Continuum shielding and flow dynamics in active galactic nuclei

Abstract

We study the ionization, thermal structure and dynamics of active galactic nuclei (AGN) flows that are partially shielded from the central continuum. We utilize a detailed non-local thermo- dynamic equilibrium photoionization and radiative transfer code using exact (non-Sobolev) calculations. We find that shielding has a pronounced effect on the ionization, thermal structure and the dynamics of such flows. Moderate shielding is especially efficient in accelerating flows to high velocities because it suppresses the ionization level of the gas. The ionization structure of shielded gas tends to be distributed uniformly over a wide range of ionization levels. In such gas, radiation pressure due to trapped line photons can dominate over the thermal gas pressure and have a significant effect on the thermal stability of the flow. Heavily shielded flows are driven mainly by line radiation pressure, and so line locking has a large effect on the flow dynamics. We show that the observed 'Lα ghost' is a natural outcome in highly ionized flows that are shielded beyond the Lyman limit. We suggest that high-velocity AGN flows occupy only a small fraction of the volume and that their density depends only weakly on the velocity field.