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What is dark matter? Dwarf galaxies offer new insight

It was first measured a hundred years ago and yet we still don’t know what it is: dark matter. By looking at stars in dwarf galaxies PhD candidate Bas Zoutendijk is trying to gain new insight into this mysterious substance.

Much of the matter in the universe is dark matter. It cannot be seen because it does not emit light. It does have mass, however, which means it exerts gravity on luminous matter. That is how Zoutendijk can measure dark matter. ‘I look at the movements of stars and if I see they are moving faster, I know the pull on them is harder. That must be the dark matter.’

Exploding stars

For his PhD research, Zoutendijk looked at the speed of stars in small galaxies, or ultra-faint dwarf galaxies as they are known. Compared with large galaxies these contain a relatively large amount of dark matter. ‘This gives you a purer measurement of the gravity that dark matter exerts on a star. There is less chance of the measurements being distorted by supernovae (exploding stars, ed.).’

Research into the speeds of stars in ultra-faint dwarf galaxies has only recently become possible due to an improved measurement method. ‘This is the first time measurements have been made in the centre of ultra-faint dwarf galaxies,’ says Zoutendijk. ‘Most of the stars are in the centre, and you can measure differences and thus confirm or disprove theories about dark matter.’

Cold dark matter

There are different theories to explain the characteristics of dark matter. One of these is cold dark matter. Zoutendijk: ‘Cold dark matter predicts that there will be a strong increase in the density of dark matter as you go from the outside to the centre of a galaxy. Two other theories predict that this increase will be less strong.’

Dwarf galaxies offer new insight into dark matter
The circled stars belong to Eridanus 2, one of the dwarf galaxies that Zoutendijk studied.

Previous research in larger dwarf galaxies showed little increase in dark matter in the centre. Zoutendijk is one of the first to repeat this research but then in ultra-faint dwarf galaxies. He found a relatively large amount of dark matter in the centre. ‘That is consistent with the standard theory on dark matter, namely cold dark matter. The conclusion from my research is that if little dark matter is measured in the centre of large dwarf galaxies, this is caused by something else. What is most probable is that other effects take place that are linked to there being more stars and consequently more supernovae.’ These supernovae therefore cause more noise in the measurements, Zoutendijk concludes.

Text: Tom Janssen
Banner image: ESA

Image in text: Bas Zoutendijk

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