Dynamics of outflows driven by winds from supermassive black holes

Abstract

Feedback is a key element of galaxy evolution, and the accretion of gas onto a central supermassive black hole (SMBH) is an important source of feedback. Modelling the energy released as an optically thick wind (with speed vw) allows observable relationships between galactic properties to be derived. Models of wind feedback predict that the momentum-flux of an outflow at large scales will exceed that of the wind source. This so-called momentum boosting of large-scale outflows and the presence of small-scale winds have both been observed in many local active galaxies. This thesis analyses the dynamical properties of wind-driven shells in order to investigate momentum-boosting in active galaxies and the observed correlation between SMBH mass (MBH) and stellar velocity dispersion (s).
The effects of ambient gas pressure on wind-driven shells are analysed and it is shown that shells can be become confined by this pressure. The inclusion of ambient pressure is also shown to formally alter previously derived MBH - s relations. For energy-driven shells at large scales it is found that there is an upper limit to momentum-boosting for a given MBHvw combination, and that maximum possible boosting occurs for a fixed ratio of wind and shell kinetic energies. It is demonstrated that observed large-scale outflows have momentum-boosts which are consistent with maximal boosting, and therefore such a scenario may be commonplace for large-scale outflows. By considering maximally boosted shells an MBH - s relation is derived which allows for the interpretation of the scatter in the MBH - s data as a distribution in momentumboosts.
These conclusions and the dynamics of shells are further examined for the case of a growing SMBH and therefore a non-steady wind. It is shown that infalling shells are capable of resuming outward motion due to the ever-growing wind force, and that conditions required for shells to be driven to large radii are not significantly different from the steady wind case. The conclusions regarding maximal momentum-boosting for steady winds are demonstrated to still be valid for non-steady winds.