Spectral analysis of the multiple populations in Omega Centauri with MUSE

Globular clusters are spherical stellar clusters, containing up to millions of old stars which are typically metal-poor. They are so old that they are even used to give estimates of the age of the Universe itself and, for a long time, they were considered as the best example of simple stellar populations. However, since the presence of multiple stellar populations was confirmed in both the Galactic and extra-galactic environment, globular clusters now represent a main challenge: characterizing the formation and the evolution of their populations is still an open issue in the astrophysical research. The most extreme and massive case in our Galaxy is represented by ω Centauri: the disentanglement of its huge number of stellar populations and the determination of their ages and chemistry have proven to be particularly challenging for decades. Given this complexity, ω Centauri is believed to be the nuclear star cluster of an accreted dwarf satellite of the Milky Way. The aim of this thesis is to analyze the spectra of the most massive Galactic globular cluster, with a particular focus on 5278 red giant branch (RGB) stars within its core radius, in order to qualitatively compare the chemical features of ω Centauri’s nine populations. The spectra are taken with the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT), whereas the populations have been previously identified by taking advantage of the Hubble Space Telescope (HST) photometry. To do this analysis, the technique of spectral stacking is applied: hundreds of individual spectra are combined into a single S/N-averaged "stacked" spectrum, which is representative of each population. The stacking is performed not only per population, but also per magnitude bin, so as to combine stars with more homogeneous atmospheric parameters and to confirm that the chemical properties of a specific population do not depend on stellar magnitude. After normalizing the stacked spectra to the continuum, a closer look at the most prominent absorption lines reveals that the populations have different line depths of many metals, such as Mg, Ca, Ba, Al and Na. Consequently, the spectral stacking proves to be an effective technique that makes abundance measurements accessible, in spite of the low S/N of the individual low resolution MUSE spectra. After developing this thesis, an accurate quantitative determination of the elemental abundances, by means of spectral synthesis, is forthcoming.