Evidence for Radiation Pressure Compression in the X-ray Narrow Line
  Region of Seyfert galaxies
  The observed spatial and kinematic overlap between soft X-ray emission and
the Narrow Line Region (NLR) in obscured Active Galactic Nuclei (AGN) yields
compelling evidence that relatively low-density gas co-exists with higher
density gas on scales as large as 100s of pc. This is commonly interpreted as
evidence for a constant gas pressure multiphase medium, likely produced by
thermal instability. Alternatively, Radiation Pressure Compression (RPC) also
leads to a density distribution, since a gas pressure (and hence density)
gradient must arise within each cloud to counteract the incident ionising
radiation pressure. RPC leads to a well-defined ionization distribution, and a
Differential Emission Measure (DEM) distribution with a universal slope of
$\sim-0.9$, weakly dependent on the gas properties and the illuminating
radiation field. In contrast, a multiphase medium does not predict the form of
the DEM. The observed DEMs of obscured AGN with XMM-Newton RGS spectra (the
CHRESOS sample) are in striking agreement with the predicted RPC DEM, providing
a clear signature that RPC is the dominant mechanism for the observed range of
densities in the X-ray NLR. In contrast with the constant gas pressure
multiphase medium, RPC further predicts an increasing gas pressure with
decreasing ionization, which can be tested with future X-ray missions using
density diagnostics.
