Quasar Rain: the Broad Emission Line Region as Condensations in the Warm
  Accretion Disk Wind
  The origin of the broad emission line region (BELR) in quasars and active
galactic nuclei is still unclear. 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, 10,000
K, gas in quasars, the low ionization phase of the warm absorbers (WAs) and the
clouds causing X-ray eclipses. The cool clouds will condense quickly (days to
years), before the WA outflows reach escape velocity (which takes months to
centuries). 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 narrow range of 2-phase equilibrium densities
may explain the scaling of the BELR size with the square root of luminosity,
while the scaling of cloud formation timescales could produce the Baldwin
effect. These dense clouds have force multipliers of order unity and so cannot
be accelerated to escape velocity. They fall back on a dynamical timescale
(months to centuries), producing an inflow that rains down toward the central
black hole. As they soon move at Mach ~40 with respect to the WA outflow, these
'raindrops' will be rapidly destroyed within months. This rain of clouds may
produce the elliptical BELR orbits implied by velocity resolved reverberation
mapping in some objects, and can explain the opening angle and destruction
timescale of the narrow 'cometary' tails of the clouds seen in X-ray eclipse
observations. Some consequences and challenges of this 'quasar rain' model are
presented along with several avenues for theoretical investigation.
