As they soon move at Mach ∼10-100 with respect to the WA outflow, these “raindrops” will be rapidly destroyed within months. They fall back on a dynamical timescale (months to centuries), producing an inflow that rains down toward the central black hole. These dense clouds have force multipliers of order unity and so cannot be accelerated to escape velocity. The narrow range of two-phase equilibrium densities may explain the (luminosity) 1/2 scaling of the BELR size, while the scaling of cloud formation timescales could produce the Baldwin effect. Cool clouds form in equilibrium with the warm phase of the wind because the rapidly varying X-ray quasar continuum changes the force multiplier, causing pressure waves to move gas into stable locations in pressure-temperature space. The cool clouds will condense quickly (days to years), before the WA outflows reach escape velocity (which takes months to centuries).
I propose that condensations form in the warm, radiation-pressure-driven, accretion disk wind of quasars creating the BEL clouds and uniting them with the other two manifestations of cool (∼10 4 K) gas in quasars, the low ionization phase of the warm absorbers (WAs) and the clouds causing X-ray eclipses.
The origin of the broad emission line region (BELR) in quasars and active galactic nuclei is still unclear.
