Astronomers have discovered a series of unusual globular star clusters with masses far exceeding what would be expected under the standard model for the celestial structures. The clusters are located in the giant elliptical galaxy Centaurus A, and seem to hint at an enigmatic dark presence that cannot yet be accounted for.
A globular cluster is essentially an enormous spherical collection of thousands of stars that orbit like satellites around large galaxies. It is believed that Centaurus A may play host to as many as 2,000 clusters, a significant amount of which exhibit the same strange, massive characteristics.
Astronomers were able to estimate the mass of a sample of 125 of the globular clusters using the FLAMES instrument mounted on the ESO's Very Large Telescope at the Paranal Observatory in Chile. The masses were then compared to the luminosity of the clusters, from which the team were then able to deduce the rough number of stars contained in the spherical structures. It was discovered that the mass of the clusters far exceeded the estimated number of stars.
In terms of appearance, the newly-discovered clusters bear no distinction to the 150 ordinary specimens orbiting the Milky Way. However, the strange readings hint at a mysterious, as yet unaccounted for, celestial presence.
Possible explanations for the unexpected mass include the presence of black holes, a collection of stellar remnants at the core of the clusters, or even a concentration of dark matter. The latter option, while enticing, seems unlikely, as globular clusters are generally considered to be singularly lacking of the enigmatic material.
"We have stumbled on a new and mysterious class of star cluster!" states PhD student Matt Taylor of the Pontificia Universidad Catolica de Chile, Santiago, Chile, and lead author of a paper on the study. "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."
Astronomers will continue to search for further examples of the new class of dark cluster.