ӣ��Ӱ��

Topic

The Chemo-dynamics and Origin of Extended Stellar Structures in Milky Way Dwarf Galaxy Satellites

Department of Physics and Astronomy

Date & location

• Friday, November 28, 2025
• 2:00 P.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Sara Ellison, Department of Physics and Astronomy, ӣ��Ӱ�� (Co-Supervisor)
• Dr. Alan McConnachie, Department of Physics and Astronomy, UVic (Co-Supervisor)
• Dr. Thomas Darcie, Department of Electrical and Computer Engineering, UVic (Outside Member)

External Examiner

• Dr. Daniel Zucker, School of Mathematical and Physical Sciences, Macquarie University

Chair of Oral Examination

• Dr. Quentin Mackie, Department of Anthropology, UVic

Abstract

Dwarf galaxies are fundamental to our understanding of galaxy formation and evolution. As the most numerous galaxies in the Universe, they are the building blocks for more massive systems (like the Milky Way) and are considered the foundation of galaxy formation. As they are accreted, the debris of these galactic building blocks are tidally stripped, leaving trails of stars throughout the host’s stellar halo. Though much observational evidence of this hierarchical assembly exists in our own Galaxy, it remains uncertain whether dwarf galaxies themselves host extended haloes of previously accreted stars. Observing these low-density features is complicated by the dwarf’s intrinsic faintness, particularly the lowest-mass systems such as ultra-faints. Though detection of dwarf stellar haloes is challenging (and remains largely unexplored territory), identifying and cataloguing these substructures holds significant promise to further our understanding of galaxy formation and small-scale cosmology.

This thesis investigates the detection and characterization of extended stellar substructure in dwarf galaxies, via the chemo-dynamical properties of their constituent stars. In these works, I study the sample of dwarf galaxy satellites in the Milky Way (MW) whose proximity allows us to explore a wide range of stellar masses (i.e., down to the faintest galaxies known) and furthermore provides us with astrometric data obtained from the space-based telescope, Gaia. In my PhD, I first addressed the detectability of these extended features in our MW dwarfs by updating an existing Bayesian-based algorithm that combines spatial, photometric, and astrometric information from Gaia to (i) characterize stellar memberships and (ii) identify extended stellar features in every MW dwarf. Applied to ∼60 systems, this method produced the first systematic census of extended structure in our nearby dwarf galaxies. Out of the entire sample, nine dwarfs were identified as exhibiting an extended stellar component.

Building on this foundation of this work, I then focused on one specific MW dwarf galaxy (Boötes 3) whose stellar density profile indicated an excess of stars in the dwarf’s outskirts. Using our Gaia-selected members, new CaHK photometry taken at the Canada-France-Hawaii Telescope, and matched filter searches in SDSS and DELVE photometric catalogues, I aimed to characterize whether this stellar excess could be explained by tidal influence with the MW. Interestingly, and contrary to previous works, I do not find evidence of lengthy tidal tails in this system. However, the dynamical analysis conducted in this work indicates that Boötes 3’s orbit may be strongly influenced by MW tides. I argue that the appearance of lengthy tidal tails may be truncated by (i) the steep distance gradient of the dwarf’s nearly radial orbit, and/or (ii) the dissipation of stars due to a recent interaction with the Galactic bar. This work highlights the necessity of combining astrometry, chemistry, and dynamics to confirm progenitor-stream associations.

A substantial outcome of this work, regarding the identification of candidate stars in each MW dwarf satellite, has been the science that has been enabled by these data. For every dwarf satellite observable by Gaia, I have produced target lists of stellar candidates for future follow-up programs. The observational campaigns that have been facilitated by my data include (i) smaller scale spectroscopic programs for one, or a few, dwarf galaxies, and (ii) selecting targets as part of commissioning the Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South Observatory. These data also form the basis for a larger scale, homogeneous, high-resolution study of southern MW dwarfs as part of GHOst Ultra-faint Legacy Survey (GHOULS). Collectively, these programs underscore the broader scientific impact and lasting utility of the algorithm’s candidate lists in future studies of MW dwarfs.

The outskirts of dwarf galaxies are not only key to understanding their individual evolutionary histories, but also serve as sensitive probes to small-scale cosmology. By cataloguing extended stellar features and constraining their likely origin with follow-up observations, this work provides crucial observations to better understand our cosmological perspective. The faintest systems are arguably the most useful to answer fundamental questions about the nature of dark matter, and their outskirts are key to understanding the smallest scales of galaxy formation.