A comparative study of AGN feedback algorithms

Abstract

Modelling AGN feedback in numerical simulations is both technically and theoretically challenging, with numerous approaches having been published in the literature. We present a study of five distinct approaches to modelling AGN feedback within gravitohydrodynamic simulations of major mergers of Milky Way-sized galaxies. To constrain differences to only be between AGN feedback models, all simulations start from the same initial conditions and use the same star formation algorithm. Most AGN feedback algorithms have five key aspects: the black hole accretion rate, energy feedback rate and method, particle accretion algorithm, black hole advection algorithm and black hole merger algorithm. All models follow different accretion histories, and in some cases, accretion rates differ by up to three orders of magnitude at any given time. We consider models with either thermal or kinetic feedback, with the associated energy deposited locally around the black hole. Each feedback algorithm modifies the region around the black hole to different extents, yielding gas densities and temperatures within r ∼ 200 pc that differ by up to six orders of magnitude at any given time. The particle accretion algorithms usually maintain good agreement between the total mass accreted byṀ dt and the total mass of gas particles removed from the simulation, although not all algorithms guarantee this to be true. The black hole advection algorithms dampen inappropriate dragging of the black holes by twobody interactions. Advecting the black hole a limited distance based upon local mass distributions has many desirably properties, such as avoiding large artificial jumps and allowing the possibility of the black hole remaining in a gas void. Lastly, two black holes instantly merge when given criteria are met, and we find a range of merger times for different criteria. This is important since the AGN feedback rate changes across the merger in a way that is dependent on the specific accretion algorithm used. Using the M BH -σ relation as a diagnostic of the remnants yields three models that lie within the one-sigma scatter of the observed relation and two that fall below the expected relation. The wide variation in accretion behaviours of the models reinforces the fact that there remains much to be learnt about the evolution of galactic nuclei.

Herschel-ATLAS: the connection between star formation and AGN activity in radio-loud and radio-quiet active galaxies

“…with our results reported here (see Wurster & Thacker (2013) for details of these models.) 4.2 Probable reasons for the difference between SFR/SSFR of radio-loud and radio-quiet AGN…”

Gravitational torque-driven black hole growth and feedback in cosmological simulations

“…Different improvements to the treatment of black hole dynamics have been proposed (e.g. Wurster & Thacker 2013) but this scheme is sufficient to compute the mass growth of "wellbehaved" central black holes in our simulations. We thus assume that dynamical friction is efficient enough to maintain black holes close to the center of galaxies but note that this may not properly capture the orbital decay of black holes in low mass galaxies and/or at high redshift (Tremmel et al 2015).…”

Off the beaten path: a new approach to realistically model the orbital decay of supermassive black holes in galaxy formation simulations

“…For example, placing the SMBH at the position of the lowest potential gas particle around it (e.g. Springel et al 2005b;Booth & Schaye 2009) causes chaotic motions, especially when relative velocity constraints are put on the gas particles (Wurster & Thacker 2013). Utilizing a large 'tracer mass' with which the SMBH gravitationally interacts with its surroundings (Debuhr et al 2011) affects the morphology in the galactic center as well as the accretion history of the SMBH (Wurster & Thacker 2013).…”

“…Springel et al 2005b;Booth & Schaye 2009) causes chaotic motions, especially when relative velocity constraints are put on the gas particles (Wurster & Thacker 2013). Utilizing a large 'tracer mass' with which the SMBH gravitationally interacts with its surroundings (Debuhr et al 2011) affects the morphology in the galactic center as well as the accretion history of the SMBH (Wurster & Thacker 2013). Pushing the SMBH a certain distance in the direction of either the local stellar density gradient (Okamoto et al 2008) or the local center of mass (Wurster & Thacker 2013) can avoid chaotic motions but adds an additional free parameter, the distance the SMBH is pushed each step.…”

“…Utilizing a large 'tracer mass' with which the SMBH gravitationally interacts with its surroundings (Debuhr et al 2011) affects the morphology in the galactic center as well as the accretion history of the SMBH (Wurster & Thacker 2013). Pushing the SMBH a certain distance in the direction of either the local stellar density gradient (Okamoto et al 2008) or the local center of mass (Wurster & Thacker 2013) can avoid chaotic motions but adds an additional free parameter, the distance the SMBH is pushed each step. Even disregarding their individual drawbacks, none of these solutions are ideal, as they all require the explicit assumption that SMBHs rapidly decay into and remain stable at the center of their host galaxies , which is often not accurate (Bellovary et al 2010).…”