He abundances in disc galaxies. I. Predictions from cosmological chemodynamical simulations

Accepted for publication in A&AWe investigate how the stellar and gas-phase He abundances evolve as a function of time within simulated star-forming disc galaxies with different star formation histories. We make use of a cosmological chemodynamical simulation for galaxy formation and evolution, which includes star formation as well as energy and chemical enrichment feedback from asymptotic giant branch stars, core-collapse supernovae, and Type Ia supernovae. The predicted relations between the He mass fraction, Y, and the metallicity, Z, in the interstellar medium of our simulated disc galaxies depend on the galaxy star formation history. In particular, dY/dZ is not constant and evolves as a function of time, depending on the specific chemical element that we choose to trace Z; in particular, dY/dX O and dY/dX C increase as a function of time, whereas dY/dX N decreases. In the gas-phase, we find negative radial gradients of Y, due to the inside-out growth of our simulated galaxy discs as a function of time; this gives rise to longer chemical enrichment timescales in the outer galaxy regions, where we find lower average values for Y and Z. Finally, by means of chemical-evolution models, in the galactic bulge and inner disc, we predict steeper Y vs. age relations at high Z than in the outer galaxy regions. We conclude that for calibrating the assumed Y-Z relation in stellar models, C, N, and C+N are better proxies for the metallicity than O because they show steeper and less scattered relations.Peer reviewedFinal Published versio

Unveiling the History and Nature of the Milky Way with Galactic Surveys and Numerical Simulations

The galaxy within which we reside, the Milky Way, offers perhaps the highest fidelity training ground for models of galaxy formation, providing insights into its history of formation and evolution on a star-by-star basis. Such insights are only truly useful for constraining disc galaxy formation models if we properly understand the wider context of the Milky Way among the plethora of galaxies in the Universe. This thesis aims to make progress toward answering the question of whether the Milky Way is a typical disc galaxy. Through the effort to answer this question, this thesis presents new measurements of the structure of our Galaxy and new insights into aspects of its history of assembly and evolution which have a strong influence on its α-element abundances. Providing a baseline constraint on future models for the formation of the Galaxy, I present a dissection of the Milky Way disc spatial structure as a function of [Fe/H], [α/Fe] and stellar ages (based on the surface abundances of Carbon and Nitrogen), as measured by the APOGEE survey. I measure the disc density profile, fitting the scale heights and lengths of mono-age, mono-[Fe/H] sub-populations in the high and low [α/Fe] disc. The fitted disc vertical scale height distribution is smooth when weighted by surface-mass density, suggesting that the high and low [α/Fe] populations are not vertically distinct at the solar radius, as would be expected if they were interchangeable with the geometric thin and thick disc components. I find that the surface density profile of low [α/Fe] mono-age, mono-[Fe/H] populations is best fit by a broken exponential, such that their density increases with R to a peak radius, and declines thereafter, whereas the high [α/Fe] populations are better described by a single, declining exponential within the range of Galactocentric radii observed by APOGEE. The trends of the density profile parameters as a function of age and metallicity provide insights into the structural evolution of the Galaxy, and provide strong constraints on future models of its formation and evolution. In particular, a main finding of this study is that the high and low [α/Fe] discs have a relatively distinct radial structure. To understand the origin of the structures found in the above study, and to generate novel and predictive models for the formation of the Galaxy, I perform an analysis of element abundances in Milky Way like galaxy discs in the EAGLE simulation. I concentrate on the abundance of α-elements in these galaxies, with a view to understanding the origin of the bimodality in [α/Fe] at fixed [Fe/H] which is apparent in both the Milky Way and in EAGLE Galactic analogues. I show that EAGLE reproduces broad expectations for the production of α-elements from simple chemical evolution models, namely that stars with enhanced [α/Fe] are born from gas in rapidly star forming environments at high density. These environments are conducive to forming α-enhanced stars because their gas consumption timescale is considerably shorter than the characteristic delay timescale for Type Ia supernovae. I further show that such environments are only achieved in EAGLE haloes when the rate of dark matter accretion is faster at early times than the majority of galaxies with similar stellar mass at z=0. The necessity for this atypical and rapid early assembly means that galaxies hosting discs with [α/Fe] bimodality are extremely rare, forming in ≈ 6% of galaxies at the Milky Way stellar mass range. Following the prediction of EAGLE that galaxies which host [α/Fe] bimodality like that of the Milky Way should have atypical histories of assembly, I then perform a study of Milky Way halo stars in common between APOGEE and Gaia DR2, with a view to placing constraints on the accretion history of the Galaxy. I present a detailed characterisation of the kinematics and abundances of the recently discovered Gaia-Enceladus association, which is proposed to be the debris of a singular and massive accretion event. By comparing the kinematics and abundances of Gaia-Enceladus to the debris of satellites accreted onto Milky Way analogues in the EAGLE simulations, I make a quantitative prediction of the stellar mass of the Milky Way satellite to be 10^8.5 < M* < 10^9 Msun and predict its earliest possible time of accretion to be z≈1.5. I also show that such mergers are uncommon in the simulations, in agreement with the prediction that the Milky Way assembly history should be atypical. The above results outline a new view of the Galaxy in which it is potentially not a good example of a `typical' disc galaxy, playing host to structural components which are linked to its chemistry in a complex manner, which may indicate that its history of assembly is not in common with other galaxies at the same stellar mass. In finding that the Galaxy is atypical in this way, this thesis has uncovered new aspects of the evolutionary history of the Milky Way, which pave the way for future work towards the goal of fully reconstructing the history of our Galaxy and using that understanding to formulate robust and general models for the formation of disc galaxies

A variational encoder-decoder approach to precise spectroscopic age estimation for large Galactic surveys

Constraints on the formation and evolution of the Milky Way Galaxy require multi-dimensional measurements of kinematics, abundances, and ages for a large population of stars. Ages for luminous giants, which can be seen to large distances, are an essential component of studies of the Milky Way, but they are traditionally very difficult to estimate precisely for a large dataset and often require careful analysis on a star-by-star basis in asteroseismology. Because spectra are easier to obtain for large samples, being able to determine precise ages from spectra allows for large age samples to be constructed, but spectroscopic ages are often imprecise and contaminated by abundance correlations. Here we present an application of a variational encoder-decoder on cross-domain astronomical data to solve these issues. The model is trained on pairs of observations from APOGEE and Kepler of the same star in order to reduce the dimensionality of the APOGEE spectra in a latent space while removing abundance information. The low dimensional latent representation of these spectra can then be trained to predict age with just $\sim$ 1,000 precise seismic ages. We demonstrate that this model produces more precise spectroscopic ages ($\sim$ 22% overall, $\sim$ 11% for red-clump stars) than previous data-driven spectroscopic ages while being less contaminated by abundance information (in particular, our ages do not depend on [$\alpha$/M]). We create a public age catalog for the APOGEE DR17 data set and use it to map the age distribution and the age-[Fe/H]-[$\alpha$/M] distribution across the radial range of the Galactic disk

Family influences on eating behaviour in low income households with pre-school children

Healthy eating advice aimed at families has traditionally been targeted at women in the belief that it is they who make the decisions about food consumption within the family. The overall aim of this study was to explore the family influences on eating behaviour in low income households with pre-school children. It focused on both the man and the woman in the family. It was considered particularly important to include men since the literature showed that little relevant research had been conducted with men on low incomes. The study consisted of four phases. The first involved interviewing 10 couples in their home. The second phase comprised of two single sex focus groups. The third phase provided a negative case analysis of women who were not prepared to be interviewed with their partners but were willing to be interviewed on their own. The final phase involved 22 couples interviewed in their home to ensure the trustworthiness of the results of the previous phases. The study was conducted from an interpretivist perspective and the findings were analysed, drawing on the principles of Grounded Theory. The male partner and the children were found to be particularly influential on eating behaviour, as was the cost of food and the time available for preparing meals. Families changed their eating behaviour over time and this was especially true once the couple began to co-habit and again when their children were old enough to make choices about what they ate. During the process of the research, the concept of the 'life course' emerged as a major theme and was explored in greater depth using the nutrition career as a theoretical framework. Following on from this, different family cultures were mapped out alongside the life course. These two themes were found to relate dynamically and a 'Life Course and Family Culture' (LCFC) model was developed. It is suggested that this model could be used as a basis for developing health promotion needs assessment tools and a questionnaire developed in the study is suggested as a means of facilitating this

The Galactic Interstellar Object Population: A Framework for Prediction and Inference

The Milky Way is thought to host a huge population of interstellar objects (ISOs), numbering approximately $10^{15}\mathrm{pc}^{-3}$ around the Sun, which are formed and shaped by a diverse set of processes ranging from planet formation to galactic dynamics. We define a novel framework: firstly to predict the properties of this Galactic ISO population by combining models of processes across planetary and galactic scales, and secondly to make inferences about the processes modelled, by comparing the predicted population to what is observed. We predict the spatial and compositional distribution of the Galaxy's population of ISOs by modelling the Galactic stellar population with data from the APOGEE survey and combining this with a protoplanetary disk chemistry model. Selecting ISO water mass fraction as an example observable quantity, we evaluate its distribution both at the position of the Sun and averaged over the Galactic disk; our prediction for the Solar neighbourhood is compatible with the inferred water mass fraction of 2I/Borisov. We show that the well-studied Galactic stellar metallicity gradient has a corresponding ISO compositional gradient. We also demonstrate the inference part of the framework by using the current observed ISO composition distribution to constrain the parent star metallicity dependence of the ISO production rate. This constraint, and other inferences made with this framework, will improve dramatically as the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) progresses and more ISOs are observed. Finally, we explore generalisations of this framework to other Galactic populations, such as that of exoplanets.Comment: Accepted to A

The evolution of the Milky Way's thin disc radial metallicity gradient with K2 asteroseismic ages

The radial metallicity distribution of the Milky Way's disc is an important observational constraint for models of the formation and evolution of our Galaxy. It informs our understanding of the chemical enrichment of the Galactic disc and the dynamical processes therein, particularly radial migration. We investigate how the metallicity changes with guiding radius in the thin disc using a sample of red-giant stars with robust astrometric, spectroscopic and asteroseismic parameters. Our sample contains $668$ stars with guiding radii $4$ kpc < $R_\mathrm{g}$ < $11$ kpc and asteroseismic ages covering the whole history of the thin disc with precision $\approx 25\%$. We use MCMC analysis to measure the gradient and its intrinsic spread in bins of age and construct a hierarchical Bayesian model to investigate the evolution of these parameters independently of the bins. We find a smooth evolution of the gradient from $\approx -0.07$ dex/kpc in the youngest stars to $\approx -0.04$ dex/kpc in stars older than $10$ Gyr, with no break at intermediate ages. Our results are consistent with those based on asteroseismic ages from CoRoT, with that found in Cepheid variables for stars younger than $1$ Gyr, and with open clusters for stars younger than $6$ Gyr. For older stars we find a significantly lower metallicity in our sample than in the clusters, suggesting a survival bias favouring more metal-rich clusters. We also find that the chemical evolution model of Chiappini (2009) is too metal-poor in the early stages of disc formation. Our results provide strong new constraints for the growth and enrichment of the thin disc and radial migration, which will facilitate new tests of model conditions and physics.Comment: 15 pages, 16 figures. Accepted for publication in MNRA

The building blocks of the Milky Way halo using APOGEE and Gaia -- or -- Is the Galaxy a typical galaxy?

We summarise recent results from analysis of APOGEE/Gaia data for stellar populations in the Galactic halo, disk, and bulge, leading to constraints on the contribution of dwarf galaxies and globular clusters to the stellar content of the Milky Way halo. Interpretation of the extant data in light of cosmological numerical simulations suggests that the Milky Way has been subject to an unusually intense accretion history at z >~ 1.5