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The Nucleus of the Sagittarius Dwarf Spheroidal Galaxy: M54

Alfaro Cuello, Mayte

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Abstract

Nuclear star clusters are the densest stellar systems in the universe, hosted by galaxies across the entire Hubble sequence, including a high fraction of dwarf galaxies. The most massive, chemically complex globular clusters in the Milky Way exhibit similar characteristics as nuclear star clusters in dwarf galaxies. This raised the idea that these globular clusters are actually former nuclei of galaxies accreted by the Milky Way. In this context, M54 – the nuclear star cluster of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) – offers a unique opportunity to understand this presumed direct connection between globular clusters and nuclear star clusters, and low-mass galaxy nucleation. The Sgr dSph is currently being disrupted by the tidal field of the Milky Way, leaving a long stellar stream as evidence of its advanced degree of disruption. M54 still lies at the center of its host, becoming a potential stripped nucleus, and presenting an outstanding example of this class of objects. From its discovery – long before the detection of the Sgr dSph – M54 was classified as a globular cluster, the second most massive in the Milky Way after ω Cen. M54 shows a high spread in iron abundance of its member stars, pointing towards an extended and complex star formation history. This Thesis presents a large Multi-Unit Spectroscopic Explorer (MUSE) data set covering a region of ∼2.5 times the effective radius of M54. The single spectra of more than 6 500 member stars extracted from the exceptional data set led to the recovery of the star formation history of this nuclear star cluster through age and metallicity information. This allowed disentangling the presence of – at least – three stellar subpopulations, whose kinematics show clear differences. The chemo-dynamical characterization of these subpopulations suggests that they originated in different star formation events. This work shows the complexity of M54 which appears to be a nuclear star cluster in a highly disturbed environment rather than a simple globular cluster. The evidence suggests that M54 is the result of the two proposed mechanisms for the formation of nuclear stars clusters, that happen at different stages of M54’s evolution: (i) at least two globular clusters are driven to the center of the host and merge to form a single high-mass cluster with a large age and metallicity spread, followed by (ii) in-situ star formation from enriched gas in the nucleus. The unprecedented details of this study help to understand low-mass galaxy nuclei, for which less information is available in contrast to the higher mass regime.

Item Type: Dissertation
Supervisor: Neumayer, Dr. Nadine
Place of Publication: Heidelberg
Date of thesis defense: 25 July 2019
Date Deposited: 01 Oct 2019 13:46
Date: 2019
Faculties / Institutes: The Faculty of Physics and Astronomy > Dekanat der Fakultät für Physik und Astronomie
Subjects: 520 Astronomy and allied sciences
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