Evidence for radiation pressure compression in the X-ray narrow-line region of Seyfert galaxies

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 ∼ −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.

On the origin of X-ray oxygen emission lines in obscured AGN

“…In this paper we present the Catalogue with the emission-line luminosities in the soft X-ray band. The details on reduction procedure, spectral analysis and emission-line measurements are fully described in Bianchi et al (2019), where part of the CHRE-SOS sample has been used in the context of RPC analysis. Briefly, each line was fit with a Gaussian profile on top of a power-law continuum.…”

“…Notes: (i) Name of the source. (ii) to (x) Line luminosities in units of log(erg• s −1 ; the errors on the line luminosity are quoted at the 1σ level (Bianchi et al 2019). Places marked as "-" represent undetected lines.…”

“…In this sense, Bianchi et al (2019) recently showed that the RPC mechanism (Dopita et al 2002;Draine 2011), which naturally drives a density stratification in the gas clouds exposed to an ionizing field, can account for the soft X-ray spectra of CHRESOS's sources. Bianchi et al (2019) developed a theoretical differential emission-measure (DEM 1 ) for RPC context which nicely corresponds with the observed DEM in CHRESOS.…”

“…The presence of narrow Radiative Recombination Continua, typical signatures of low temperature plasma (Liedahl & Paerels 1996), as well as plasma diagnostics based on the He-like triplets indicate that the X-ray emitting gas is photo-ionized, most likely by the AGN radiation field. Recently, Radiation Pressure Compression (RPC) has been proposed as a universal scenario for the gas in the AGN nuclear environment, from the sub-kpc scale of the Broad Line Regions and the torus (Stern et al 2014) to the ENLR (Bianchi et al 2019). Deep high-resolution imaging with Chandra unveiled that a large fraction of the flux in the comparatively large RGS aperture is extended on scales of a few arcseconds.…”

“…Notes: (i) Name of the source. (ii) to (viii) Line luminosities in units of log(erg• s −1 ); the errors on the line luminosity are quoted at the 1σ level (Bianchi et al 2019). ( ix The optical [OIII]λ5007 emission line is an indicator of the AGN ionizing power.…”