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Spectroscopic Analysis and Chemodynamic Exploration of the Milky Way with Million-Star Surveys
The Milky Way is traditionally depicted as a composite of three main stellar components: the halo, disk, and bulge. The closer we look and the more information we gather, this picture becomes more complicated. The components are in fact overlapping in stellar properties, such as positions, kinematics, ages, and chemical composition. The most promising way to further our understanding of the formation of the Milky Way, the main aim of Galactic Archaeology, is to explore chemical information and stellar ages with large stellar surveys. In Chapter 1, we motivate this exploration in more detail and describe the challenges that come with the collection of big data from spectroscopic surveys. In Chapter 2, we introduce the large-scale spectroscopic `Galactic Archaeology with HERMES' (GALAH) survey, whose spectra were analysed in the course of this Thesis in order to estimate stellar properties. The very large data flow of stellar surveys has been referred to as the industrial revolution of Galactic archaeology and the analysis requires new efficient and automated techniques. This Thesis describes the spectroscopic analyses of the more than 650,000 stars in the GALAH survey, which deliver up to 30 abundances with unprecedented accuracy on such scales, based on the papers by Buder et al. (2018) and Buder et al. (in prep. b). In Chapter 3, we use the chemical information from GALAH together with dynamical information and stellar ages to analyse the Galactic disk in the solar neighborhood, which has shown to be assembled from two populations. This work is published by Buder et al. (2019) and we find that the two populations of the disk can be separated more clearly when using stellar chemical composition and age, rather than phase-space information. In Chapter 4, we use chemodynamic information and stellar ages to analyse the transition between the disk and halo. This work will be submitted by Buder et al. (in prep. a) and we confirm that the old disk overlaps significantly with the halo. With our performed chemodynamic decompositions we are able to link the disk population that is enhanced in alpha-element abundances with the high-alpha halo population identified by Nissen & Schuster (2010). We further show that the accreted halo population of the `Sausage', identified by Belokurov et al. (2018), is strongly correlated with the low-alpha halo population identified by Nissen & Schuster (2010). Our stellar age estimates suggest that the halo components and the oldest high-alpha disk stars are coeval and show no strong age gradient, which would rule out several formation scenarios of the Milky Way. In Chapter 5, we conclude the work of this Thesis and outline further ways to continue the research in the field of Galactic archaeology with large stellar surveys.
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520 Astronomie
520 Astronomy and allied sciences