How did our Galaxy form? What were the major events that shaped its formation and evolution? Can we use the present-day positions, motions, chemical compositions and ages of stars in the Milky Way to trace its past formation history? These are some of the big questions driving the study of our Milky Way, commonly known as Galactic Archaeology as part of the VIA LACTEA project.
In the current cosmological model, galaxies form hierarchically through the condensation of baryons into stars at the center of dark matter halos. A galaxy like our Milky Way is thought to have experienced many mergers in its early formation history when the Universe was only 2-4 billion years old and later evolved more quiescently to the present-day. However, a number of signs seem to suggest that its evolution may not be as quiescent as previously thought and this may also have important implications for how our Sun was even born. Understanding the formation of the Milky Way and its history throughout cosmic time, is ultimately tied to a fundamentally existential question: "Where do we come from?".
In the stellar halo of the Milky Way, Gaia has uncovered signs of a potentially massive ancient galaxy which merged with our own some 10 bilion years ago. Much is still uncertain about the significance of this event. How massive was the galaxy? Are we certain it was just one galaxy? How was our Galaxy affected by it? What were the implications of this event for the formation and later evolution of the disc of the Milky Way? Did it mark a new phase in the formation of the disc? Did the GSE merger give rise to the thick disc or was it born thick to begin with? Did the GSE merger bring the necessary gas to trigger the formation of the chemical bimodality seen in the Milky Way disc?
In the disc, close to our Sun, Gaia has shown clear signs of disequilibrium which have been mapped out to large distances even into the stellar halo. This points out that our Galaxy is not in a steady-state equilibrium as often assumed opening new opportunities in defining the tale of its formation history as well as the distribution and nature of dark matter. Two candidate galaxies have been proposed to be the culprit for all this recent state of Galactic unrest: the Magellanic Clouds and the Sagittarius dwarf galaxy. How did our Galaxy respond and evolve subject to its interaction with these two alien intruders? What other signs have these interactions left in the chemical, age and phase-space structure of the disc and stellar halo?
Together, these questions motivate the objectives of the VIA LACTEA project which aims to gain a better understanding of the role of massive accretion events in shaping the age, chemical and structural/kinematical properties of the Galaxy from very high-redshift mergers (e.g. Gaia-Sausage-Enceladus) to the present-day subject (e.g. Magellanic Clouds and the Sagittarius dwarf galaxy), test whether we can tell the formation tale of the Milky Way Galaxy from observations of its present-day stellar populations and ultimately test how dark matter behaves on Galactic scales.
