Star clusters may harbor dark matter

ESO/Digitized Sky Survey; Acknowledgement: Davide de Martin

Star clusters may harbor dark matter

Globular clusters—bunches of a few thousand stars that orbit around much larger galaxies—may be harboring a dark secret. Our galaxy, the Milky Way, has about 150 globular clusters around it and, until recently, these were the only ones near enough to study in detail. Now, a team of astronomers has taken a close look at some of the 2000 clusters (circled in picture above) around Centaurus A, our nearest giant galaxy. The team used the Fibre Large Array Multi Element Spectrograph (FLAMES) attached to Europe’s Very Large Telescope in Chile. FLAMES is equipped with 130 optical fibers, which astronomers position precisely at the point where the telescope forms an image. That allows light from 130 different targets to be captured and sent separately to a spectrograph, which measures how bright they are at different wavelengths. Using FLAMES, the team could measure the movement of stars in 125 different clusters around Centaurus A to get estimates of their masses. In most cases, those mass estimates chimed closely with how bright the clusters appeared—as expected from observing the Milky Way’s clusters. But, as the researchers report online today in The Astrophysical Journal, some deviated from this rule and seemed to be several times more massive than their brightness suggests. These seemingly overweight clusters could contain black holes or other dark remnants of old stars, but the team believes that can’t totally explain the observations. Could it be that some clusters are harboring dark matter, the mysterious stuff that provides galaxies with enough gravity to hold together? Theorists don’t currently think that globular clusters contain much dark matter, but because very little is known about the stuff it could be the answer to this unexpected breed of star cluster.