'Perfect' Spiral Galaxy May Harbour Dark Secret

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<b>'Perfect' Spiral Galaxy May Harbour Dark Secret</b><br /><br />LINK<br /><br />A picture-perfect spiral galaxy may harbour two colossal black holes instead of the usual one, new observations suggest. But if it does, astronomers are mystified as to how the second black hole got there without ruining the galaxy's spiral structure.<br /><br />Most galaxies are thought to contain a single large black hole with a mass proportional to that of its galaxy. But galaxies grow through collisions, and astronomers believe their black holes also merge over time. <br /><br />Two galaxies appear to contain pairs of black holes. These galaxies emit telltale X-rays when superhot matter falls towards the massive objects. <br /><br />But now a trio of astronomers, led by Damián Mast of Argentina's National University of Cordoba, says there may be two black holes inside the galaxy M83, which lies about 15 million light years away - even though it does not emit these X-rays.<br /><br />Such a possibility was first raised in 2000 by another team in Germany. That team studied the motion of the galaxy's stars along the line of sight between Earth and M83, discovering two concentrations of mass, or "nuclei". One shone brightly while the other - which lay closer to the apparent centre of the galaxy - was dim. The team suggested that gas and dust were blocking light from the second object, which they thought might be the galaxy's true centre - around which gas and stars circle.<br /><br />Millions of Suns<br />Now, Mast’s team has studied the motion of ionised gas near M83's centre. They used the data to better locate the two knots of matter, which appear to lie about 200 light years from one another. <br /><br />They also estimated the knots' masses, with the bright one containing the equivalent of five million Suns and the dark one 10 million Suns. By comparison, the Milky Way's central black hole has the mass of about four milli