Near-Field Cosmology with Deep Hubble Space Telescope Imaging of Ultra-Faint Dwarf Galaxies

This dissertation presents an optimized sample of ultra-faint dwarf galaxies (UFDs) and source catalogs, with observations taken using uniform techniques as part of a Hubble Space Telescope (HST) Treasury Program. Chapter 2 includes the first study based on photometric catalogs that I derive from these HST observations. For this work, I develop an aperture photometry pipeline to find and measure the magnitudes of sources within each image. Seven catalogs created from this aperture photometry are used to investigate the possible differences in star-formation histories from LMC-associated UFDs (“satellites of satellites”) and non-LMC-associated UFDs. I report that LMC-associated satellites, on average, could have been forming stars for 600 million years longer than the non-LMC-associated galaxies. This could suggest that environment in the early Universe may have affected the star-forming abilities of UFDs.

In Chapter 3, I continue to investigate the effects of environment and possible satellite associations, using similarly derived aperture photometry catalogs to study two specific UFDs: Pegasus III (Peg III) and Pisces II (Psc II). These are the most distant UFDs in our sample and have been postulated to have had a previous gravitational interaction with each other. I search for any morphological signs of an interaction (e.g., irregular shapes, extended regions), as well as examine how the two galaxies' sizes and luminosities compare to simulated UFDs and observed UFDs closer to the Galactic center. I find no evidence of a significant gravitational interaction in the lifetime of Peg III and Psc II and no major differences between these more distant UFDs and the closer MW satellites. Both groups also overlap in parameter space with a subset of simulated field UFDs. As expected, simulations have been unable to reproduce the smallest and faintest observed UFDs.

Following these initial studies based on aperture photometry, I created a point-spread-function (PSF) photometry pipeline, which allowed me to perform artificial star tests. Artificial star tests involve inserting fake stars based on real stellar PSFs into the observed images and then running the modified images through the same photometry pipeline. Based on the percentage and measurements of the fake stars recovered, we can learn valuable information related to the statistics and uncertainties of our photometry of the real stars.

Chapter 4 reports morphological measurements for six of the seven targets discussed in Chapter 2 and presents photometry for an additional four targets. With the use of supplemental measurements from the literature, I compare the LMC-associated and non-LMC-associated UFDs against each other in the magnitude-size plane and find no systematic differences between the two groups. Expanding on my work in Chapter 3, I also compare observed UFD properties with those of hydrodynamically simulated UFDs in the magnitude-size plane. Some of the simulations that contain these UFD analogs are based on alternative dark matter models, such as self-interacting and warm dark matter, rather than the current cold dark matter paradigm. There are no trends in the magnitude and size properties of the simulated UFDs, however, that would support this parameter plane being useful for constraining the nature of dark matter.

My work concludes in Chapter 5, with a presentation of the full details and methodology of the creation of photometric source catalogs and artificial star tests for 30 MW satellites. These catalogs will be posted and shared with the astronomical community as High Level Science Products through the Mikulski Archive for Space Telescopes, thus allowing even more researchers to benefit from this wealth of data.