Dark Matter in the Darkest Galaxies: Exploring the Structure of Low Surface Brightness Spirals
Date
2022-07-01
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Abstract
Dark matter in our universe is not evenly distributed: some galaxies are lacking it entirely, and some galaxies have it in abundance. While its existence is generally agreed on, its distribution within galaxies is hotly debated. Low Surface Brightness (LSB) spirals are a class of galaxy with peculiarly low star-formation rates, and one possible explanation for this points to an overabundance of dark matter relative to normal (baryonic) matter. LSB spirals tend to be dark matter-dominated at all radii, which makes them an ideal place to probe the structure of galactic dark matter haloes. In contrast, “normal” High Surface Brightness (HSB) spirals are baryon-dominated at the core, which obscures the central structure and muddles the apparent mass distribution of the galaxy. Currently, LSB spirals appear to be compatible with a “cored” halo, possibly due to early central star formation driving baryons out of the center, which dragged dark matter out along with it, thereby transforming a cuspy halo into a cored one.
This thesis explores both the mystery of LSB spirals and the mystery of dark matter, pairing them together to shed light on the evolutionary difference between LSB and HSB spiral galaxies and begin examining cusp-core transformation. Optical emission line-derived velocity fields are used to determine the dynamical mass at different radii in a sample of eight LSB spirals, and archival Spitzer IRAC images allow the stellar mass at different radii in these galaxies to be determined. This results in a robust estimation of the structure of each galaxy's dark matter halo. We conclude that LSB spirals indeed tend to be dark matter-dominated at all radii, and highlight UGC 4179 as a likely case of solid-body rotation. Additionally, a peculiar trend in the [OII] to [OIII] ratio in these spectra is apparent, possibly supporting the truncated IMF hypothesis to explain LSB spirals.
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astronomy, dark matter, galaxies