Radiation Pressure Confinement in Active Galaxies
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The pressure from the radiation of active galactic nuclei (AGN) can
exceed the typical gas pressure in the interstellar medium by many
orders of magnitude. We show that in luminous AGN, this radiation
pressure likely confines the optically thick photoionized gas in the
host galaxy. Radiation pressure confinement produces a unique
hydrostatic solution, which is independent of the ambient pressure.
The gas density within the photoionized gas scales as the distance
from the nucleus to the power of -2. Thus, the AGN radiation pressure
sets the density of the illuminated gas in the host galaxy. We show
that this density vs. distance relation is observed over a dynamical
range of ~10^4 in distance and ~10^8 in gas density. The unique
hydrostatic solution implies a highly ionized X-ray emitting surface
layer, and a lower ionization inner layer which emits optical lines.
This slab structure can explain the observed overlap of the extended
X-ray and optical line emission. We also compare the predicted ratios
and widths of the narrow emission lines with available observations.