Under Pressure: UV Emission Line Ratios as Barometers of AGN Feedback Mechanisms

TL;DR

This study uses UV and optical emission line ratios from AGN-driven outflows to determine whether radiation pressure or hot winds primarily drive feedback mechanisms, finding radiation pressure generally dominates but hot winds can also be significant.

Contribution

It provides the first multi-object analysis of FUV spectra in obscured quasars to constrain AGN feedback drivers using emission line ratios and theoretical models.

Findings

Radiation pressure dominates in most targets.

Hot winds may also contribute in some cases.

Results support feedback scenarios involving radiation pressure or transient hot wind phases.

Abstract

Feedback from active galactic nuclei (AGN) is widely acknowledged to regulate the growth of massive galaxies, though its driving mechanisms are debated. Prevailing theories suggest that AGN-driven outflows are driven either by radiation pressure acting directly on the dusty interstellar medium (ISM) or by hot winds entraining cooler ISM gas, but the relative contribution of each mechanism remains uncertain. By combining optical emission line measurements with highly ionized UV emission lines, it is possible to constrain whether the pressure source applied to ionized clouds is primarily radiation or primarily hydrodynamic, and thus constrain the dominant driver. This study presents the first multi-object analysis of far-ultraviolet (FUV) spectra from galactic-scale AGN-driven outflows in obscured quasars, based on Cosmic Origins Spectrograph observations of five low-redshift targets. By comparing narrow-line region UV emission line ratios to theoretical models that vary the importance of the two pressure sources, we find three out of five targets fall within the radiation pressure-dominated regime. A fourth target exhibits intermediate emission-line ratios that suggest radiation pressure and pressure from a hot wind are both dynamically important. Finally, the lowest-luminosity object in our sample may have a dynamically important hot wind component, but non-detections prevent a clear conclusion in this case. These results suggest radiation pressure dominates circum-nuclear narrow-line region cloud dynamics, but pressure from a hot wind also plays a role in some cases. This is consistent with AGN feedback scenarios mediated by radiation pressure or a short-lived hot wind phase that dissipates after initially accelerating outflows.