Abstract
The evolution of galaxies is shaped by feedback from Active Galactic Nuclei (AGN) and massive stars, however there is little observational insight into feedback within individual galaxies which are typical of the general galaxy population. The majority of previous studies have instead focused on more extreme systems with higher star formation rates (SFRs) and/or more luminous AGN, or rely on stacking methods to enhance the relatively weak (and thus hard-to-detect) galactic wind signatures from typical systems. In order to advance our understanding of the impact of feedback on galaxy evolution, we must further complement our empirical studies with theoretical models where feedback effects are appropriately accounted for, given that our observations only provide an instantaneous snapshot of the state of the galaxy during its evolution.This thesis takes a two-pronged approach to investigate feedback in normal galaxies. First, feedback is directly probed through an investigation of outflows (a display of feedback in action) within individual galaxies in the nearby Universe. Second, feedback is indirectly probed though the development of an analytical model of galaxy formation which produces a population of galaxies with properties consistent with what is seen typically in the Universe, whilst accounting for ejective and preventative feedback effects.
In the 'direct approach', I investigate the ionized and neutral gas phases of galactic winds launched from (z ~ 0.04) galaxies in the MaNGA survey, which are probed using the ionized line-emission and neutral sodium absorption features in the galaxy spectra. I find ~12% of line-emitting MaNGA galaxies show evidence for ionized outflows, and ~5% show evidence for neutral gas outflows, where this offset can be at least partially attributed to the fact that neutral winds are preferentially seen in galaxies with dustier central regions. By looking at the warm, ionized component, I quantify strong correlations between mass outflow rates and the mechanical drivers of the outflows, and show low-luminosity AGN and star formation contribute jointly to the observed outflow phenomenology in MaNGA galaxies. By comparing to the neutral phase, I show the ionized outflow kinematics to be in line with what we measure in the neutral phase, demonstrating that, despite their small contributions to the total outflow mass budget, there is value to collecting empirical measurements of the ionized wind phase to provide information on bulk motion in the outflow. I further investigate the geometry of the winds, providing estimates for the radial extent of the outflows and showing evidence for outflows typically having a biconical structure.
For the 'indirect approach', I construct the Bath Tracing Underachieving Baryons (BathTUB) model. The key feature distinguishing this model from previous analytic models, is that BathTUB traces the size growth of galaxies. I find that in order to reproduce the shapes of empirically-derived galaxy scaling relations in the literature, both preventative and ejective feedback mechanisms need to be incorporated into the model, where the mass-loading of the ejective outflow scales negatively with stellar mass, and the preventative term reduces the efficiency of cold gas accretion onto galaxies with an amplitude that scales positively with halo mass.
Overall this thesis provides new insights from both an observational and theoretical perspective into the effects of feedback in typical, nearby galaxies.
| Date of Award | 14 Sept 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Stijn Wuyts (Supervisor) & Carole Mundell (Supervisor) |