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

## TL;DR

This paper presents evidence that Radiation Pressure Compression (RPC) explains the density and ionization structure of the X-ray Narrow Line Region in Seyfert galaxies, supported by observed emission measure distributions matching RPC predictions.

## Contribution

The study provides observational evidence that RPC, rather than multiphase pressure equilibrium, governs the density distribution in the X-ray NLR of Seyfert galaxies.

## Key findings

- Observed DEMs match RPC predictions with a universal slope of ~-0.9.
- RPC predicts increasing gas pressure with decreasing ionization, testable by future X-ray missions.
- Evidence favors RPC as the dominant mechanism over multiphase models.

## Abstract

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.