The complex effect of gas cooling and turbulence on AGN-driven outflow properties

TL;DR

This study uses hydrodynamical simulations to explore how turbulence and cooling influence the properties of AGN-driven outflows, revealing complex interactions that depend on AGN luminosity and aligning with observational data.

Contribution

It introduces a detailed analysis of cooling and turbulence effects on AGN outflows using idealised simulations, highlighting their impact on outflow energetics and coupling mechanisms.

Findings

Cooling significantly reduces outflow energy in smooth and turbulent cases.

Turbulence enhances wind-gas coupling at high AGN luminosity, but not at low luminosity.

Simulated outflow properties align qualitatively with observations, but observational estimates tend to underestimate true values.

Abstract

(abridged) Accretion onto supermassive black holes (SMBHs) at close to the Eddington rate can influence the host galaxy via powerful winds. Theoretical models of such winds can explain observational correlations between SMBHs and their host galaxies and the powerful multi-phase outflows observed in a number of active galaxies. Analytic models usually assume spherical symmetry and a smooth gas distribution with an adiabatic equation of state. However, the interstellar medium in real galaxies is clumpy and cooling is important, complicating the analysis. We used a suite of idealised hydrodynamical simulations to isolate the effects of turbulence and cooling on the development and global properties of AGN wind-driven outflows on kiloparsec scales. We measured the outflow velocity, mass outflow rate and momentum and energy loading factors as the system evolved over 1.2 Myr and estimated plausible observationally derived values. We find that adiabatic simulations approximately reproduce the analytical estimates of outflow properties independently of turbulence or clumpiness. However, cooling reduces the outflow energy rate by 1-2 orders of magnitude in the smooth simulations and by up to one order of magnitude in the turbulent ones. The interplay between cooling and turbulence depends on AGN luminosity: in Eddington-limited AGN, turbulence enhances the coupling between the AGN wind and the gas, while the opposite happens in lower-luminosity simulations. This occurs because dense gas clumps are resilient to low-luminosity AGN feedback but get driven away by high-luminosity AGN feedback. The overall properties of multi-phase outflowing gas in our simulations qualitatively agree with observations of multi-phase outflows. We also find that using `observable' outflow properties leads to their parameters being underestimated by a factor of a few compared with real values.