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Topic

Chemodynamical Analyses of the Milky Way Satellites Sagittarius II and Aquarius II with GHOST: Challenges in Detecting Dark Matter in Ultra-Faint Systems

Department of Physics and Astronomy

Date & location

• Thursday, August 14, 2025
• 12:00 P.M.
• Virtual Defence

Examining Committee

Supervisory Committee

• Dr. Kim Venn, Department of Physics and Astronomy, ßÉßɱ¬ÁÏ (Supervisor)
• Dr. Alan McConnachie, Department of Physics and Astronomy, UVic (Member)
• Dr. Julio Navarro, Department of Physics and Astronomy, UVic (Member)

External Examiner

• Dr. Holger Baumgardt, School of Mathematics and Physics, University of Queensland

Chair of Oral Examination

• Dr. Simon Glezos, Department of Political Science, UVic

Abstract

Deep photometric surveys have revealed numerous faint Milky Way satellites with absolute 𝑉-band magnitudes 𝑀_𝑉≳−5. Distinguishing whether these systems are dark matter-dominated ultra-faint dwarf galaxies (UFDs) or purely baryonic globular clusters (GCs) remains a key challenge. In this thesis, we explore the diagnostic power of detailed chemical abundances using high-resolution Gemini/GHOST spectra of five stars in two low surface brightness satellites – Sagittarius II (Sgr2) and Aquarius II (Aqu2). For Aqu2, the kinematics and metallicities of two stars support a dark matter-dominated UFD origin. The abundance patterns – low Na, Sr, Ba, and enhanced K – indicate inefficient star formation from only a few supernovae, possibly with enrichment from super-AGB stars. In contrast, Sgr2 shows unresolved velocity and metallicity dispersions, and its stars exhibit typical metal-poor abundance ratios with little spread. One exception is the discovery of an r-process-enhanced star (Sgr2584, [Eu/Fe] = +0.7 ± 0.2), which does not clarify its nature, as such stars are found in both UFDs (Tuc III, Tuc IV, Grus II) and GCs (M15 and M92). However, radial mass segregation offers modest support for a GC classification. Motivated by this ambiguity, we examine the “Valley of Ambiguity“ – the region below 𝑀_𝑉≳−5 in the size–luminosity (𝑟_ℎ−𝑀_𝑉) plane – by dividing it into five zones of shared properties. We identify observational caveats and propose diagnostics to address UFD–GC overlap in each. We argue that only a multi-faceted approach – combining kinematic (e.g., velocity dispersions of member stars, tidal streams kinematics), dynamical (e.g., tidal resilience, mass function slopes, detections of mass segregation), and chemical diagnostics (e.g., metallicity spreads, heavy elemental deficiencies, carbon enhancement) – offers the most robust path to classification. Coupled with 𝑁-body modeling, this strategy is essential for interpreting ambiguous systems in the era of large-scale photometric and spectroscopic surveys.