The analysis of stellar populations in the Milky Way and beyond

Carrillo, Andreia Jessica Arabani
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No other astronomical object can unlock the mysteries of the Universe more than stars. Studying the crowded, unresolved stellar populations of nearby galaxies aids in understanding what brings about their observed properties, morphology, activity, and assembly history. Studying the Milky Way’s resolved stellar populations, specifically their detailed chemical abundances and kinematics, provides us with an unparalleled, zoomed-in view of galaxy formation. Additionally, understanding Milky Way stellar populations can supplement our knowledge in other fields in Astronomy, such as exoplanet populations, as the gas that forms a star also forms the planets around it. This is the focus of my dissertation: analyzing stellar populations--both resolved in the Milky Way (observed and simulated) and unresolved in a nearby galaxy--to holistically understand galaxy formation, the Milky Way assembly, and the Galactic context of exoplanet demographics. This Thesis is structured going from the largest and farthest of scales e.g. external galaxy, to the smallest of scales e.g. planet-hosting stars. With unresolved, ensemble stellar populations, I investigate the assembly of the different components: the bulge, bar, and disc, of the nearby galaxy NGC 2903, using the VIRUS-P Exploration of Nearby Galaxies (VENGA) Integral Field Spectroscopy (IFS) survey. This work benefited from high signal-to-noise, spatially-resolved spectroscopic data that enabled me to construct a more comprehensive picture of NGC 2903’s formation history by understanding the growth of its different components. Moving closer to the Milky Way and unveiling the history of its halo, I present a detailed chemical study for stars from one of Milky Way’s most significant mergers dubbed Gaia-Enceladus-Sausage (GES), aided by high-resolution optical spectra from McDonald Observatory and Magellan Telescope. I contrast these stars’ abundance trends to those found in the Milky Way and its surviving satellites to understand how its chemical signatures compare to other stellar populations and what this tells us about its star formation history. As emphasized in this Thesis, it is important to investigate galaxy assembly through the lenses of different galaxy components and in an interdisciplinary way. Therefore, I also aim to understand the formation of the Milky Way disc. I do this by turning to a zoom-in cosmological simulation of a Milky Way mass galaxy from the FIRE-2 suite and where I determine how the ages, metallicities, and detailed chemical abundances of stars relate to each other and to their current and birth locations. Specifically, I investigate if the stars in the simulations exhibit a tight age-abundance trend, similar to what is found in observations. Further, I explore how the dispersion found around this trend, at different metallicities and locations in the galaxy, relates to the star formation history of the simulated Milky Way. Lastly, taking advantage of the power of Milky Way large surveys, I kinematically and chemically characterized targets from the Transiting Exoplanet Survey Satellite (TESS) to understand the Galactic context of planet-hosting stars. This is especially important as we find more exoplanets in different parts of the Galaxy, enabling us to understand if and how planet formation and demographics are different for different Milky Way stellar populations. The accomplishments of this Thesis have contributed to a broad range of fields in Astronomy, but all tied together by the analysis of stellar populations in the Milky Way and beyond.