AGN outflows and feedback twenty years on

TL;DR

This paper reviews twenty years of research on AGN-driven outflows and their role in galaxy evolution, emphasizing observational challenges and caution in interpreting feedback efficiencies.

Contribution

It critically examines the observational and theoretical challenges in measuring AGN outflow feedback, highlighting the need for careful interpretation of kinetic coupling efficiencies.

Findings

Observational uncertainties significantly affect outflow property measurements.

Assumptions in deriving kinetic efficiencies can lead to misinterpretation.

Extreme caution is needed when comparing observations to models.

Abstract

It is the twentieth anniversary of the publication of the seminal papers by Magorrian et al. and Silk & Rees which, along with other related work, ignited an explosion of publications connecting active galactic nuclei (AGN)-driven outflows to galaxy evolution. With a surge in observations of AGN outflows, studies are attempting to directly test AGN feedback models using the outflow properties. With a focus on outflows traced by optical and CO emission lines, we discuss significant challenges which greatly complicate this task from both an observational and theoretical perspective. We highlight observational uncertainties involved, and the assumptions required, when deriving kinetic coupling efficiencies (i.e., outflow kinetic power as a fraction of AGN luminosity) from typical observations. Based on recent models we demonstrate that extreme caution should taken when comparing observationally-derived kinetic coupling efficiencies to coupling efficiencies from fiducial feedback models.