Radiation pressure confinement - IV. Application to broad absorption line outflows

TL;DR

This paper proposes that radiation pressure confinement naturally creates dense, thin gas structures in quasar outflows, explaining observed absorption features and ionization states, and predicts conditions for broad absorption line quasars.

Contribution

It demonstrates that radiation pressure confinement leads to dense gas sheets in quasar outflows, providing a solution to the overionization problem consistent with observations.

Findings

RPC produces dense gas sheets with large density gradients.

Predicted ion column densities match observed X-ray and UV absorption.

Outflows with L/L_Edd > 0.1 are consistent with broad absorption line quasars.

Abstract

A fraction of quasars present broad absorption lines, produced by outflowing gas with typical velocities of 3000 - 10,000 km/s. If the outflowing gas fills a significant fraction of the volume where it resides, then it will be highly ionized by the quasar due to its low density, and will not produce the observed UV absorption. The suggestion that the outflow is shielded from the ionizing radiation was excluded by recent observations. The remaining solution is a dense outflow with a filling factor $f<10^{-3}$. What produces such a small $f$? Here we point out that radiation pressure confinement (RPC) inevitably leads to gas compression and the formation of dense thin gas sheets/filaments, with a large gradient in density and ionization along the line of sight. The total column of ionized dustless gas is a few times $10^{22}$ cm$^{-2}$, consistent with the observed X-ray absorption and detectable P V absorption. The predicted maximal columns of various ions show a small dependence on the system parameters, and can be used to test the validity of RPC as a solution for the overionization problem. The ionization structure of the outflow implies that if the outflow is radiatively driven, then broad absorption line quasars should have $L/L_{\rm Edd} \gtrsim 0.1$.