Galactic stellar haloes
The galactic stellar halo is critical for understanding the assembly history of the Milky Way because it serves as a repository of fossil evidence from the galaxy's formation and evolutionary processes. I study a lot of the composition of the Milky Way stellar halo, mainly from a theoretical point of view, using cosmological and tailored simulations.
In a series of papers based on simulations of the Local Group - HESTIA, I have investigated the kinematics, structure, and chemical composition of galactic stellar haloes. Specifically, I explored the formation of the in-situ stellar halo component and the impact of mergers on the primary disk component, including its star formation history. Additionally, I examined the intricate structure of stellar merger debris and compared these findings to known features in the Milky Way's stellar halo, such as Gaia-Sausage-Enceladus (GSE). Our results indicate that Milky Way and M31 analogues exhibit similar stellar halo structures, often concealing evidence of multiple accreted systems. Furthermore, we demonstrated that the overlapping stellar merger debris structures evolve over time, even in traditionally assumed conserved chemo-kinematic domains and integrals-of-motion spaces, challenging conventional assumptions about their long-term stability. Finally, I investigated the chemical abundances and star formation histories of accreted systems, highlighting their distinct differences from the surviving satellite galaxies observed at z=0. This contrast underscores the critical importance of understanding the nature of satellite galaxies at z>1−2 to construct a robust and accurate accretion history for the Milky Way using data from the next-generation spectroscopic surveys, like 4MOST and Milky Way Mapper (SDSS-V). The images on top show Galactic and Extragalactic perspectives on the Milky Way (left) and M31 (right) analogues in one of the HESTIA simulations.
While cosmological simulations provide a wealth of information, investigating specific details of merger debris remains challenging and often lacks precise control. Furthermore, globular clusters—despite their widespread use in reconstructing the Milky Way's merger tree—cannot yet be fully incorporated into self-consistent models. To address these challenges, I collaborate with the GEPI team at Paris Observatory to study the kinematic properties of accreted and in-situ globular cluster systems using tailored N-body simulations of galaxy mergers. The accompanying animation illustrates one of our simulations, showcasing the structure of in-situ (left) and accreted (right) stellar populations, with accreted globular cluster orbits highlighted by solid lines.
I am a member of the e-TidalGCs project, led by Salvatore Ferrone (Paris Observatory), which aims at modelling and predicting the extra-tidal features surrounding Galactic globular clusters with available 6D phase-space information. These simulations reveal a diverse range of morphologies for extra-tidal structures, from elongated tidal tails to complex halo-like shapes, and compare these predictions with well-studied streams. By exploring different Galactic potentials, the study estimates the globular cluster population has lost 7-55% of its mass over the last 5 Gyr, with most of this material concentrated in the inner Galaxy, providing a foundation for future comparisons with high-precision data from ESA's Gaia mission and spectroscopic surveys.
The animation on the right illustrates the 3D morphology of stellar streams originating from Galactic globular clusters in one of the models.