Black Hole and Host Galaxy Co-Evolution

Thorne, Matthew and Simmons, Brooke (2026) Black Hole and Host Galaxy Co-Evolution. PhD thesis, Lancaster University.

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Abstract

The origins and the evolution of the observed correlations between supermassive black holes (SMBHs) and their host galaxies are still under debate. A merger-driven co-evolution framework has been a popular hypothesis, and suggests that mergers fuel both star formation and SMBH accretion. However, there is now evidence that the majority of black hole growth has occurred in the absence of mergers; moreover, a merger-driven scenario does not account for the rapidly accreting SMBHs, known as active galactic nuclei (AGN), observed in galaxies with morphologies indicative of a secular evolutionary history. The nature of co-evolution in disc-dominated galaxies is unclear, with different studies of unobscured (Type 1) AGN drawing contradictory conclusions. We analyse three such samples, applying their selection functions to a mock population of Type 1 AGN. We find that the different samples from the published studies are consistent with being drawn from the same underlying population. The mock AGN population is agnostic to the specifics of galaxy mass accretion and assembly. The fact that the disc galaxy samples are in agreement with the mock sample supports the growing consensus that merger-free black hole growth is typical and can lead to co-evolution between supermassive black holes and galaxies. We then use multicomponent 2D decomposition to examine the morphological dependence, as traced by bulge-to-total (B/T ) ratio, of the correlations between SMBH mass and both total and bulge stellar mass for a sample of 415 Type 1 AGN at z < 0.35. We use mock synthetic AGN host galaxies, generated by inserting point sources into inactive galaxies, to show that the decomposition process is reliable with small systematics. We find no morphological dependence on the SMBH mass-total stellar mass relation, while we find that the SMBH mass-bulge stellar mass relation is a shallower relation and dependent on B/T , with SMBHs in disc-dominated galaxies being offset above SMBHs hosted in bulge-dominated galaxies for a given bulge stellar mass. This further supports the hypothesis that secular evolution plays a strong role in driving co-evolution. In summary, regardless of the mechanism driving bulge growth, coevolution of the galaxy and SMBH clearly occurs, and is more strongly tied to the total stellar mass than to bulge properties. Thus, AGN growth may be more related to the total gravitational potential, which is better traced by the total stellar mass than the bulge stellar mass. The new and upcoming surveys are well placed to further this work with larger or higher resolution samples.