We present hydrodynamical simulations of major mergers of galaxies and study the effects of winds produced by active galactic nuclei (AGN) on interstellar gas in the AGN’s host galaxy. We consider winds with initial velocities ∼10 000 km s−1 and an initial momentum (energy) flux of ∼τw L/c (∼ 0.01  τw L), with . The AGN wind sweeps up and shock heats the surrounding interstellar gas, leading to a galaxy‐scale outflow with velocities ∼1000 km s−1, peak mass outflow rates comparable to the star formation rate and a total ejected gas mass of ∼3 × 109 M⊙. Large momentum fluxes, τw≳ 3, are required for the AGN‐driven galactic outflow to suppress star formation and accretion in the black hole’s host galaxy. Less powerful AGN winds (τw≲ 3) still produce a modest galaxy‐scale outflow, but the outflow has little global effect on the ambient interstellar gas. We argue that this mechanism of AGN feedback can plausibly produce the high‐velocity outflows observed in post‐starburst galaxies and the massive molecular and atomic outflows observed in local ultraluminous infrared galaxies. Moreover, the outflows from local ultraluminous infrared galaxies are inferred to have τw∼ 10, comparable to what we find is required for AGN winds to regulate the growth of black holes and set the MBH ‐ σ relation. We conclude by discussing theoretical mechanisms that can lead to AGN wind mass loading and momentum/energy fluxes large enough to have a significant impact on galaxy formation.

We perform hydrodynamical simulations of major galaxy mergers using new methods for calculating the growth of massive black holes (BH) in galactic nuclei and their impact on the surrounding galaxy. We model BH growth by including a subgrid model for accretion produced by angular momentum transport on unresolved scales. The impact of the BH's radiation on surrounding gas is approximated by depositing momentum into the ambient gas, which produces an outward force away from the BH. We argue that these phenomenological models for BH growth and feedback better approximate the interaction between the BH and dense gas in galaxies than previous models. We show that this physics leads to self‐regulated BH growth: during the peak of activity, the accretion rate on to the BH is largely determined by the physics of BH feedback, not the subgrid accretion model. The BH significantly modifies the gas dynamics in the galactic nucleus (≲300 pc), but does not generate large‐scale galactic outflows. Integrated over an entire galaxy merger, BH feedback has little effect on the total number of stars formed, but is crucial for setting the BH’s mass.

We investigate the gravitational interactions between live stellar discs and their dark matter haloes, using cold dark matter haloes similar in mass to that of the Milky Way taken from the Aquarius Project. We introduce the stellar discs by first allowing the haloes to respond to the influence of a growing rigid disc potential from z = 1.3 to 1.0. The rigid potential is then replaced with star particles which evolve self-consistently with the dark matter particles until z = 0.0. Regardless of the initial orientation of the disc, the inner parts of the haloes contract and change from prolate to oblate as the disc grows to its full size. When the disc's normal is initially aligned with the major axis of the halo at z = 1.3, the length of the major axis contracts and becomes the minor axis by z = 1.0. Six out of the eight discs in our main set of simulations form bars, and five of the six bars experience a buckling instability that results in a sudden jump in the vertical stellar velocity dispersion and an accompanying drop in the m = 2 Fourier amplitude of the disc surface density. The bars are not destroyed by the buckling but continue to grow until the present day. Bars are largely absent when the disc mass is reduced by a factor of 2 or more; the relative disc-to-halo mass is therefore a primary factor in bar formation and evolution. A subset of the discs is warped at the outskirts and contains prominent non-coplanar material with a ring-like structure. Many discs reorient by large angles between z = 1 and 0, following a coherent reorientation of their inner haloes. Larger reorientations produce more strongly warped discs, suggesting a tight link between the two phenomena. The origins of bars and warps appear independent: some discs with strong bars show little disturbances at the outskirts, while the discs with the weakest bars show severe warps.

We study the growth of massive black holes (BHs) in galaxies using smoothed particle hydrodynamic simulations of major galaxy mergers with new implementations of BH accretion and feedback. The effect of BH accretion on gas in its host galaxy is modelled by depositing momentum at the rate of ∼τL/c into the ambient gas, where L is the luminosity produced by accretion on to the BH and τ is the wavelength‐averaged optical depth of the galactic nucleus to the AGN’s radiation (a free parameter of our model). The accretion rate on to the BH is relatively independent of our subgrid accretion model and is instead determined by the BH’s dynamical impact on its host galaxy: BH accretion is thus self‐regulated rather than ‘supply limited’. We show that the final BH mass and total stellar mass formed during a merger are more robust predictions of the simulations than the time dependence of the star formation rate or BH accretion rate. In particular, the latter depend on the assumed interstellar medium physics, which determines when and where the gas fragments to form star clusters; this in turn affects the fuel available for further star formation and BH growth. Simulations over a factor of ∼30 in galaxy mass are consistent with the observed MBH–σ relation for a mean optical depth of τ∼ 25. This requires that most BH growth occur when the galactic nucleus is optically thick to far‐infrared radiation, consistent with the hypothesized connection between ultraluminous infrared galaxies and quasars. We find tentative evidence for a shallower MBH–σ relation in the lowest‐mass galaxies, σ≲ 100 km s−1. Our results demonstrate that feedback‐regulated BH growth and consistency with the observed MBH–σ relation do not require that BH feedback terminate star formation in massive galaxies or unbind large quantities of cold gas.

During galaxy mergers the gas falls to the center, triggers star formation, and feeds the rapid growth of supermassive black holes (SMBHs). SMBHs respond to this fueling by supplying energy back to the ambient gas. Numerical studies suggest that this feedback is necessary to explain why the properties of SMBHs and the formation of bulges are closely related. This intimate link between the SMBH's mass and the large scale dynamics and luminosity of the host has proven to be a difficult issue to tackle with simulations due to the inability to resolve all the relevant length scales simultaneously. In this paper we simulate SMBH growth at high-resolution with FLASH, accounting for the gravitational focusing effects of nuclear star clusters (NSCs), which appear to be ubiquitous in galactic nuclei. In the simulations, the NSC core is resolved by a minimum cell size of about 0.001 pc or approximately 10 −3 of the cluster's radius. We discuss the conditions required for effective gas funneling to occur, which are mainly dominated by a relationship between NSC velocity dispersion and the local sound speed, and provide a sub-grid prescription for the augmentation of central SMBH accretion rates in the presence of NSCs. For the conditions expected to persist in the centers of merging galaxies, the resultant large central gas densities in NSCs should produce drastically enhanced embedded SMBH accretion rates -up to an order of magnitude increase can be achieved for gas properties resembling those in large-scale galaxy merger simulations. This will naturally result in faster black hole growth rates and higher luminosities than predicted by the commonly used Bondi-Hoyle-Lyttleton accretion formalism.