Solo Galaxy Has Dark Matter Cloak

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<b>Solo Galaxy Has Dark Matter Cloak</b><br /><br />LINK<br /><br />The discovery of three "naked" galaxies - not cloaked in dark matter - shocked astronomers in 2003 and prompted an urgent hunt for more. But a detailed new X-ray observation of an isolated galaxy has not reproduced the finding, leading researchers to suggest galaxies may form or evolve in multiple ways.<br /><br />Conventional theories propose that galaxies form within clumps - or halos - of dark matter. Although it cannot be seen directly by astronomers, dark matter can be detected by its gravitational effect on objects in or around these galaxies.<br /><br />In 2003, astronomers led by Aaron Romanowsky, then at the University of Nottingham, UK, used an optical telescope to study how gas clouds in blob-shaped "elliptical" galaxies move. The team found three galaxies that appeared to have little or no dark matter around them.<br /><br />Two of the galaxies were grouped with other galaxies, making it conceivable that their dark matter halos had been stripped away by interactions with their neighbours. But one naked galaxy was out on its own - apparently too isolated to have ever interacted with other galaxies<br /><br /><br />Lonely as a cloud <br /><br /><br />That lone galaxy prompted Ewan O'Sullivan, at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, to track down other isolated galaxies - which are rare compared to those in groups - to discover whether they had dark matter halos. <br /><br />He used NASA's Chandra X-ray Telescope to study one in detail - a galaxy called NGC 4555 which shows no large galaxies within about 400 million light-years. The galaxy is bathed in a hot, X-ray-emitting gas cloud, implying the galaxy is swaddled in a dark matter halo with 300 times the cloud's mass. <br /><br />"This is the first time anyone's done decent X-ray studies of isolated ellipticals," O'Sullivan told New
 
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