May 19, 2015
Astronomers have discovered a new kind of globular clusters that are more massive than their luminosity suggests. Discovery was made with ESO’s Very Large Telescope (VLT) by observing globular clusters around Centaurus A galaxy, also known as NGC 5128.
It’s not exactly a galaxy or dwarf galaxy, since globular clusters are significantly smaller and they are found in galaxies’ halos. They usually have hundreds of thousands of stars, compared to hundreds of billions in galaxies.
Typically, the stars in a globular cluster are only 1 light-year away from each other and at the core of the cluster, it gets really crowded with only 200 astronomical units (1 AU is the distance between the Sun and Earth) between stars. Stars in globular clusters are usually oldest in the galaxy and created at the same time.
The brighter a globular cluster is, the more stars it contains i.e. the more massive it is. This is of course true for galaxies too, but globular clusters are more dense, their stars are of similar age and composition and there is thus stronger relation between their luminosity and mass. Recent discovery, however, has brought the last statement into question.
Team of astronomers led by Matt Taylor, Pontificia Universidad Catolica de Chile, Santiago, Chile used ESO’s VLT and studied 125 out of some 2000 globular clusters around Centaurus A galaxy. Most of these 125 were brighter the more mass they had, but some globular clusters were considerably more massive than they looked. Something other than stars gave mass to these clusters. Additionally, the more massive these odd clusters were, the higher the percentage of their mass came from this mysterious source.
“Globular clusters and their constituent stars are keys to understanding the formation and evolution of galaxies. For decades, astronomers thought that the stars that made up a given globular cluster all shared the same ages and chemical compositions — but we now know that they are stranger and more complicated creatures,” said Taylor.
Some of the invisible mass could come from black holes or neutron stars, but this wouldn’t account for all of the dark mass. Another candidate is dark matter, but current theories actually predicted that globular clusters are devoid of dark matter. After this discovery, another theory could be necessary, especially if further observations found more globular clusters with mysterious dark mass.
“We have stumbled on a new and mysterious class of star cluster! This shows that we still have much to learn about all aspects of globular cluster formation. It’s an important result and we now need to find further examples of dark clusters around other galaxies,” added Taylor.