Milky Way's disk structures as a memory of its turbulent past

Abstract

The Milky Way (MW) has historically served as a template for understanding the formation and evolution of late-type massive disk galaxies in general. Combining astrometric data from the Gaia mission, spectroscopic and photometric surveys has allowed us to map the entire history of our Galaxy, from a turbulent proto-MW at early times to the barred spiral we see today. This history was contributed to by numerous merger events with dwarf satellite galaxies that left scars on the Galaxy's disk and halo. These scars are generally formed by accreted stellar populations or chemodynamically affected by these interactions. With the advent of new databases that provide chemical composition and kinematics for millions of objects in the MW, we can identify these stellar populations and map the consequences left by these events, seeking to piece together the puzzle of how structures formed and evolved in our Galaxy. In this thesis, we are particularly interested in the disk of the MW.The disk of our Galaxy is a complex structure formed by two main populations: the thin and the thick disks. These two components present spatial, chemical, and kinematic differences, indicating distinct formation histories. In addition, the last two decades have been filled with discoveries of substructures present throughout the disk, known as stellar overdensities. These structures consist of regions of greater stellar density when compared to the surroundings, which have been associated with perturbations generated by the passage of satellite galaxies during the merger process. This shows how these events impact the construction and evolution of galaxies in general. In the MW, in particular, we can use these regions as important sources of study to understand how these mergers occur and what the consequences are for the structure of the Galaxy.Most of the disturbed satellite galaxies are low mass and cause more localized impacts on the disk. However, there is evidence that the early times of the MW were also populated by so-called major mergers, which caused more drastic and decisive disturbances in the Galaxy. The last major merger is dated to approximately 9-11 billion years ago, known as Gaia-Sausage/Enceladus (GSE), coinciding with the epoch of the MW disk formation. It is believed to have played a key role in creating the dichotomy between thin and thick disks. Initially, with few age measurements for MW stars, previous studies noted that the average age of the thick disk was much higher than that of the thin disk, which could indicate a sequential formation. To explain the different chemical, kinematic and age characteristics between the disks, a model was proposed in which the GSE would be responsible for injecting gas into the Galaxy and triggering the formation of the thin disk, managing to explain the observational data and the few age estimates. However, the advent of the Gaia era allowed ages to be measured for thousands of stars in the MW disk, revealing a population of thin disk with ages older than the merger, that is, a scenario of co-formation between the disks in the early times of the Galaxy. This contradicted the most accepted scenarios and started a new era of searching for explanations about how the thin and thick disks formed.In this context, we aim to use a combination of spectroscopic, astrometric and age estimates to study how the dichotomy between thin and thick disks was formed and how the MW merger scenario directly impacts this process. In addition, we intend to use simulated data to better understand the mechanisms involved in these processes, seeking to reproduce our observational results. At the end of this project, we aim to publish four papers in high-impact journals.