The existence of these spherical clusters of millions of stars has been known since the invention of the telescope in the 17th century. These round collections of stars are found in virtually all galaxies, and nearly 200 have been identified around the Milky Way. Although globular clusters appear to be a natural part of galaxies, their origin has long eluded astronomers. No theoretical models have been able to fully explain how they form.
However, in a new article published in Nature, the international research collaboration EDGE (Engineering Dwarfs at Galaxy formation’s Edge), in which the galaxy formation group at Lund University participates, has succeeded in solving the mystery. The study, led by the University of Surrey in the UK and based on groundbreaking cosmological supercomputer simulations, shows that dwarf galaxies – small, faint galaxies with relatively few stars – play a crucial role.
'When galaxies collide with each other, it becomes possible for gas between the galaxies to be compressed and create globular star clusters. Galaxy collisions were very common in the early universe, and small galaxies are much more common than large galaxies. These two factors make dwarf galaxies veritable factories for globular star clusters' says Oscar Agertz, astronomy researcher at Lund University.
A major problem for researchers has previously been that globular clusters appear to be completely devoid of dark matter. Dark matter is believed to be the dominant component, in terms of mass, in the universe. Explaining how star-forming gas could gather so densely in the early universe without the help of dark matter has not been obvious.
'But our computer models show that this is not a problem at all. Globular clusters do not form where dark matter is densest, in the centre of the galaxy. They can form outside the centre of the galaxy, and even far outside their host galaxy when it collides with other galaxies. This allows star-forming gas to be compressed to the high densities needed' says Oscar Agertz.
Reveals new objects
Thanks to high resolution and advanced models for astrophysical processes, researchers can show how dwarf galaxies formed and developed from the Big Bang to the present day. The new simulations also reveal a whole new class of objects that researchers call globular cluster-like dwarf galaxies. These have properties that lie between globular clusters and dwarf galaxies. The objects, in turn, provide new insights into both the evolution of galaxies and the conditions during the cosmic dawn when the universe was young.
'The formation of globular clusters has been a mystery for hundreds of years, so being able to add further context to how they form is fantastic. It is also very exciting to find a whole new class of objects in the simulations, especially since we have already identified a handful of candidates that exist in the Milky Way,' says researcher Ethan Taylor at Surrey's School of Mathematics and Physics, who led the study.
In addition to Lund University and the University of Surrey, the following institutions participated: Durham University, the University of Bath, the University of Hertfordshire, Carnegie Observatories, the American Museum of Natural History, and the University of Barcelona.
The study is published in the scientific journal Nature: “The emergence of globular clusters and globular cluster-like dwarfs”. (external website).
Oscar Agertz's profile in Lund University's research portal.