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

© 2024 The Author(s). Published by EDP Sciences. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/Context. Accretion onto supermassive black holes at close to the Eddington rate is expected to drive powerful winds, which have the potential to majorly influence the properties of the host galaxy. Theoretical models of such winds can simultaneously explain observational correlations between supermassive black holes and their host galaxies, such as the M − σ relation, and the powerful multi-phase outflows that are observed in a number of active galaxies. Analytic models developed to understand these processes usually assume simple galaxy properties, namely 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. Aims. We wish to determine how gas turbulence, uneven density distribution, and cooling influence the development of active galactic nucleus (AGN) wind-driven outflows and their global properties on kiloparsec scales. Methods. We calculated a suite of idealised hydrodynamical simulations of AGN outflows designed to isolate the effects of turbulence and cooling, both separately and in combination. All simulations initially consisted of a 1 kpc gas shell with an AGN in the centre. We measured the main outflow parameters – the velocity, the mass outflow rate (Ṁout), and the momentum (ṗoutc/LAGN) and energy (Ėout/LAGN) loading factors – as the system evolves over 1.2 Myr and estimated plausible observationally derived values. Results. We find that adiabatic simulations approximately reproduce the analytical estimates of outflow properties independently of the presence or absence of turbulence and clumpiness. Cooling, on the other hand, has a significant effect, reducing the outflow energy rate by one to two 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 in lower-luminosity simulations, the opposite is true. This mainly 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, although there are some quantitative differences. We also find that using ‘observable’ outflow properties leads to their parameters being underestimated by a factor of a few compared with real values. Conclusions. We conclude that the AGN wind-driven outflow model is capable of reproducing realistic outflow properties in close-to-realistic galaxy setups and that the M − σ relation can be established without efficient cooling of the shocked AGN wind. Furthermore, we suggest ways to improve large-scale numerical simulations by accounting for the effects of AGN wind.Peer reviewe

Do AGN outflows quench or enhance star formation?

Active galactic nucleus (AGN) outflows can remove large quantities of gas from their host galaxy spheroids, potentially shutting off star formation. On the other hand, they can compress this gas, potentially enhancing or triggering star formation, at least for short periods. We present a set of idealized simulations of the AGN outflows affecting turbulent gas spheres, and investigate the effect of the outflow and the AGN radiation field upon gas fragmentation. We show that the AGN outflows of sufficient luminosity shut off fragmentation while the nucleus is active, but gas compression results in a burst of fragmentation after the AGN switches off. Self-shielding of gas against the AGN radiation field allows some fragmentation to occur during outbursts, but too much shielding results in a lower overall fragmentation rate. For our idealized simulation setup, there is a critical AGN luminosity, which results in the highest fragmentation rate, with outflows being too efficient at removing gas when $L$ > $L_{crit}$ and not efficient enough to compress the gas to high densities otherwise. These results, although preliminary, suggest that the interaction between AGN and star formation in their host galaxies is particularly complex and requires careful study to interpret observations correctly.KZ is funded by the Research Council Lithuania through the National Science Programme grant no. LAT-09/2016. MAB acknowledges support by the ERC starting grant 638707 ‘BHs and their host galaxies: co-evolution across cosmic time.’ Simulations were performed on resources at the High Performance Computing Center HPC Sauletekis in Vilnius University Faculty of Physics

A simple way to improve AGN feedback prescription in SPH simulations

Active galactic nuclei (AGN) feedback is an important ingredient in galaxy evolution, however its treatment in numerical simulations is necessarily approximate, requiring subgrid prescriptions due to the dynamical range involved in the calculations. We present a suite of smoothed particle hydrodynamics simulations designed to showcase the importance of the choice of a particular subgrid prescription for AGN feedback. We concentrate on two approaches to treating wide-angle AGN outflows: thermal feedback, where thermal and kinetic energy is injected into the gas surrounding the supermassive black hole (SMBH) particle, and virtual particle feedback, where energy is carried by tracer particles radially away from the AGN.We show that the latter model produces a far more complex structure around the SMBH, which we argue is a more physically correct outcome. We suggest a simple improvement to the thermal feedback model - injecting the energy into a cone, rather than spherically symmetrically - and show that this markedly improves the agreement between the two prescriptions, without requiring any noticeable increase in the computational cost of the simulation.KZ is funded by the Research Council of Lithuania grant no. MIP-062/2013. MAB and SN acknowledge an STFC grant. MAB is funded by an STFC research studentship. We thank Justin Read for the use of SPHS. This research used the DiRAC complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the UK National E-Infrastructure