Abstract
The dominant cosmological models of the Universe suggest that most matter is dark matter, which is typically thought to be a fundamental particle that does not interact with light. Because of this, dark matter is undetectable through our traditional observational techniques. Nearly all galaxies are thought to lie within dark matter "halos", which may be approximated as clumps of mass many times more expansive and orders of magnitude more massive than the galaxies' observed, luminous components. These halos are the backbone of cosmic-scale structure: At large scales, dark matter-based models are able to reproduce and explain observed galaxy properties – such as their numbers, how they cluster, and their evolution – quite well. However, at smaller scales, such as the size of the Local Group, observations are much more discordant with theory. At these scales, the substructure of these halos becomes relevant, including the smaller "subhalos" that host dwarf satellite galaxies around larger galaxies like the Milky Way. Therefore, a broad understanding of cosmology and the origins of the Universe requires a detailed understanding of dark matter at these small, local scales.
Within the Local Group, one promising way to probe dark matter is through galaxy and stellar motions, which are directly governed by halo properties, such as mass. Yet, owing to the immense distances between these galaxies and us, precise velocities are extremely difficult to measure, with proper motions (the angular, 2D motions of these galaxies on the plane of the sky) having magnitudes < 0.1 milliarcseconds per year (sub-pixel motions). More recently, data from the Gaia satellite have allowed these measurements to be made for nearly all satellite galaxies of the Milky Way. However, greatly improving the precision of these measurements or expanding the availability of these measurements to satellites around other galaxies (such as our neighbor M31) requires careful analysis with high-precision data from space-based observatories, such as the Hubble Space Telescope and the James Webb Space Telescope.
In this dissertation, I present a series of methodological techniques for measuring the proper motions of dwarf galaxies in and around the Local Group. In Chapter 3, I show how two epochs of HST imaging can be combined with data from Gaia to measure the absolute motions of dwarf galaxies even if no stars in those galaxies have been observed by Gaia itself. In Chapter 5, I show that these measurements can be further improved if linked to careful measurements of the stationary positions of the hundreds of background galaxies in these images. In Chapter 7, I show that archival HST images can be combined with JWST observations to make proper motion measurements, extending the validity of this science past the life of HST and providing a bridge to the next generation of space observatories.
