Are BAL outflows powered by radiation pressure on dust?

Abstract

Broad absorption line (BAL) outflows are commonly detected in active galactic nuclei (AGN), but their driving mechanism remains poorly constrained. Here we investigate whether radiation pressure on dust can adequately explain the BAL phenomenon observed in quasars. In the framework of our AGN radiative dusty feedback scenario, we show that dust-driven outflows can reach BAL wind-like velocities ($v \sim 10^4$ km/s) on galactic scales ($r \lesssim 1$ kpc). This is consistent with recent observations indicating that BAL acceleration typically occurs on scales of $\sim 10$ pc, and that the majority of BAL outflows are located at galactocentric radii greater than $\sim 100$ pc. We derive the outflow radial velocity profile and compute the associated outflow momentum rate and kinetic power, which are found to be in agreement with the outflow energetics measured in BAL quasars. Therefore radiation pressure on dust may account for the observed BAL outflow dynamics and energetics. Furthermore, we consider BAL clouds/clumps (leading to a clumpy BAL flow characterised by a wide range of outflowing velocities), and we analyse how the resulting covering factors affect the shape of the absorption line profiles. We conclude that dust-driven BAL outflows may provide a significant contribution to AGN feedback on galactic scales.