Dark Star Clusters: A ‘that’s weird’ Moment

It sounds like something out of a science fiction novel.  The incredible and rare dark star cluster, hiding the evil super villain’s headquarters.  A dark star cluster is something I would imagine as a spooky, eerie type of place where everything you see changes when you enter its space.

Science fiction aside, a dark star cluster is real, and it’s a new type of cluster that is similar to the mighty dense globular clusters that orbit most galaxies.  Globular clusters orbit in a halo of space around the centres of galaxies, and though our Milky Way harbours 150 of them, there are far more in the massive galaxies of the Universe.

Globular Cluster NGC 6388 Credit: ESO, F. Ferraro (University of Bologna)

The Elliptical galaxy NGC 5128, also known as Centaurus A, is the closest massive galaxy to the Milky Way, and is thought to harbour over 2000 globular clusters, many of which are much larger and more massive than the Milky Way’s globulars.  New observations of 125 of the clusters orbiting Centaurus A were taken with the ESO’s Very Large Telescope (VLT), and used to infer their observed motion.  From their motion we can determine their mass, and when astronomers compared this to the visible matter in the clusters, and found a striking discrepancy.

Ceentaurus A Credit: ESO/Digitized Sky Survey. Acknowledgement: Davide de Martin

Most of the clusters showed what was expected, that the more massive they were, the brighter they would appear, and vice-versa.  More stars equal more brightness and more total mass.  But some of the clusters were very different.  They had a mass that was many times larger than what was expected from looking at their brightness.  And for the ones that fit this category, the more massive they were, the larger their fraction of unseen material.  Where is the missing mass?

Could black holes be to blame? Its possible, but even black holes wouldn’t account for enough of the missing mass.  What about dark matter? Globular clusters contain very little dark matter traditionally, but perhaps these clusters are retaining dark matter? It would explain the observations, but it runs against conventional theory.

This is one of those moments in science when we’ve said ‘that’s strange.’ It will lead to a new focus on the globular clusters of massive galaxies, to see if more of these ‘dark clusters’ can be found.  Will they constitude a new class of star cluster? Or will they require a new idea of the formation and evolution of all globular clusters? Could it be both?

We will have to wait and see!


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