The diversity of galactic stellar halos
Title: The diversity of galactic stellar halos
SNIC Project: SNIC 2020/14-59
Project Type: SNIC Small Storage
Principal Investigator: Martin Rey <>
Affiliation: Lunds universitet
Duration: 2020-08-19 – 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. 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. 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.