Speaker
Description
Massive, large-scale galactic outflows are well known to be important to galaxy evolution. They shape the high-mass end of the mass function, enriching the circumgalactic and intergalactic medium, preventing the cooling catastrophe in galaxy clusters and so on. They also presumably establish the observed correlations between supermassive black holes and their host galaxies, such as the M-sigma relation. A simple analytical model posits that this relation arises when the Eddington-limited luminosity of the AGN, powered by accretion on to a black hole of mass M, provides just enough momentum to the surrounding gas to drive it out of the galaxy’s gravitational potential characterised by the velocity dispersion sigma. This model predicts the slope and scaling of the relation that are very close to observed values. However, real outflows are most likely driven by not only the momentum, but also the energy input from the AGN wind. Naively, this should lead to gas removal from the galaxy at much lower luminosities and, consequently, much lower black hole masses than observed.
The solution to this conundrum lies in the inherent non-spherical and multiphase nature of the interstellar medium. AGN energy-driven outflows expand due to the high pressure of the shocked AGN wind bubble. This is much easier to do in directions of lower density. So the energetic outflow expands through low-density channels, leaving high-density clouds in its wake. These clouds can continue to efficiently feed the black hole, unless the momentum of the AGN wind is high enough to push them away. That way, the M-sigma relation is reestablished.
I will present the results of a set of tightly controlled, progressively more detailed numerical simulations designed to capture the details of AGN energy-driven outflow propagation in a turbulent medium and its interaction with dense clouds. I will show that cooling of the outflowing gas leads to energy-driven outflows being weaker than the simple analytical model predicts, bringing their properties more in line with observations. I will also show that the dense clouds are pushed away and ablated by outflows driven by black holes having masses appropriate for the M-sigma relation. Finally, I will show how the observed relationships between AGN luminosity and outflow parameters arise from complex interplay between black hole masses and AGN Eddington ratios, and suggest how these properties can help us better understand the distribution of AGN Eddington ratios in galaxies with known large-scale outflows.
