Supermassive black holes may be lurking at the centre of most ultra-compact dwarf galaxies, according to a new study which potentially doubles the number of black holes known in the universe. Scientists have discovered supermassive black holes in two ultra-compact dwarf galaxies. Earlier, the researchers had discovered that an ultra- compact dwarf galaxy contained a supermassive black hole, then the smallest known galaxy to harbour such a giant black hole. Together, the three examples suggest that black holes lurk at the centre of most of these objects, potentially doubling the number of supermassive black holes known in the universe. The black holes make up a high percentage of the compact galaxies' total mass, supporting the theory that the dwarfs are remnants of massive galaxies that were ripped apart by larger galaxies. "We still don't fully understand how galaxies form and evolve over time.
These objects can tell us how galaxies merge and collide," said Chris Ahn, doctoral candidate at University of Utah in the US. "Maybe a fraction of the centres of all galaxies are actually these compact galaxies stripped of their outer parts," said Ahn. The researchers measured two ultra-compact dwarf galaxies, named VUCD3 and M59cO, that lie far beyond the spiral arms of our Milky Way, orbiting massive galaxies in the Virgo galaxy cluster. They detected a supermassive black hole in both galaxies; VUCD3's black hole has a mass equivalent to 4.4 million suns, making up about 13 per cent of the galaxy's total mass, and M59cO's black hole has a mass of 5.8 million suns, making up about 18 per cent of its total mass. By comparison, the monstrous black hole at the centre of the Milky Way has a mass of 4 million suns, but makes up less than 0.01 per cent of the galaxy's total mass. "These ultra-compact dwarfs are around 0.1 per cent the size of the Milky Way, yet they host supermassive black holes that are bigger than the black hole at the centre of our own galaxy," said Ahn. To calculate the ultra-compact dwarf galaxies' mass, the astronomers measured the movement of the stars using the Gemini North telescope located on Mauna Kea volcano in Hawaii. They found that the motion of the stars at the centre of the galaxies moved much faster than those on the outside, a classic signature of a black hole. VUCD3 and M59cO are the second and third ultra-compact dwarf galaxies found to contain a supermassive black hole, suggesting that all such dwarfs may harbour similarly massive light-sucking objects. Astronomers discovered ultra-compact dwarf galaxies in the late 1990s. The objects are made up of hundreds of millions of stars densely packed together on an average of 100 light years across.
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