Hydrostatic Photoionization Models and Radiation Pressure Confinement (RPC)
In Stern et al. (2014a, b) and Baskin et al. (2014a, b) together with collaborators at the Technion I developed idealized radiation transfer solutions for photoionized gas dominated by quasar radiation pressure, using both analytic approximations and numerical calculations. These type of solutions assume that the photoionized gas is in quasi-hydrostatic equilibrium with the incident radiation pressure, an approximation justified in the figure below. We demonstrated that the dominance of radiation pressure can solve a host of puzzles raised in the last two decades by studies of Active Galactic Nuclei, including:
- The broad range of ionization levels seen in ‘warm absorbers’ (see Stern et al. 2014b).
- The spatial overlap of extended X-ray emission and narrow OIII emission (see Stern et al. 2014a).
- The broad absorption line (BAL) ‘over-ionization problem’ (see Baskin et al. 2014b).
- The ionization level of the broad line region and its dependence on distance from the BH (see Baskin et al. 2014a).
The figure shows a radiative hydrodynamic (RHD) simulation of a dusty cloud irradiated by a quasar, using a modified version of PLUTO which includes radiation (adapted from Stern, Oñorbe, & Kuiper, in prep.). The quasar is located at r = 0. Radiation pressure is set to be the dominant pressure source, as suggested by the figure above. Left panel shows the initial conditions. The right panel shows the cloud 104 years after exposure to the quasar radiation. A quasi-static density gradient develops at the HII surface layer (zoomed on in the inset), which justifies the hydrostatic approximation mentioned above. The length scale of this gradient is ~0.01 pc, far below the resolution of galaxy-scale and cosmological simulations.