Radiation Pressure Confinement -- III. The origin of the broad
  ionization distribution in AGN outflows
  The winds of ionized gas driven by Active Galactic Nuclei (AGN) can be
studied through absorption lines in their X-ray spectra. A recurring feature of
these outflows is their broad ionization distribution, including essentially
all ionization levels (e.g., Fe^0+ to Fe^25+). This characteristic feature can
be quantified with the absorption measure distribution (AMD), defined as the
distribution of column density with ionization parameter |dN / dlog xi|.
Observed AMDs extend over 0.1 < xi < 10^4 (cgs), and are remarkably similar in
different objects. Power-law fits (|dN /dlog xi| ~ N_1 xi^a) yield N_1 =
3x10^{21} cm^-2 +- 0.4 dex and a = 0 -- 0.4. What is the source of this broad
ionization distribution, and what sets the small range of observed $N_1$ and
$a$? A common interpretation is a multiphase outflow, with a wide range of gas
densities in a uniform gas pressure medium. However, the incident radiation
pressure leads to a gas pressure gradient in the photoionized gas, and
therefore to a broad range of ionization states within a single slab. We show
that this compression of the gas by the radiation pressure leads to an AMD with
|dN / dlog xi| = 8x10^21 xi^0.03 cm^-2, remarkably similar to that observed.
The calculated values of $N_1$ and $a$ depend weakly on the gas metallicity,
the ionizing spectral slope, the distance from the nucleus, the ambient
density, and the total absorber column. Thus, radiation pressure compression
(RPC) of the photoionized gas provides a natural explanation for the observed
AMD. RPC predicts that the gas pressure increases with decreasing ionization,
which can be used to test the validity of RPC in ionized AGN outflows.
