What is the origin of the Milky Way? This question forms the basis of a field within Astronomy called Galactic Archeology. Its name comes from the fact that astronomers in this field look at the present state of our galaxy to infer its past, just as archeologists do with fossils in a way. The answer to the aforementioned question is, of course, rather complex and still incomplete. Disk galaxies like the Milky Way are complicated systems formed by billions of stars, interstellar gas, and huge amounts of dark matter. These components interact with each other through many different processes. Some of them are slow and their effects can only be seen over the course of a few billion years, and some are quite violent and fast (for astronomical standards) and change the properties of galaxies in just a few million years or even less. Stars migrate throughout the galaxy, changing the orbits they were born with; giant gas clouds collapse to give birth to new stars; massive stars blow up as supernovae and inject their surroundings with kinetic energy, chemically enriching the new star generations; dark matter blobs hit and merge with galaxies, changing their kinematics and morphology as a whole; and this is just the beginning. To reconstruct all these different events and put them together to build a coherent picture of how the Milky Way formed and evolved is an intricate endeavour.
However, there are ways to get around this complicated puzzle. Astronomers have known for a while that galaxies grow hierarchically: small galaxies accrete onto each other to form and grow bigger galaxies. If the accreting galaxy is big enough compared to the size of the main galaxy -over a tenth as big as the main galaxy-, the merge of the two will be dramatic, leaving an imprint on the orbits and velocities of the stars born up to that point. This imprint is especially noticeable in the thick disk of galaxies – the most vertically extended and least dense part of the disk. Therefore, by looking at how stars with different ages are spread along the thick disk of the Milky Way, we can say something about its merger history (particularly massive mergers!).
In the current phase of my PhD with Dr. Marie Martig at the Astrophysics Research Institute, I am looking at a large sample of simulated disk galaxies, and analysing how different stars with different ages populate the thick disk. Similar thick disk structures should come from similar merger histories. Therefore, the goal is to characterise different thick disk structures in order to create a classification system, linking every category within that classification to a particular merger history. After this, I will look into the thick disk structure of the Milky Way and see which category from the classification it falls into. Since we will know the main characteristics of the merger history of that particular group, we will be able to draw conclusions about the merger history of the Milky Way itself.
Although I am working exclusively with simulations at the moment, my PhD project will lead me to work with observations in the future. Only now are new astronomical surveys like Multi Unit Spectroscopic Explorer (MUSE) starting to provide more detailed information about the thick disk structure of nearby galaxies. Perhaps we could also apply this classification to these nearby galaxies and start to have a better idea of how hierarchical galaxy formation works in general: which merger histories are more common? Which ones are rare? What are the mechanisms that favour some over others?
The field of Galactic Archeology is at a very exciting point, where new technologies in both simulations and observations are changing dramatically the way we see and understand the structure and evolution of the Milky Way (as well as galaxies in general). To carry out a PhD project in which I will see both sides of the coin will make me have a round perspective of the field as well as open the door to many opportunities in the future.