While following up on an earlier discovery, astronomers using the Hubble Space Telescope (HST) have spotted over 10,000 globular clusters lurking in a vast swarm near the heart of a giant cluster of galaxies. At 2.5 billion light years away, these globulars are so far away that even the hubble can only see the brightest of them, and astronomers estimate that there as many as 160,000 waiting to be found. By comparison, our own Milky Way galaxy has only 150. This discovery is an important development of earlier work studying the correlation between the amound of dark matter in a galaxy’s halo and the number of globular clusters orbiting it.Ten years ago, astronomer John Blakeslee was studying gravitational lensing in an image of galactic cluster Abell 1689, which he had captured using Hubble’s newly installed Advanced Camera for Surveys. While examining the image, he found a large number of point-like objects. He knew that these objects could not be stars, and later found that these objects were globular clusters. Late last year, he conducted a new survey, using the HST to star deeper into Abell 1689 to better understand the globulars. The results of this paper were published in the AstroPhysical Journal.
The new image was sensitive enough to reveal over 10,000 new globular clusters, concentrated near the centre of the galactic cluster. The faintest of these was a mere 29th magnitude, some 50 million times fainter than the dimmest star visible to the naked eye, but even these were only the brightest of the globulars in the swarm. By looking at the range of brightnesses of the globulars, and using statistical methods to compare how populations of globular clusters are normally distributed, Blakeslee calculated that there are actually a total of 160,000 globulars in Abell 1689, concentrated in the centre of the galactic cluster.
“The globular clusters are fossils of the earliest star formation in Abell 1689, and our work shows they were very efficient in forming in the denser regions of dark matter near the center of the galaxy cluster,” Blakeslee said. “Our findings are consistent with studies of globular clusters in other galaxy clusters, but extend our knowledge to regions of higher dark matter density.”
Earlier work by Australian astronomers revealed a link between dark matter and globular clusters: for medium-sized galaxies and larger, you can predict the amount of dark matter in a galaxy’s halo by counting the globular clusters. Blakeslee’s team have now expanded on that knowledge: “In our study of Abell 1689, we show how the relationship between globular clusters and dark matter depends on the distance from the galaxy cluster’s center,” explained team member Karla Alamo-Martinez of the Center for Radio Astronomy and Astrophysics of the National Autonomous University of Mexico in Morelia. “In other words, if you know how many globular clusters are within a certain distance, we can give you an estimate of the amount of dark matter.”
The average brightness of the globulars within this swarm is estimated to be about 31st magnitude, which puts them out of reach of the HST. But the Hubble’s successor instrument, the James Webb Telescope, some five times larger and currently scheduled for launch in 2018, will easily detect them and should return even more spectacular images of the distant universe.