How do Milky Way galaxies grow their stellar halos?
Title: How do Milky Way galaxies grow their stellar halos?
SNIC Project: SNIC 2020/5-181
Project Type: SNIC Medium Compute
Principal Investigator: Thomas Bensby <>
Affiliation: Lunds universitet
Duration: 2020-04-01 – 2021-02-01
Classification: 10305


The diffuse halo of stars surrounding galaxies provides a unique window on their formation and evolution in our Universe. The stellar halo is a tracer of a galaxy’s past assembly, originating from the accretion and stripping of stars previously formed in smaller objects. However, such mergers and accretion events are stochastic for each galaxy: they are seeded by random early-universe perturbations inflated to cosmological scales. This stochasticity then couples with the complex non-linear physics of galaxy formation, generating large diversity in stellar halo properties. Advances in low-surface-brightness imaging have now enabled systematic surveys of extragalactic stellar halos, revealing the extent of this diversity. In this project, we will use a new computational approach to causally connect the variety of observed stellar halo properties to different merger scenarios. We will simulate the formation and evolution of five Milky-Way-like galaxies with high-resolution, dark-matter-only cosmological simulations. We will post-process these simulations with “particle tagging” to obtain simulated stellar halos, i.e. semi-analytically tying the dynamical evolution of stars to their neighbouring dark matter particles. The smaller computational cost associated to such semi-numerical approach will allow us to probe a larger diversity of objects and merger histories than would be possible with hydrodynamical simulations. The key novelty of our approach lies in our ability to re-simulate these five reference simulations using the “genetic modification” technique. This framework enables us to modify a specific aspect of a galaxy’s history, while reproducing all other untargeted features. For example, it allows us to systematically grow or downsize a chosen merger event, while reproducing the same large-scale environment and earlier formation for the galaxy. Our baseline scenario will generate four alternative merger scenarios for each galaxy, for a total of 20 dark-matter-only simulations. Comparing reference merger scenarios with their related, modified counterparts then creates controlled experiments in a fully cosmological context. The end product of the project will be a simulation library cleanly testing the mapping between a galaxy’s merger history and the building of its stellar halo. We foresee at least two publications arising from the results of this project, which will be submitted to leading astronomical journals. We further plan to use this library as a base for Tier-0 proposals (e.g. PRACE), which will re-simulate the most interesting objects and mass growth histories with hydrodynamics and galaxy formation physics.