A Simple Disk Wind Model for Broad Absorption Line Quasars

TL;DR

This paper presents a simple, geometric model of quasar disk winds that can produce broad absorption lines, providing insights into quasar outflows and their role in galaxy evolution.

Contribution

It introduces a straightforward benchmark model of equatorial disk winds in QSOs, linking wind parameters to observable broad absorption features.

Findings

Model reproduces broad, blue-shifted absorption lines at certain viewing angles.

X-ray weakness is necessary to prevent overionization of the wind.

Outflows could carry enough energy for significant feedback in galaxy evolution.

Abstract

Approximately 20% of quasi-stellar objects (QSOs) exhibit broad, blue-shifted absorption lines in their ultraviolet spectra. Such features provide clear evidence for significant outflows from these systems, most likely in the form of accretion disk winds. These winds may represent the "quasar" mode of feedback that is often invoked in galaxy formation/evolution models, and they are also key to unification scenarios for active galactic nuclei (AGN) and QSOs. To test these ideas, we construct a simple benchmark model of an equatorial, biconical accretion disk wind in a QSO and use a Monte Carlo ionization/radiative transfer code to calculate the ultraviolet spectra as a function of viewing angle. We find that for plausible outflow parameters, sightlines looking directly into the wind cone do produce broad, blue-shifted absorption features in the transitions typically seen in broad absorption line QSOs. However, our benchmark model is intrinsically X-ray weak in order to prevent overionization of the outflow, and the wind does not yet produce collisionally excited line emission at the level observed in non-BAL QSOs. As a first step towards addressing these shortcomings, we discuss the sensitivity of our results to changes in the assumed X-ray luminosity and mass-loss rate, Mdot(wind). In the context of our adopted geometry, Mdot(wind) ~ Mdot(acc) is required in order to produce significant BAL features. The kinetic luminosity and momentum carried by such outflows would be sufficient to provide significant feedback.