One merger to rule them all From galaxies to black hole merges

Galaxies are observed at individual moments in cosmic history, while the processes that shape them unfold over much longer timescales. Their evolution is influenced by both their internal properties and their environments, from the ionized regions surrounding the first galaxies to the interactions and mergers that drive later evolution and the growth of supermassive black holes. Connecting these snapshots to the physical structures and histories behind them is essential for understanding how galaxies and black holes evolve. This thesis aims to make this connection using imaging, spectroscopy, and cosmological simulations.
We first study a small sample of luminous Lyman-α emitters during the Epoch of Reionization and find an excess of neighbouring galaxies around them, suggesting that these sources can trace overdense regions where large ionized bubbles may form. With advances in observational facilities and broader spectral coverage, we then extend this study to a larger spectroscopic sample. We find that Lyman-α emitters are not found in denser environments on average than the general galaxy population, suggesting that their visibility is influenced not only by their environment, but also by other factors.
Finally, we use cosmological simulations to connect observed galaxy pairs to galaxy and supermassive black hole mergers. We find that galaxy and black hole merger histories are closely connected, while predictions of black hole merger rates depend strongly on merger timescales and on how black hole mergers are modelled. These results connect observed galaxy populations to their environments, merger histories, and future gravitational-wave sources.