European Southern Observatory’s (ESO) Very Large Telescope (VLT) spotted a new class of “dark” globular star clusters around the giant galaxy Centaurus A. These mysterious objects look similar to normal clusters, but contain much more mass and may either harbor unexpected amounts of dark matter, or contain massive black holes - neither of which was expected nor is understood.
Globular star clusters are huge balls of thousands of stars that orbit most galaxies. They are among the oldest known stellar systems in the universe and have survived through almost the entire span of galaxy growth and evolution.
Matt Taylor, a Ph.D. student at the Pontificia Universidad Católica de Chile, Santiago, Chile, and holder of an ESO Studentship, is lead author of the new study. According to a press release, he set the scene: “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.”
Taylor and his team have now made the most detailed studies so far of a sample of 125 globular star clusters around Centaurus A. They used these observations to deduce the mass of the clusters and compare this result with how brightly each of the clusters shines.
Taylor sums up the situation: “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.”
Reference:
“Observational Evidence for a Dark Side to NGC 5128’s Globular Cluster System,” M. Taylor et al., 2015, to appear in the Astrophysical Journal [https://apj.aas.org, preprint: https://arxiv.org/abs/1503.04198, PDF: https://www.eso.org/public/archives/releases/sciencepapers/eso1519/eso1519a.pdf]. The team is composed of Matthew A. Taylor (Pontificia Universidad Católica de Chile, Santiago, Chile; ESO, Santiago, Chile), Thomas H. Puzia (Pontificia Universidad Católica de Chile), Matias Gomez (Universidad Andres Bello, Santiago, Chile) and Kristin A. Woodley (University of California, Santa Cruz, California, USA).
