Supermassive Black Hole Feeding in Galactic Nuclei

In this thesis we present numerical and analytical models of supermassive black hole (SMBH) feeding, via deposition of gas, in galactic nuclei. Through simulations, we consider the environment of galactic centres, starting at sub-parsec scales within our own Milky Way, and moving upwards in scale and outwards in generality to scales of hundreds of parsecs in typical galaxies and finally to dark matter halos within which galaxies reside. We find that the stellar features observed in our own Galactic centre are likely explained by a collision between two molecular clouds at a distance of a few parsecs from the central black hole, Sgr A*. The amount of gas transported to small radii is large, occurring on a timescale close to dynamical. The disordered nature of the flow leads to the formation of a gaseous disc around Sgr A* that in some cases remains small-scale, undergoing complex, time-varying evolution in its orientation. Such a disc would efficiently feed the SMBH, if replenished from larger scales. We develop a model for ballistic accretion onto an SMBH at the centre of a typical galaxy, from scales of ~ hundreds of parsecs. We invoke turbulence in the gas, assumed to be driven by feedback from supernovae, as the means to create such a flow. The accretion mode is again dominated, soon after the initial turbulent kick, by the dynamical timescale for the gas in the angular momentum loss-cone, resulting in an accretion rate at or near Eddington, >~ 1Mסּ yr−1. At the largest scale, we critically evaluate the current state-of-the-art prescription for SMBH growth in cosmological simulations, finding that in general it lacks a physically consistent basis. We propose an alternative, motivated by our analytical estimates and numerical simulations, that is based on the free-fall time.