The role of galaxy mergers in fuelling active galactic nuclei

: (Alternative Format Thesis)

Abstract

It is a well-known fact in astronomy that nearly all massive galaxies have a supermassive black hole (SMBH) lurking at their centre. Some show observational signatures of rapid growth, called active galactic nuclei (AGN). Determining which mechanisms drive AGN accretion is essential to understanding how SMBHs reached their present size. Galaxy mergers lead to gas cloud collisions and rapidly changing gravitational potential, driving inflows that may fuel an AGN. Conversely, intergalactic gas may accrete smoothly onto the galaxy and migrate to the centre via slowly-evolving disc instabilities. Observational studies over the past few decades have tried to determine which of these AGN triggering mechanisms dominates, but findings have been inconclusive. This is mainly due to the difficulty surrounding merger identification and morphological analysis of galaxies containing a bright central compact source. This PhD makes its novel contribution here, using deep learning to characterise AGN host galaxy morphologies.

The first part of this work uses supervised learning to identify observed galaxies in the local Universe that resemble simulated post-mergers. This approach finds similar overall merger fractions among host galaxies of low-luminosity, obscured AGN as among inactive controls, indicating that mergers are not the dominant cause of local AGN activity. It does, however, identify more post-mergers among star-forming AGN hosts than matched controls, indicating that mergers may boost AGN fuelling when the galaxies involved have rich reservoirs of cold gas.

The second part takes a largely-unsupervised approach to study galaxies containing unobscured AGN, showing that incorporating mock point sources into a contrastive learning framework decreases bias towards classifying galaxies as ellipticals that is otherwise very strong. Applying this method to a sample of local, low-luminosity AGN hosts finds that a majority occupy spiral galaxies, which are associated with a major-merger-free history. The fraction does not vary significantly with obscuration. These results support the conclusion that most nearby AGN are fuelled by secular evolution rather than galaxy mergers.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Carolin Villforth (Supervisor) & Stijn Wuyts (Supervisor)