Chandra Image of NGC 4555
Credit: NASA/CXC/SAO/E. O'Sullivan & T. Ponman, Monthly Notices of the Royal Astronomical Society, Vol. 354, p. 935

An Isolated Case of Dark Matter

One of the biggest mysteries about the Universe is what, exactly, is it made of? Most of the material in the Universe seems to be invisible, something astronomers call "dark matter". Dark matter is defined by its gravitational attraction on surrounding luminous material like stars. A picture obtained by the Chandra X-ray Observatory shows the presence of dark matter in an unusual context. The image above right shows an optical image of the elliptical galaxy NGC 4555, while on the left is a Chandra image of the galaxy. The Chandra image shows a large mass of glowing X-ray gas which is somehow bound to the galaxy, even though the luminous matter doesn't possess enough gravitational force to hold onto the gas. Astronomers believe that dark matter in the galaxy provides extra gravity to hold the gas bound; this is unusual, since most X-ray halos are found around large groups of galaxies, while NGC 4555 is isolated in space. Where did all the dark matter come from?

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Page Author: Dr. Michael F. Corcoran
Last modified Friday, 20-Apr-2012 15:23:12 EDT