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The observed spatial and kinematic overlap between soft X-ray emission andthe Narrow Line Region (NLR) in obscured Active Galactic Nuclei (AGN) yieldscompelling evidence that relatively low-density gas co-exists with higherdensity gas on scales as large as 100s of pc. This is commonly interpreted asevidence for a constant gas pressure multiphase medium, likely produced bythermal instability. Alternatively, Radiation Pressure Compression (RPC) alsoleads to a density distribution, since a gas pressure (and hence density)gradient must arise within each cloud to counteract the incident ionisingradiation pressure. RPC leads to a well-defined ionization distribution, and aDifferential Emission Measure (DEM) distribution with a universal slope of$\sim-0.9$, weakly dependent on the gas properties and the illuminatingradiation field. In contrast, a multiphase medium does not predict the form ofthe DEM. The observed DEMs of obscured AGN with XMM-Newton RGS spectra (theCHRESOS sample) are in striking agreement with the predicted RPC DEM, providinga clear signature that RPC is the dominant mechanism for the observed range ofdensities in the X-ray NLR. In contrast with the constant gas pressuremultiphase medium, RPC further predicts an increasing gas pressure withdecreasing ionization, which can be tested with future X-ray missions usingdensity diagnostics.

Evidence for Radiation Pressure Compression in the X-ray Narrow Line Region of Seyfert galaxies