title: The chemical evolution of galaxies and nuclear clusters - a synergetic view from cosmological numerical simulations and analytical models
Creators: name: Mason, A
Publisher: name: Liverpool John Moores University
Year: 2025

description: Over the past century, grand strides in methods and instrumentation have allowed us to observe the Universe -- especially our Local Group of galaxies -- in exquisite detail such that we can constrain the assembly of these galaxies by the study of their chemical compositions. Using sophisticated simulations of galaxy formation and evolution, we can also assess how representative the Local Group is today of other systems at $z=0$. This thesis aims to assess the connection between the assembly of galaxies and star clusters, and the chemical compositions of their stellar populations, using a combination of observational datasets, cosmological numerical simulations, and analytical models of galaxy chemical evolution. This thesis provides a new interpretation of the formation of the so-called `$\upalpha$-Fe knee' in nearby galaxies, a feature on the $\upalpha$-Fe planes of galaxies that is thought to arise due to the onset of SN Ia contributing Fe to the ISM. I perform an analysis of the $\upalpha$-abundance patterns of galaxies populating a high-resolution realisation of the EAGLE cosmological numerical simulations. I focus on the cosmic evolution of the abundance ratio [$\upalpha$/Fe] as a function of [Fe/H], with a view to constraining: {\it i)} the origin by which the $\upalpha$-knee forms in simulated galaxies, {\it ii)} why the stellar populations of some observed galaxies do not display this feature on the $\upalpha$-Fe plane, and {\it iii)} if there is a tight scaling relation between stellar mass and the metallicity of the $\upalpha$-knee, as well as what drives the scatter in this relation. I demonstrate that EAGLE predicts a diversity of possible distributions of stellar particles on the $\upalpha$-Fe plane in simulated galaxies, much like we see in the Local Group. We compare and contrast galaxies that exhibit no $\upalpha$-knee (`single slopes') on the $\upalpha$-Fe plane and those that do (`classical knees'), finding that the former are characterised by extended histories of roughly constant star formation. The latter exhibit sharp declines in their SFRs after an earlier rise, which is coincident with the formation of the $\upalpha$-knee. This is because a sharp decline in the SFR causes a prompt decrease in enrichment by SN II, leaving the SN Ia from antecedent star formation episodes to dominate the Fe-enrichment. Since single sloped galaxies do not experience a similar drop in star formation, they exhibit gently declining distributions of [$\upalpha$/Fe] vs. [Fe/H] over the whole range of [Fe/H] with no change in slope. I further demonstrate that the scaling relation between the stellar mass of a galaxy and the metallicity of the $\upalpha$-knee in the simulations is similar to that observed in the Local Group dwarf galaxies, including its scatter. Our analysis shows that the scatter in this relation is caused by variation in the amount of star formation taking place before the peak of the star formation history. As a result, galaxies with low [Fe/H]$_{\rm knee}$ at fixed $M_\star$ formed less massive plateaus before the decline in the SFR, and vice versa. Finally, I compare the typical assumptions of one-zone GCE models regarding the treatment of the star formation efficiency to what is seen in the EAGLE simulations. I find that the star formation efficiency in EAGLE galaxies vary, by an order of magnitude (from low to high) as a function of cosmic time in knees. That is in stark contrast to the constant star formation efficiency typically involved in one-zone GCE models, an assumption which implies the presence of a readily-enriched, low-mass gas reservoir. Instead, the mixing mass in EAGLE galaxies tends to be much higher, and thus SN Ia can still make significant contribution to the enrichment of the plateau. A decline in the star formation rate produces a clear $\upalpha$-knee, consequently. In the second half of the thesis, I performed a detailed chemical study of the complex Galactic stellar system $\omega$ Centauri, a system suspected to be the accreted nuclear star cluster of a dSph galaxy that underwent a merger with the Milky Way at an earlier cosmic epoch. We identified three distict populations in the cluster, which are labelled P1, P2, and IM (for intermediate). We find that the P1 population exhibits field-like abundance patterns, whereas P2 and IM are characterised by the chemical abundance anomalies typical of poulations seen in other Galactic globular clusters. Using complementary photometric data derived from HST observations, we show that the complexity of the distribution of $\omega$ Cen stars in the so-called `chromosome map' is accounted for by the range of light-element abundances covered by the P1, P2, and IM populations as well as the chemical evolution of P1 and P2. We further hypothesise that the IM population resulted from the inspiral of GCs from $\omega$ Cen's progenitor galaxy, which may also have deposited gas in the centre off the cluster. The subsequent star formation episode may then have formed the P2 population, which may also have experienced significant loss of gas or stars during this episode. The above outlines a new interpretation of the abundance patterns of the Local Group dwarfs and their histories of star formation, as well as a prediction for what could be a useful scaling relation for galaxies in the Local Group. Furthermore, the hypothetical scenario to explain the abundance patterns of $\omega$ Cen is one well-motivated by the likely nature of the cluster as a nuclear star cluster (NSC).