Dark matter: searching for the invisible in the universe’s tiniest galaxies

Dark matter – that strange, invisible substance believed to hold galaxies together – remains one of the greatest astrophysical mysteries. Although it makes up 27 % of the universe, it’s exceptionally hard to spot. This is because, unlike ordinary matter, dark matter doesn’t absorb, reflect or emit light. In fact, so far, researchers have only been able to deduce that it exists from the gravitational effect it appears to have on visible matter.

However, a new method of measuring dark matter in the centre of dwarf galaxies is opening new vistas in the pursuit of this elusive substance. With support from the EU-funded CLUSTERS project, a team of astrophysicists developed this technique by focusing on star clusters. Their findings have been published in ‘Monthly Notices of the Royal Astronomical Society’.

Star clusters, and more specifically globular clusters, are old systems of thousands to hundreds of thousands of stars that are held together by gravitational attraction. Researchers realised that if they were to study how these clusters behaved within a galaxy, they could learn more about dark matter.

The best candidates for exploring star clusters are the universe’s smallest galaxies, called dwarf galaxies. Galaxies like these, found orbiting the Milky Way, are dominated by dark matter. The tiniest of the dwarf galaxies, the ultra-faint dwarfs, are made up of at most tens of thousands of stars –a drop in the ocean compared to our Milky Way’s 200-400 billion stars.

Probing dark matter

In their efforts to find out what dark matter is made up of, scientists have been using detailed models to compare its distribution in galaxies. If dark matter could be successfully measured, it would be a start towards solving the mystery of its nature. But the absence of gas and the small number of stars in ultra-faint dwarfs made such measurements impossible. That is, until the CLUSTERS researchers developed this new method.

The key element in the scientists’ method was the use of dense star clusters that orbit close to the centre of the dwarf galaxy. Unlike galaxies, these star clusters are so dense that their stars gravitationally scatter from one another, making them expand. The project team realised that the gravitational field that the star cluster orbits in and, consequently, the distribution of dark matter in the host galaxy play a role in how fast a cluster expands.

Computer simulations showed that the structure of star clusters is sensitive to whether dark matter is smoothly distributed or densely packed at the centre of galaxies. The astrophysicists tested their new method on the ultra-faint dwarf Eridanus II. One of the smallest known galaxies, it has a lone star cluster about 147 light years from its centre. Although the scientists’ results pointed to a dark matter core for Eridanus II, there was much less dark matter than expected.

Professor Justin Read, a co-author on the study, commented on their findings on the Phys.org news portal: “One possibility is that the dark matter at the very centre of Eridanus II was ‘heated up’ by violent star formation, as suggested by some recent numerical models. More tantalising, however, is the possibility that dark matter is more complex than we have assumed to date.”

The CLUSTERS (Galaxy formation through the eyes of globular clusters) dark matter findings are being used to further knowledge on globular clusters.

For more information, please see:
CORDIS project web page