The chemical evolution of galaxies in semi-analytic models and observations

The chemical compositions of the stars and gas in galaxies play a
significant role in all their key evolutionary processes, from gas
cooling, through star formation, to the production of new heavy
elements that are released back into the gas as stars die in
supernova explosions. A theoretical explanation of the production
of elements heavier than helium (known simply as `metals' in
astrophysics) in stars and its distribution throughout galaxies has
been developing since the first postulation of stellar
nucleosynthesis in the 1920s. However, there are still a number of
unanswered questions in the field of galactic chemical evolution
(GCE). For example, what is the most accurate way to measure the
metallicities in galaxies? What are the relative contributions to
GCE from different types of stars? How is this metal-rich material
circulated throughout the various components of a galaxy? And how
can we explain the seemingly incompatible chemical properties
observed in different galaxies in the local Universe? This thesis
provides an investigation into the chemical enrichment of galaxies,
by utilising both observations of nearby galaxies and sophisticated
GCE models within a semi-analytic model of galaxy evolution. Its
core aims are a) to better quantify the chemical properties seen in
low-redshift galaxies and explain there likely causes, and b) to
develop an improved GCE model that can simultaneously reproduce the
diverse chemical properties seen in different types of galaxies in
the local Universe. With these aims in mind, Chapter 1 outlines the
key background knowledge required for such an investigation. It
discusses the different methods used for measuring the metallicity
of real galaxies, and their various shortcomings. It also describes
simple, analytic GCE models, and the sophisticated semi-analytic
model, L-Galaxies, that is used to simulate galaxy evolution in
detail. In Chapters 2 and 3, I provide an investigation into the
relation between stellar mass (M*), star formation rate (SFR), and
gas-phase metallicity (Zg) in galaxies. It is shown that the
L-Galaxies model reproduces the positive correlation between SFR
and Zg in massive galaxies that is seen when using sophisticated,
theoretical metallicity diagnostics. This lends support to the use
of such diagnostics over simpler, emission-line ratios. It is
further shown that, in the semi-analytic model, this SFR-Zg
correlation is due to the gradual dilution of the gas in low-SFR,
elliptical galaxies, after a gas-rich merger event. A number of
signatures of this particular evolution can be seen in these model
galaxies at redshift zero, including low gas fractions and low
values of (Zg-Z*). Crucially, all of these properties are also seen
in nearby elliptical galaxies in the Sloan Digital Sky Survey
(SDSS), providing indirect evidence that such an evolutionary
process is also occurring in the elliptical galaxy population in
the real Universe. In Chapter 4, I present a new, sophisticated GCE
model implemented into L-Galaxies, that significantly improves on
the previous scheme. It does this by accounting for the delayed
enrichment of many chemical elements from stars, of various initial
masses and metallicities, via stellar winds and supernovae. This
new scheme enables a much more detailed study of the chemical
evolution of galaxies, and enables a comparison with a larger range
of observational data. In Chapter 5, I demonstrate that this new
model is able to simultaneously reproduce the chemical properties
observed in a) the gas of local, star-forming galaxies, b) the
photospheres of G dwarfs in the Milky Way disc, and c) the
integrated stellar populations of nearby elliptical galaxies.
Furthermore, the model is able to do this without any significant
deviation from the standard framework of galaxy formation in the
canonical paradigm of hierarchical structure formation. This can be
seen as a significant achievement, which has allowed us to form a
much more comprehensive view of GCE than was possible before.