Hubble: Abell 1689 is Stuffed with Globular Clusters

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The depths of the famous Abell 1689 galaxy cluster has been studied by the Hubble Space Telescope, revealing the faint light of 10,000 previously undiscovered globular clusters. This has led researchers to estimate that over 160,000 of these small clusters of stars are lurking inside Abell 1689 -- the largest population of globular clusters ever seen in the Cosmos.

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Abell 1689 is known for its huge number of galaxies all swarming together to form a vast galactic cluster. The collective mass of the cluster warps spacetime and, from our vantage point nearly 2.2 billion light-years away, the light from distant galaxies beyond the cluster appears as huge arcs.

These arcs are artifacts of a relativistic effect known as gravitational lensing. From this phenomenon, a measure of the mass of dark matter contained within the cluster can also be estimated.

Hubble has now stared deep into Abell 1689 to uncover a huge population of globular clusters -- proverbial stellar fossils. Globular clusters, which are dense groups of hundreds of thousands of ancient stars, are thought to have formed only 1-2 billion years after the Big Bang. These clusters are the precursors to the massive galaxies -- containing hundreds of billions of stars -- that dominate the modern universe.

The massive Abell 1689 galactic cluster as observed by Hubble -- the image to the right shows the previously hidden globular clusters contained within the galactic cluster. Click image to see full-resolution version.
NASA, ESA, J. Blakeslee (NRC Herzberg Astrophysics Program, Dominion Astrophysical Observatory), and K. Alamo-Martinez (National Autonomous University of Mexico)

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There's strong link between globular clusters and dark matter, and with the help of Hubble's Advanced Camera for Surveys, researchers have found that groups of globular clusters (such as the population found inside Abell 1689) act as a tracer for the presence of dark matter.

"We show how the relationship between globular clusters and dark matter depends on the distance from the center of the galaxy grouping," said Karla Alamo-Martinez, of the Center for Radio Astronomy and Astrophysics at 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."

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"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," said John Blakeslee, of National Research Council Canada's Herzberg Institute of Astrophysics at the Dominion Radio Astrophysical Observatory in Victoria, British Columbia, and team leader. "Our findings are consistent with studies of globular clusters in other galaxy clusters, but extend our knowledge to regions of higher dark matter density."

The light from the 10,000 newly discovered globular clusters are 1 one-billionth of the brightness of the faintest star you can see in the night sky.

A paper detailing this discovery will be published in the Sept. 20 edition of The Astrophysical Journal.

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