title: The Assembly History Of The Milky Way Stellar Halo
Creators: name: Kisku, S
Publisher: name: Liverpool John Moores University
Year: 2024

description: The Milky Way as a galaxy is one that holds particular interest to me because it is the one in which we exist. Understanding the origins of the Galaxy is key to figuring out our importance in the Universe and whether there is a purpose for our existence or whether we are merely a series of accidents. Being able to study the Milky Way from such a close distance, since we are in it, allows us to analyse its different components on a star-by-star basis. The stellar halo, inparticular, due to the large dynamical timescales, yields the most information about the early stage sof the Galaxy. This large dynamical timescale means that sub-structures in the Milky Way halo are preserved.By understanding the formation history of our own Galaxy, we can place constraints on the formation of other disc galaxies. And knowledge of the formation history of other galaxies lets us place the Milky Way in a larger context and gets us closer to answering the question: do we live in a special place in the Universe? This thesis aims to provide insight in to the formation history of theGalaxy, achieved through the analysis of some of its sub-structures. Recent evidence based on APOGEE data for stars within a few kpc of the Galactic Centre suggests that dissolved globular clusters contribute significantly to the stellar mass budget of the inner halo. I enquire into the origins of tracers of globular cluster dissolution,N-richstars, that are located in the inner 4 kpc of the Milky Way. From an analysis of the chemical compositions of these stars, I establish that about 30 percent of the N-rich stars previously identified in the inner Galaxy may have an accreted origin. This result is confirmed by ananalysis of the kinematic properties of my sample. The specific frequencyof N-rich stars is quite large in the accreted population, exceeding that of its in situ counterparts by near an orderof magnitude, in disagreement with predictions from numerical simulations.We hope that our numbers provide a useful test to models of globular cluster formation and destruction. The heated disc of the Galaxy, known as the Splash, has been predicted and recently discovered due to its halo-like kinematics and disc-like chemistry. Using data from APOGEE and Gaia,I examine the spatial and chemo dynamical properties of the Splash compared to the disc. In addition, I analyse the Splash population of MilkyWay-like galaxies in the ARTEMIS simulations, comparing galaxies with Gaia Enceladus/Sausage (GE/S)-like mergers and those without. The analysis performedr eveals a statistically significant chemical difference between theSplash and the disc,showing that theSplash is an older population with a shorter period of star formation. I also find a smooth correlation in the high-α population of both the Milky Way and simulated galaxies, regardlessof if they’ve undergone a GE/S-like merger, when comparing α-abundance to kinematics. Finally,a simple comparison of the Splash fraction of the galaxies reveals that even galaxies with only minor mergers can create a Splash comparable to galaxies with GE/S-likemergers. I conclude that whileaGE/S-like merger is shown to cause a Splash, it is not necessary.The orbital direction of the accreted population iswhat also plays a big role increating a Splash. The above results place constraints on our understanding of the effects of mergers on the stellar populations in the MilkyWay. From discovering the existence of accreted globular cluster members, leading to the question of whether the environment of dwarf galaxies can cause the destruction of globular clusters, to understanding how a merger affects thedisc of the Galaxy and comparing the results of this interaction in different Milky Way-like simulated galaxies. The findings of this thesis pave the way for future works towards the goal of fully reconstructing the formation history and evolutionof the Milky Wayand placing it in a larger context to better understand and compare it with other galaxies.