From the Cosmic Web to the Bar: The Multiscale Drivers of Galaxy Spin Evolution

Abstract

The assembly and evolution of angular momentum in galaxies is shaped by a combination of internal and external processes. Integral field spectroscopy has revolutionised our ability to study stellar kinematics across thousands of systems. In this thesis, we investigate the physical drivers of galaxy spin evolution. We use marked correlation functions to find an anti-correlation of \lre\ with environment, and show it is not driven by stellar mass or slow rotators, and agrees with results from the EAGLE simulations.

We examine how mergers drive galaxy spin-down. From deep HSC SAMI galaxy images, we identify low surface brightness tidal features. Younger galaxies with tidal shells exhibit lower spin and higher slow rotator fractions. We conclude that radial major mergers are the primary driver of spin-down in young early-type SAMI galaxies, with the lack of shells in older systems reflecting early Universe.

We derive orbital distributions for a subsample of SAMI early-types, using Schwarzschild models. We find that the orbital fractions correlate strongly with \lre, with the strongest relation arising from the merger-generated hot + counter-rotating fraction. Hot and cold orbits correlate with stellar age and tidal shells, while warm orbits do not, implying mergers cause stars to transition directly from cold to hot orbits. These results indicate that merger-driven heating dominates the spin-down of massive galaxies.

We next extend this to late-type galaxies. We use GECKOS-MUSE observations of edge-on discs to assess how the presence of dust affects JAM modelling. When dust is masked, we find discs are fit well. Analysis of residual velocity fields reveals coherent excesses in two galaxies that are aligned with bar orbits and supported by photometric bar signatures. Collectively, this research contributes to a multiscale understanding of angular momentum evolution, connecting cosmological environment, mergers, age and the internal orbital structure of galaxies.