PhD ceremony Ewoud Wempe

On Tuesday 25 November 2025 Ewoud Wempe defends his thesis called Reconstructing the Local Group in a cosmological context

Summary of his thesis:
The Milky Way and the Andromeda Galaxy are the two largest galaxies of our immediate cosmic neighbourhood, known as the Local Group. Together with over a hundred smaller dwarf galaxies, they form a unique laboratory for studying how galaxies form and evolve due to exceptionally detailed observations. In this research, I created cosmological simulations that accurately reproduce the Local Group's present-day configuration within the framework of the standard cosmological model, ΛCDM. This was achieved by generating a statistically representative ensemble of possible universes, where each realisation is jointly constrained by observations of the dynamics of our Local Group, including the masses, the precise positions and velocities of the Milky Way and Andromeda, as well as the observed quiet large-scale flow in their surroundings.

Analysis of this ensemble shows that the total mass of the two main galaxies is about three trillion solar masses, and that they are moving toward each other along a nearly radial trajectory. This close-to-radial orbit is a direct consequence of the quiet external flow, which limits the orbital angular momentum that could be acquired from massive nearby systems. The simulations further reveal that the mass distribution surrounding the Local Group is strongly flattened, confirming the dynamical existence of the so-called Local Sheet. This flattened structure is a necessary dynamical feature to reconcile the substantial mass implied by the Local Group's dynamics with the flow observed in its surroundings. Additionally, Milky Way analogues tend to assemble slightly earlier than isolated counterparts, due to the tidal influence of the more massive Andromeda galaxy. The methodology developed provides a new, statistically robust way to connect cosmological theory directly to specific observations of the Local Group.

Link to Ewoud's thesis

If available, you can follow the PhD ceremony via this livestream