Universiteit Leiden

nl en

Tracking galaxies from a few glowing pixels

In 2018, astronomer Jorryt Matthee won the C.J. Kok Jury Prize for the best dissertation of the Faculty of Science. He succeeded in finding a number of rare galaxies from the early Universe. One of them received the same initials as football player Cristiano Ronaldo: CR7.

‘What initially looked like a few glowing pixels on a photo later turned out to contain a very complex structure: an extremely bright galaxy that we see when it is created by the merging of as many as three, possibly four, smaller galaxies.’ This is how astronomer Jorryt Matthee briefly describes the results of his Leiden PhD project, which he concluded in 2018 with the award-winning dissertation.

A complete process

Not only was the content of Matthee's thesis impressive; with 425 pages it was also respectable in size. It is, therefore, the result of a complete process that he managed to complete during his promotion, he says. ‘Finding a number of rare galaxies in the early Universe in the first months of my PhD trajectory, then confirming their distances with new observations a year later, and finally uncovering their structure with follow-up observations that we have planned and performed ourselves.’

Information overload

In addition to the completeness of the process, Matthee is proud of the analysis he did on a series of computer simulations that were quite challenging due to ‘a huge information overload.’ ‘During the analysis I often asked myself: what exactly do we want to measure? This turned out not to be easy to answer.’

New connections

Nevertheless, he succeeded in making a number of interesting connections between completely different properties of galaxies. ‘For example, it appears that small differences in the chemical composition of stars in galaxies depend on the structure of the Universe on a large scale because this influences the growth of galaxies on long time scales. This result also affects my perception side of the research, because the chemical composition of stars influences the interpretation of the light that we perceive,’ Matthee says.

Stay patient

Apart from the tough analyses, Matthee sometimes found it difficult to stay patient during his promotion, he admits. ‘There is often about a year, sometimes even two or three, between requesting an observation and obtaining the data. This is of course still short compared to other groundbreaking experiments, but it is a relatively long time scale compared to a PhD trajectory or a postdoctoral appointment.’

The research sometimes also provided good moments. ‘The most exciting thing is when a new photo or measurement arrives and you open the data on your computer for the first time. Is our suspicion confirmed or not?’

The properties of the early Universe

Matthee remains driven to discover the properties of stars in the early Universe. ‘These stars resemble the ancestors of our own sun and their properties can, therefore, tell us something about our own distant history. In addition, I am also very interested in the question of why some galaxies grow faster than others and what the consequences are.’ He believes that the challenge is to find out theoretically what properties of stars and galaxies we must (and can) measure in order to deepen our understanding, and then to try to map these properties with telescopes.


Matthee now works as a postdoc at a Zwicky fellowship at the ETH in Zürich. He also won a prestigious IAU PhD Prize for his dissertation. He considers the C.J. Kok Juryprijs a very nice recognition of his work. ‘This recognition is currently very motivating and stimulating, and therefore helps me with the confidence to conduct new daring research.'

3 tips from C.J. Kok Prize winner Jorryt Matthee to new PhD students

  1. Find a good sparring partner with whom you can exchange ideas as often as possible.
  2. In order to familiarise yourself with a topic, conference proceedings or public slides of presentations are often more accessible than articles.
  3. Exchange your research methods and the problems you encounter with others, including for instance PhD students from another group. For example, I used software designed to compare large catalogs of stars (galaxies) to analyse computer simulations, and have benefited greatly from it.
This website uses cookies.  More information.