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Chemical analysis of the Fornax Dwarf galaxy

Letarte, B., 2007, s.n.. 160 p.

Research output: ThesisThesis fully internal (DIV)Academic

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  • Bruno Letarte
This thesis is entitled “Chemical Analysis of the Fornax Dwarf Galaxy”, and it’s main goal is to determine what are the chemical elements present in the stars of this galaxy in order to try and understand it’s evolution. Galaxies are not “static” objects, they move, form stars and can interact with other galaxies. Studying the stars composing a galaxy can in principle, inform us about its past. Some stars can be as old as the galaxy itself, some can be much younger and we can use this information to study how the stellar spectra have varied with time over the entire history of star formation in this galaxy. Dwarf galaxies are in principle the most simple and straightforward type of galaxy and their study can be used to test numerous theories of the formation and evolution of stars and galaxies in a range of environments. Dwarf spheroidal galaxies are small, roughly spherical galaxies that are typically found in the vicinity of larger galaxies, such as the Milky Way. They typically do not have any ongoing star formation, nor do they appear to have any gas associated to them. The abundance ratios of different elements in individual stars with a range of ages provide a detailed insight into the various chemical enrichment processes (e.g., supernovae, stellar winds) which in turn improves our understanding of the global processes of formation and evolution of a galaxy as a whole. An important aspect of this thesis is the pipeline developed to analyse a large number of stellar spectra (~100) in a consistent and statistically robust manner, using tools that are typically used on spectra with twice the resolution and larger wavelength coverage. This required bringing together several complex tasks, including accurate stellar atmospheric models, atomic data for the absorption lines, codes of line formation, EW measurements and signal extraction methods, all of which need to be properly included and treated in order to obtain accurate results. The pipeline delivers stellar parameters and abundances in a controlled man- ner. This involved developing error analysis and diagnostics to carefully test the robustness of the results. The Fornax dwarf spheroidal galaxy contains five globular clusters (GCs) with a range of properties. Using VLT/UVES I have obtained the first detailed chemical abundances for nine individual stars in three of its GCs. From our results it is clear that they were formed promptly and early in the history of Fornax dSph, as were the Milky Way GCs. Thus, despite their very different mass, morphology and global star formation history, the abundance patterns of individual stars in the Fornax GCs are almost identical to those found in Milky Way globular clusters, including abundance patterns that are specific to GC stars (deep-mixing) and rare anomalies (europium-rich) also observed in other GCs. This suggests that stars in GCs are the same regardless of the type or size of the galaxy in which the GC is hosted. Thanks to the multi-fibre capability of VLT/FLAMES I have been able to make detailed abundance measurements of 81 RGB stars in the central part of Fornax, which is a significant, even dramatic, improvement on the previous UVES sample of three individual field stars. This sample of Fornax field stars exhibits unusually low [α/Fe] ratios, and its dependence with metallicity is different from the Milky Way, showing a different efficiency in gas enrichment. Fornax field stars are clearly predominantly enriched by s-process elements at high metallicity, showing the strong role of (metal poor) AGB stars. Our sample is dominated by a relatively young, relatively metal rich population. This means that we have obtained the most detailed picture of the chemical enrichment of Fornax during the last ~4 Gyrs. There is only one field star in our sample which appears old and metal poor, and it has abundance properties almost indistinguishable from the globular clusters in Fornax. These results confirm and deepen the difficulties found in earlier more limited surveys in understanding the role (if any) of this and of similar galaxies in the build up of our Milky Way. These results also challenge our understanding of basic nucleosynthetic processes, with for example, ratios of [Ni/Fe] that are well below what was typically thought possible.
Original languageEnglish
QualificationDoctor of Philosophy
Supervisors/Advisors
Publisher
Print ISBNs9036729270
Publication statusPublished - 2007

    Keywords

  • Proefschriften (vorm), Dwergsterrenstelsels, Chemische analyses, extragalactische stelsels

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