gas near a very luminous source must be hot with |$T_e\approx10^7-10^8 \enspace \text K$|⁠. So long as such gas flows supersonically, stand-off shocks inevitably form around all obstacles to the flow. The high-pressure, superheated gas in the shocked layer cools rapidly by inverse-Compton cooling off the radiation field. If the obstacles are large enough so that the flow time around them is longer than the cooling time, cold condensations – which we identify with the BLR clouds – form in the shocked gas. Both supernovae shells and groups of stars with strong stellar winds are shown to provide such cooling obstacles. The clouds are continuously generated and need no external confinement mechanism. They are accelerated to the local flow speed by the pressure gradients in the shock and thus the line widths reflect the flow speeds and no other acceleration mechanism is required. Their maximum column density is of the order of 1022 cm−2. The total mechanical energy input required to create the obstacle–shock system is modest compared to the output in continuum radiation, and the system is shown to be self-generating and self-consistent.