Fletcher Waller

Topic

A Near Glimpse into a Distant Past: Chemical Analyses of Ancient Milky Way Satellite Galaxies using High-Resolution Stellar Spectroscopy

Department of Physics and Astronomy

Date & location

• Thursday, April 11, 2024
• 11:00 A.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Kim Venn, Department of Physics and Astronomy, University of Victoria (Co-Supervisor)
• Dr. Alan McConnachie, Department of Physics and Astronomy, UVic (Co-Supervisor)

External Examiner

• Dr. Samantha Lawler, Department of Physics, University of Regina

Chair of Oral Examination

• Dr. Adam Murray, Department of Civil Engineering, UVic

Abstract

The least-luminous galaxies ever detected, ultra-faint dwarf (UFD) galaxies, are ancient, metal-poor, dark-matter dominated, and likely represent the floor of the galaxy luminosity function. Their stars preserve the signatures from early chemical evolution, and their dark matter haloes are testing grounds for CDM cosmological models.

UFDs are subject to tidal interactions with the Milky Way and dwarfs, which visually range from disrupted streams to slightly elongated structures. An extended structure may also be the result of a minor merger of dwarfs. To further our understanding of these extended structures, we examine high-resolution spectra of stars in the outskirts of UFDs. It is expected that signatures of one or just a few early supernovae may be detectable in the chemical abundance patterns of UFD stars. High-resolution spectroscopy of the brightest individual stars allows us to recover these signatures, and piece together the chemodynamical history of these galaxies. We use Gemini/GRACES to obtain high-resolution spectra of potential members in the outskirts of three ultra-faint dwarf (UFD) galaxies: Coma Berenices (>2Rh), Ursa Major I ( 4Rh), and Boötes I ( 4Rh), as well as a new member in the central region of Ursa Major I. Targets selected with a new Bayesian technique which uses Gaia EDR3 photometry and astrometry to determine stellar membership to UFDs. All 5 stars are determined to be members by precision radial velocities and metallicities that confirm their associations with the UFD galaxies. The spectra were also used to measure absorption lines for 10 elements (Na, Mg, K, Ca, Sc, Ti, Cr, Fe, Ni, and Ba), which confirm that the chemical abundances of the outermost stars are in good agreement with stars in the central regions. The abundance ratios and chemical patterns of the stars in Coma Berenices are consistent with contributions from SN Ia, which is unusual for its star formation history and in conflict with previous suggestions that this system evolved chemically from a single core collapse supernova event. The chemistries for all three galaxies are consistent with the outermost stars forming in the central regions, then moving to their current locations through tidal stripping and/or supernova feedback. In Boötes I, however, the lower metallicity and lack of strong carbon enrichment of its outermost stars could also be evidence of a dwarf galaxy merger.

To pursue further high-resolution spectroscopic observations of UFD stars, I carried out an internship at the Gemini South telescope Observatory which was timed with the commissioning of the Gemini High Resolution Optical SpecTrograph (GHOST). Target selection, logkeeping, early data reduction, calibration tests and instrument operation tasks were completed during 17 nights of commissioning between June and September 2022.

Since commissioning, I performed bug-testing and developed streamlined methods for processing data to work towards completion and refinement of the data reduction pipeline. With this experience and relative expertise, I worked with the science team towards the goal of publishing our first science results. GHOST has been integrated into the Gemini instrumentation suite and is available for observing proposals.