NCG 720 Chandra X-ray Image
Credit: X-ray: NASA/CXC/UCI/D.Buote et al., Optical: DSS U.K.Schmidt Image/STScI

MOND Unfound

Astronomers have known for some time that the motions of galaxies usually do not follow the predictions of standard gravitational theory if all the mass is contained in bright objects like stars. Such discrepancies might suggest that the presence of large amounts of hidden, "dark" matter. But what if gravity itself is at fault? Galactic motions could be explained without "dark matter" if gravity on large distance scales behaves differently than it does on the smaller distance scales that we're more familiar with. One such theory that has had some success in matching observations is called "Modified Newtonian Dynamics" or MOND. A recent observation by the Chandra X-ray Observatory provides one of the most stringent tests of this theory. A Chandra image (above left) of the elliptical galaxy NGC 720 reveals a faint glow of X-ray emitting matter around the galaxy. What's unusual is that the shape of the X-ray emitting gas is much different than the optical image of the galaxy (shown in the image on the right). The shape of this hot gas is difficult to reconcile with MOND, and more consistent with the dark matter hypothesis.

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Page Author: Dr. Michael F. Corcoran
Last modified October 27, 2002