Miniature galaxies inhabit the central regions of two separate, huge clusters of galaxies, astronomers announced today. The objects are thought to be the remains of galaxies whose outer stars have been stripped to feed the other, larger galaxies.
Each of the diminutive galaxies contains tens of millions of stars packed into an incredibly tight space, just 120 light-years across. The Milky Way, for comparison, contains some 300 billion stars across a span of 100,000 light-years or so, and it is thought to have developed by consuming other galaxies.
The study found 46 of the ultra-compact dwarf (UCD) galaxies, as they are called, in the Fornax galaxy cluster, some 60 million light-years away. Steven Phillipps of Bristol University in the UK explained how a galaxy might become a UCD: A dwarf elliptical galaxy -- destined to become even smaller -- passes very close to a massive galaxy, such as one called NGC 1399, at the center of the Fornax cluster
"The outer stars are stripped off by what are called tidal forces -- basically the gravitational pull of the giant galaxy," Phillipps said in an e-mail interview. "All that is left in the end is the central dense nucleus of the original galaxy."
The core of a UCD is held together by the mutual gravitational pull of the densely packed stars.
The finding adds to six UCDs in Fornax that Phillipps' team found in 2000. The UCDs were presented as a new class of galaxy that may have played a key role in the evolution of larger galaxies. From that initial discovery, the researchers predicted they would find more. But they did not expect 46 of them.
They also predicted UCDs should exist in the central regions of other galaxy clusters, and in the new work they found eight in the Virgo cluster, which is also about 60 million light-years away in another part of the sky.
Together, the discoveries suggest that UCDs are common in the central regions of galaxy clusters, Phillipps told SPACE.com. But he said they might make up just a fraction of the overall number of galaxies in a cluster. He does not know if UCDs exist outside galaxy clusters.
Phillipps said UCDs could be small galaxies were never any bigger and represent a halted stage of galactic development. But that scenario is less likely, based on the stellar make-up and the dynamics of the stars in them, he said.
The observations were made with the Anglo-Australian Telescope. Michael Drinkwater of the University of Queensland in Australia was Phillipps' primary collaborator. The results were to be presented today at a meeting of the Royal Astronomical Society.
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