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Abstract

Galaxy mergers are a key process in the current paradigm of hierarchical galaxy formation. They are responsible for many galaxy transformations. For instance, they can drive changes in galaxy morphology and colour, boost the star formation activity and trigger strong inflows of pristine gas that dilute the central gas metallicity and feed the central supermassive black hole. To which extent and how galaxy mergers affect some of the galaxy properties are still open questions. In the first part of this thesis, we use simulations and observations to study the processes of merger-induced gas metallicity dilution and star formation enhancement. We find that merging and post-merger galaxies constitute a prominent outlier population in the fundamental metallicity relation, a relation that links the galaxy stellar mass with the star formation rate and the gas metallicity. In the second part of this thesis, we present a sub-grid model of supermassive black hole spin evolution in cosmological simulations of galaxy formation. We find that the accretion discs of low-mass black holes accrete mass in a coherent fashion, resulting in high spin values. On the other hand, high-mass black holes have lower spins because gas accretion is either chaotic or completely suppressed, in which case black hole binary coalescence resulting from galaxy mergers is the only relevant evolution channel. Finally, we test a hypothesis in which the black hole energy feedback transitions from a thermal mode to a kinetic mode at the onset of self-gravity fragmentation in the accretion disc, and is thus coupled to the spin evolution of the black hole. We find that our conjecture reproduces the galaxy colour bimodality and the colour-morphology relation.