What drives black hole and galaxy growth in the EAGLE simulation?

Abstract

In this thesis we investigate what drives the evolution of actively accreting central supermassive black holes and unusually active strongly star-forming galaxies using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) suite of cosmological hydrodynamical simulations. We find that many of our results are intimately tied to the complex evolutionary pathway taken by the central black holes within the simulation. This evolution can be separated into three distinct phases, each related to the mass of the host dark matter halo. In low mass haloes, stellar feedback dominates by driving an effective outflow and substantially hinders the growth of the central black hole. As haloes become more massive, the stellar feedback loses its efficiency, and the outflow stalls. This gives the first opportunity for the central black hole to grow, which is does so initially at a rapid rate. After this phase of rapid growth, the central black hole then becomes sufficiently massive to regulate the gas inflow onto the halo, resulting in both the star formation of the galaxy and any continued rapid growth of the central black hole to be substantially restricted via the outputted energy of an actively accreting supermassive black hole (referred to as an active galactic nuclei, or AGN). In Chapter 2 we discover that this complex evolutionary behaviour is integral to understanding how the growth rates of galaxies and their black holes are related to each other throughout cosmic time. We use this behaviour to explain why the current observational studies report different relationships between galaxy and black hole growth rates depending on the initial selection method used. Finally, in Chapter 3 we find that the evolutionary state of the black hole is also closely connected with high star formation rates in lower mass galaxies (~, where is the stellar mass of the galaxy). Such 'starbursting' galaxies are rare, and we argue that they are produced through a culmination of two coinciding events; (1) the galaxy must host an underdeveloped black hole (one that has not yet entered its rapid growth phase), thus ensuring that the galaxy has maintained a gas rich reservoir and contains a low mass black hole; and (2) the galaxy must undergo an interaction to kick-start the starburst process. This tells us that strongly star-forming galaxies are a predominately merger driven population that host undermassive black holes, making them fundamentally distinct from the 'typical' star-forming population.