Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Astronomy

First Advisor

van Dokkum, Pieter

Abstract

Ultra-Diffuse Galaxies (UDGs) are low-mass, large-sized galaxies that challenge traditional galaxy formation theories. Understanding how UDGs formed is crucial because each formation pathway can constrain different physical processes relevant to galaxy formation. The proposed formation pathways include early quenching, stellar feedback, and mergers, each generating predictions about the halo mass, the stellar population, and the Globular Cluster (GC) population of the galaxy. In this thesis, we develop observational techniques to distinguish between these key pathways. First, we found that UDGs can form in the absence of dark matter (DM) in a high speed merger of progenitor dwarf galaxies. Using a larger sample of field UDGs, we found evidences for a complex star-formation history consistent with the stellar feedback formation scenario. A case study of two UDGs, NGC 1052-DF2 and DF4: These two UDGs have been shown to contain little to no DM from their low velocity dispersions. We used deep Hubble Space Telescope imaging to detect the Tip of the Red Giant Branch (TRGB) at 20 Mpc for both galaxies. In addition, the GCs in these two galaxies are 1.5 mag brighter than expected, resulting in a top-heavy GC luminosity function. Combining all the anomalies, we proposed that they formed in the DM-free gas remnants of a high-speed collision between two progenitor dwarf galaxies. The Dragonfly UDG sample: To expand the sample size of UDGs in low-density environments, we searched for UDG candidates in 10,000 deg2 of imaging data from the Dragonfly Ultrawide Survey. Leveraging cloud computing, we developed a pipeline to analyze the data and obtain a large sample of UDGs in galaxy clusters, groups, and in the field. We doubled the literature sample of UDGs with detailed spectroscopic follow-up. These spectra showed that UDGs in low-density environments are “post-starburst” galaxies with multiple stellar populations, while UDGs in clusters are consistent with the early quenching formation route. Future work: Upcoming space telescopes like Euclid and Roman will provide high-resolution imaging data over a wide footprint. These datasets will enable further investigations of the GC populations in known UDGs. The number and the luminosity function of the GCs will provide deeper insights into UDG formation across different environments.

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