Unravelling the physics of multiphase AGN winds through emission line tracers

TL;DR

This study uses radiative transfer calculations in hydrodynamic simulations to understand how emission lines trace the physical properties of multiphase AGN outflows driven by hot wind bubbles, revealing non-equilibrium conditions and key line diagnostics.

Contribution

It introduces a novel approach combining hydrodynamic simulations with radiative transfer to analyze emission line tracers of AGN outflows, highlighting the importance of non-equilibrium effects.

Findings

Hot bubbles compress line-emitting gas, affecting line ratios.

Line emission often arises from out-of-pressure equilibrium gas.

Key emission lines trace over 50% of outflow mass, momentum, and energy.

Abstract

Observations of emission lines in Active Galactic Nuclei (AGN) often find fast (~1000 km s^-1) outflows extending to kiloparsec scales, seen in ionised, neutral atomic and molecular gas. In this work we present radiative transfer calculations of emission lines in hydrodynamic simulations of AGN outflows driven by a hot wind bubble, including non-equilibrium chemistry, to explore how these lines trace the physical properties of the multiphase outflow. We find that the hot bubble compresses the line-emitting gas, resulting in higher pressures than in the ambient ISM or that would be produced by the AGN radiation pressure. This implies that observed emission line ratios such as [OIV] 25 ${\mu}$m / [NeII] 12 ${\mu}$m , [NeV] 14 ${\mu}$m / [NeII] 12 ${\mu}$m and [NIII] 57 ${\mu}$m / [NII] 122 ${\mu}$m constrain the presence of the bubble and hence the outflow driving mechanism. However, the line-emitting gas is under-pressurised compared to the hot bubble itself, and much of the line emission arises from gas that is out of pressure, thermal and/or chemical equilibrium. Our results thus suggest that assuming equilibrium conditions, as commonly done in AGN line emission models, is not justified if a hot wind bubble is present. We also find that >50 per cent of the mass outflow rate, momentum flux and kinetic energy flux of the outflow are traced by lines such as [NII] 122 ${\mu}$m and [NeIII] 15 ${\mu}$m (produced in the 10^4 K phase) and [CII] 158 ${\mu}$m (produced in the transition from 10^4 K to 100 K).