Bright and dark: the accretion history of the Milky Way across scales

The merger history of the Milky Way (MW) is key to decoding its evolution, as the standard cosmological model predicts that galaxies assemble hierarchically. The ESA astrometric mission Gaia, complemented by spectroscopic surveys, e.g. APOGEE, offers a unique opportunity to pursue this research, providing 6D phase-space information coupled with chemical abundances for hundreds of thousands stars in the Galaxy. Prompted by such a prospect, in this thesis we combine theory with observations and develop a variety of methods to uncover the MW accretion history, shedding light on its progenitor galaxies across different mass scales. Stars accreted from the same progenitor are identified as featuring similar integrals of motion, such as orbital energy (E), angular momentum (Lz), and metallicity distribution function (MDF). To test this scenario, we analysed novel high-resolution N-body simulations of a massive satellite accreting onto the MW (mass ratio 1:10). Conversely to prevailing paradigms, our models show that stars accreted from a single massive merger can exhibit distinct MDFs across the E − Lz space because different regions of the progenitor, each with peculiar chemical properties, were incorporated into the Milky Way at different merger stages. This new result is essential for accurately tracing the merger history of our Galaxy. Misinterpreting this debris as originating from multiple smaller progenitors would bias the Milky Way’s merger history toward an overestimated number of low-mass accretions. Guided by these findings, we provide for the first time observational evidence of this chemo-dynamical diversity revealing that the last massive merger underwent multiple passages around the Milky Way before being fully disrupted. In light of the results above, we proceed to characterise the chemical abundances (Fe, Mg, Si, Ca, Mn, Al, and C) of the different structures that are currently recognised as ancient galaxies accreted by the MW, based on their E and Lz. To this end, we analyse the latest data releases of Gaia and APOGEE, to quantify the contamination from the last massive merger by means of Gaussian Mixture Models. We find that most of these substructures feature high chemical compatibility (≥ 50% of their stars) with the last massive merger, suggesting indeed its broad distribution in E − Lz . Within this entangled scenario, we can still attempt to identify the accretion of the least massive progenitors, which are less affected by dynamical friction. By analysing high mass resolution dark matter-only cosmological simulations, we settled realistic initial conditions to simulate the accretions of Ultra-Faint Dwarf galaxies, which build up the most metal-poor tail of the Galactic MDF, to provide predictions for upcoming spectroscopic surveys (e.g., WEAVE and 4MOST).