The Cosmic Chemical Evolution Histories of Spiral Galaxies

Greener, Michael John (2023) The Cosmic Chemical Evolution Histories of Spiral Galaxies. PhD thesis, University of Nottingham.

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Spiral galaxies have always been of particular interest to astronomers ever since it was discovered that our own galaxy, the Milky Way, is itself a spiral. Although long-slit spectroscopy of galaxies has been available for over half a century, the recent advent of large-scale integral field unit surveys such as SDSS-IV MaNGA means that spatially resolved spectroscopic observations are now available for thousands of nearby spiral galaxies. In this thesis, we will use data from the MaNGA survey to investigate the fundamental properties and the chemical evolution of spiral galaxies.

We begin with an investigation into the spatially resolved dust attenuation of a well-defined sample of star-forming spiral galaxies observed by MaNGA. By making use of software which allows the integrated spectrum of a galaxy to be decomposed into stellar population spectra of single ages and stellar metallicities, we are able to obtain spatially resolved measures of the dust attenuation affecting the stellar populations of the spirals. We compare these measurements with those obtained for the dust attenuation in the gas for the same galaxies, which we derive from analysing emission lines in the galactic spectra. Analysis of the radial profiles of the two dust attenuation measures suggests that there is a disproportionately high concentration of giant molecular clouds (incorporating gas, young stars and clumpy dust) nearer to the centres of star-forming spiral galaxies.

We then turn our attention to the main question that this thesis aims to answer: How do the chemical compositions of spiral galaxies change over cosmic time? In order to address this question, we first analyse the stellar metallicity distributions of a large sample of MaNGA galaxies, with a view to addressing the "G-dwarf problem", which essentially states that the small number of low-metallicity G-dwarf stars observed in the Milky Way is inconsistent with the simplest picture of the Galaxy forming from a "closed box" of gas. We can resolve the G-dwarf problem by treating the Galaxy as an open system that accretes gas throughout its life. This observation has classically only been made in the Milky Way, but the availability of high-quality spectral data from MaNGA and the development of new analysis techniques mean that we can now make equivalent measurements for a large sample of spiral galaxies. Our analysis shows that high-mass spirals generically show a similar deficit of low-metallicity stars, implying that the Milky Way's history of gas accretion is common. By contrast, low-mass spirals show little sign of a G-dwarf problem, presenting the metallicity distribution that would be expected if such systems evolved as pretty much closed boxes.

Following on from this research, we investigate archaeologically how the metallicity in both stellar and gaseous components of a similar sample of spiral galaxies of differing masses evolve with time. In particular, we analyse how the metallicity of these systems has changed over the last 10 billion years since "cosmic noon". We find that the chemical evolution of a given galaxy depends on its stellar mass: low-mass galaxies, which behave as pretty much closed boxes, are found to steadily increase both their stellar and gas metallicities over cosmic time; however, in high-mass galaxies, which behave as accreting boxes, the gas metallicity is found to increase over cosmic time (albeit less dramatically than in their low-mass counterparts), but we find that the metallicity of the stars may actually be lower at later times. Such disjoint behaviour is what we might expect if these more massive systems have accreted significant amounts of largely pristine gas over their lifetimes, and this material has not been well mixed into the galaxies.

While we do ultimately answer the question posed at the beginning of this thesis, further considerations can of course be taken into account that we do not address in this work. At any rate, the studies presented in this thesis illustrate the strengths of large-scale integral field unit surveys, which have provided us with new and powerful tools for studying the chemical evolution of galaxies.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Aragón-Salamanca, Alfonso
Merrifield, Michael
Keywords: Galaxies, spiral galaxies, dust attenuation, star formation, interstellar medium, chemical evolution histories, stellar metallicities, gas metallicities, G-Dwarf problem
Subjects: Q Science > QB Astronomy
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 69652
Depositing User: Greener, Michael
Date Deposited: 03 Aug 2023 11:59
Last Modified: 03 Aug 2023 11:59

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