On the complex stellar populations of ancient stellar systems

The study of ancient stellar systems in the vicinity of the Milky Way provides a wealth of information on the conditions, in the early Universe, that led to the properties we observe today in galaxies and in their constituent components. Resolved stellar populations enable us to gain detailed insights on the age and chemical composition of such stellar systems, tracing their properties on a fine spatial scale. The deep investigation of Local Group objects revealed that even very old, low mass, stellar systems host unexpected complexities in their stellar populations. Such complexities remain largely unexplained, our understanding limited by observational and theoretical limitations. Here I present work aimed at a deeper characterisation of the complex stellar popula- tions in dwarf spheroidal galaxies and Galactic globular clusters. I use a combination of observational and modelling techniques to shed light on the detailed stellar properties of these objects. Part of my investigation focuses on the horizontal branch of dwarf spheroidal galaxies. By careful modelling of the horizontal branch in the galaxy Carina, which has well known star formation history, I demonstrate that the horizontal branch contains precious information, that can be used to refine age measurements in nearby galaxies. To this aim, I develop a new modelling method that, for the first time, combines constraints from the main sequence turn-off and the horizontal branch to provide very precise measurements of the star formation history in resolved galaxies. The combined information from different regions of the colour-magnitude diagram permits to recover the value of mass loss experienced by red giant branch stars with very high precision. I test this technique on a range of synthetic populations and on the well studied galaxy Sculptor, demonstrating the increased age resolution that this approach provides. I apply my modelling tool to the distant galaxy Tucana, determining a very detailed star formation history, where multiple events of star formation can be clearly distinguished. The identification of the different stellar populations on the horizontal branch permits us to characterise the spatial distribution of the star formation events in this galaxy. I also perform a photometric study of the massive globular cluster M13, focussing on the multiple stellar populations present in this object. I identify and trace the different stellar populations out to most external regions of this cluster. The spatial distribution of these populations, which shows no sign of radial segregation, reveal the very advanced dynamical evolutionary stage of the cluster. The work presented in this manuscript constitutes a step forward to understand the formation of low mass ancient stellar systems and paves the way for deeper studies of large samples of stellar systems in the Milky Way vicinity