Packed tightly together like twinkling stellar beehives, these globular clusters are made up of hundreds of thousands of stars. Now, for the first time, a team of scientists can reveal how the ancient and mysterious star systems actually form.
The existence of these globular clusters, comprising millions of stars, has been known since the invention of the telescope in the 17th century. Globular clusters are found in virtually all galaxies, and almost 200 have now been identified around the Milky Way.
Although globular clusters seem to be a natural feature of galaxies, their origin has long baffled astronomers. No theoretical models have been able to fully demonstrate how they are formed.
However, a new article published in Nature, explains how the international research collaboration EDGE (Engineering Dwarfs at Galaxy formation's Edge), which the galaxy formation group at Lund University is a part of, has managed to solve the puzzle. The study, led by the University of Surrey and based on cutting-edge 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 allows gas between the galaxies to be compressed and create these globular clusters. Galactic collisions were very common in the early universe, and small galaxies are far more common than large galaxies. "These two factors make dwarf galaxies a factory for globular clusters," says Oscar Agertz, astronomy researcher at Lund University.
In the past, the fact that globular clusters appear to be completely devoid of dark matter has been a major problem for researchers. In terms of mass, dark matter is believed to be the dominant component of the universe. Attempting to explain how star-forming gas can accumulate so densely in the early universe, without the help of dark matter, has not been straightforward.
"Our computer simulations show that this is not a problem at all. Globular clusters do not form where dark matter is densest, at 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.
New objects also revealed
Thanks to high resolution and advanced modelling of astrophysical processes, scientists can show how dwarf galaxies formed and evolved from the Big Bang to the present day. The new simulations also reveal a whole new class of objects that scientists call globular cluster-like dwarf galaxies. These have properties that are midway between those of globular clusters and dwarf galaxies. In turn, these objects provide new insights into both the evolution of galaxies and the conditions of 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 shed more light on how they form is fantastic. On top of that, it is very exciting to be able 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," concludes Ethan Taylor, researcher at Surrey's School of Mathematics and Physics, who led the study.
Publication:
Link to the article in Nature:
The emergence of globular clusters and globular-cluster-like dwarfs
