A set of globular clusters surrounding the giant galaxy Centaurus A have a weight problem. They contain far more mass than their brightness would suggest. As of yet, we don't know why, but the problem could lead in many interesting directions.

Globular star clusters are, as the name suggests, round collections of tens or hundreds of thousands of stars. They orbit large galaxies and are usually dominated by very old stars.

In recent years, many oddities have turned up in some or all globular clusters, suggesting their process of formation was more complex than we realized. “Globular clusters and their constituent stars are keys to understanding the formation and evolution of galaxies,” said Matt Taylor, a PhD student at Pontificia Universidad Catolica de Chile, Santiago, Chile.

Centaurus A, perhaps twice the mass of the Milky Way, is thought to be accompanied by 2,000 globular clusters, some larger and brighter than any in the vicinity of the Milky Way. Taylor has conducted the most detailed study ever made of these objects, choosing 125 as representatives.

Using the FLAMES instrument on the European Southern Observatory's Very Large Telescope, Taylor was able to track the motions of individual stars within these clusters, even at a distance of more than 10 million light-years. Since these motions are determined by the gravitational field within a cluster, this provided a measure of each cluster's mass.

In a paper submitted to The Astrophysical Journal, and available online, Taylor reports that most clusters fall in line with the relationship we see between brightness and mass for almost all globular clusters in the Local Group. However, almost a dozen clusters were much more massive than their light would suggest. Two clusters fitted somewhere between the two categories, which the authors speculate might be a sign of them evolving from one to the other.

Large black holes at the center of the clusters could provide some of the mass, but Taylor says it is not credible for the hole to be that massive. This raises the intriguing possibility of clumps of dark matter, usually thought to be largely absent from globular clusters, making up the difference.

If so, some serious thought will have to go into why some globular clusters coincide with patches of dark matter while most do not. Notably, however, there was a correlation between the size of the cluster and the proportion of hidden mass.

The large clusters are interesting in other ways, with many stars having higher concentrations of metals than those in the globular clusters around the Milky Way or Andromeda.

If isolated clumps of dark matter can be confirmed to exist around large galaxies, attracting star clusters in the process, it will tell us something profound and surprising about the nature of this mysterious material. The search for an explanation may begin closer to home: Two globular clusters located near the Large Magellanic Cloud, while much smaller than those studied here, also show signs of having more mass than their brightness would suggest.