The energetics of AGN radiation pressure-driven outflows

TL;DR

This paper demonstrates that AGN radiation pressure on dust, with radiation trapping effects included, can explain observed galactic outflow energetics and scaling relations, challenging the exclusive focus on wind energy-driven models.

Contribution

It shows that radiation pressure on dust can reproduce outflow energetics and scalings, highlighting the importance of radiation trapping effects in AGN feedback models.

Findings

Radiation pressure on dust can match observed outflow energetics.

Predicted scalings: mass outflow rate ∝ L^{1/2}, kinetic power ∝ L^{3/2}.

Both radiation pressure and wind models should be considered for AGN feedback.

Abstract

The increasing observational evidence of galactic outflows is considered as a sign of active galactic nucleus (AGN) feedback in action. However, the physical mechanism responsible for driving the observed outflows remains unclear, and whether it is due to momentum, energy, or radiation is still a matter of debate. The observed outflow energetics, in particular the large measured values of the momentum ratio ($\dot{p}/(L/c) \sim 10$) and energy ratio ($\dot{E}_k/L \sim 0.05$), seems to favour the energy-driving mechanism; and most observational works have focused their comparison with wind energy-driven models. Here we show that AGN radiation pressure on dust can adequately reproduce the observed outflow energetics (mass outflow rate, momentum flux, and kinetic power), as well as the scalings with luminosity, provided that the effects of radiation trapping are properly taken into account. In particular, we predict a sub-linear scaling for the mass outflow rate ($\dot{M} \propto L^{1/2}$) and a super-linear scaling for the kinetic power ($\dot{E}_k \propto L^{3/2}$), in agreement with the observational scaling relations reported in the most recent compilation of AGN outflow data. We conclude that AGN radiative feedback can account for the global outflow energetics, at least equally well as the wind energy-driving mechanism, and therefore both physical models should be considered in the interpretation of future AGN outflow observations.