Radiation pressure confinement - I. Ionized gas in the ISM of AGN hosts

We analyse the hydrostatic effect of active galactic nucleus (AGN) radiation pressure on optically thick gas in the host galaxy. We show that in luminous AGN, the radiation pressure likely confines the ionized layer of the illuminated gas. Radiation pressure confinement (RPC) has two main implications. First, the gas density near the ionization front is 7 × 10⁴L_i,45r^-2₅₀ cm^-3, where L_{i, 45} is the ionizing luminosity in units of 10⁴⁵ erg s^-1 and r₅₀ is the distance of the gas from the nucleus in units of 50 pc. Secondly, as shown by Dopita et al., the solution of the ionization structure within each slab is unique, independent of the ambient pressure.We show that the RPC density versus distance relation is observed over a dynamical range of ~10⁴ in distance, from sub-pc to kpc from the nucleus, and a range of ~10⁸ in gas density, from 10³ to 10¹¹ cm^-3. This relation implies that the radiative force of luminous AGN can compress giant molecular clouds in the host galaxy and possibly affect the star formation rate. The unique ionization structure in RPC includes a highly ionized X-ray-emitting surface, an intermediate layer which emits coronal lines and a lower ionization inner layer which emits optical lines. This structure can explain the observed overlap of the extended X-ray and optical narrow line emission in nearby AGN. We further support RPC by comparing the predicted ratios of the narrow line strength and narrow line widths with available observations. We suggest a new method, based on the narrow line widths, to estimate the black hole mass of low-luminosity AGN.