As galaxies accrete smaller galaxies, tidal streams and shells are formed. Especially in the halos of galaxies these features are visible for a long time, as demonstrated by the simulation by Johnston et al. (2005) and the observed galaxy NGC 5907.

Research interests

Before moving into software development and testing, I worked in astronomical research. My scientific career focused on the structure and dark matter content of galaxies, the nature of galaxy halos and how they are assembled. This is of course closely related to the general picture of galaxy formation and evolution, which predicts galaxies to build up over time by merging and accretion of smaller building blocks. Over the past years, new instrumentation revealed the signatures of this process within the Milky Way and other Local Group galaxies. The study of the Milky Way, its stellar halo, and its satellite galaxies have even started to yield important constraints on cosmological models.

The majority of my work has been based on modeling color-magnitude diagrams (CMDs) of stellar populations. Since the observed color and brightness of a star depends on its age, mass, metal content and distance, comparing CMDs with models can reveal information on age, metallicity and distance distributions of stellar populations. Using this information I have reconstructed star formation histories of nearby dwarf galaxies. And using millions of stars in the Milky Way I have also been able to reconstruct a map of the stellar structure of our own Galaxy.

For my PhD thesis I worked on a microlensing survey of the nearby Andromeda Galaxy (M31). The goal of this project was to see whether the dark matter halo around M31 consists for a significant part of Massive Compact Halo Objects (MACHOs), which were considered a viable dark matter candidate at the time. Using four years worth of imaging data and the Difference Imaging Photometry method, I detected around 100,000 variable sources, from which I filtered 14(!) that showed the characteristics of microlensing events. After a comparison with model predictions, we concluded that the MACHOs did not make up a significant part of the dark matter around M31.

Cover van de Zenit van december 2005 (Stichting De Koepel)


Astronomy is not just something for scientists. On the contrary, the mysteries of the universe are intriguing to many people, and astronomy can only be practised the way it is now thanks to this general interest in the cosmos. It is therefore important to share this knowledge and fascination with the general public.

Over the years I have given many popular talks about astronomy, and about gravitational lensing in particular. I have also written several articles in Zenit, the Dutch popular scientific magazine about astronomy and meteorology. In the future I hope to keep active in astronomy outreach.