Scientists find evidence of galactic dark matter


MIT atronomers have found new evidence that galaxies contain large amounts of "dark matter" in addition to the well-known visible matter, most of which is in the form of stars similar to our sun.

Dark matter is the term given to matter that cannot be detected by any means other than by its gravitational effects on the visible matter that we can see. Theories suggest that dark matter may be the most common form of matter in the universe.

David A. Buote of Rancho Santa Fe, CA, a graduate student in the Department of Physics, and Professor Claude R. Canizares, director of the Center for Space Research, presented their findings June 7 to a meeting of the American Astronomical Society in Berkelely, CA.

This result is of particular importance because it infers the existence of dark matter from geometrical considerations, making the conclusions of the MIT researchers less dependent on prior assumptions than in previous measurements.

"We also believe our results contradict the possibility that this evidence for dark matter is instead signaling a breakdown of Newton's theory of gravity, as a few astronomers have been suggesting for the past 10 years," Professor Canizares said.

The evidence comes from observations of X-rays emitted by hot gas, at a temperature of 10 million degrees, which is trapped by the gravitational pull of the elliptical galaxy NGC 720 residing in the constellation Cetus, more than 65 million light-years away from Earth.

The observation was performed with the X-ray telescope on the German ROSAT satellite, through a joint NASA-German program.

The X-ray emitting gas effectively maps the shape of the gravitational field around the galaxy, which these observations show to be obviously elliptical, the MIT researchers said. "The hot gas," Mr. Buote said, "is a lot like water in an undisturbed bathtub. If the surface of the water was curved instead of flat, we would know that some force other than the gravity of the Earth must be acting on it."

The clue, according to the researchers, is that even at large distances from the center of the galaxy, as far as 33,000 light years, the X-ray picture shows a gravitational field that is elliptical, with a short axis only 3/4 the length of the long axis.

The 100 billion stars that make up the visible galaxy also follow an elliptical distribution, but it is well accepted that their shape is a consequence of their orbits, not the underlying gravitational field (by analogy, the sun's gravitational field is spherical in shape, but the orbits of all the planets lie in a single plane).

The argument for dark matter follows from the fact that most of the stars are located within about 20,000 light years of the galaxy's center. If the stars were the only kind of matter in the galaxy, the gravitational field would have to be nearly round at distances beyond 30,000 light years, the researchers said.

But the X-ray pictures show very significant ellipticity. The only way this could happen is if the galaxy contains an extended "halo" of dark matter with an absolute minimum of four times more mass than there is in stars. Its gravity would distort the X-ray gas to the observed shape. A more likely estimate for the amount of dark matter in the halo is about 10 times the amount in stars, the researchers said. The hot gas itself is only a small fraction of the mass of the galaxy.

"Our results are consistent with earlier estimates of the amounts of dark matter," Mr. Buote said. "The new thing is that we have nearly a pure geometric argument that requires no major assumptions about the nature of the hot gas. Since NGC 720 is very isolated from other large galaxies, the shape of the gas is unlikely to be distorted by environmental effects."

For the past decade, some astronomers have maintained that all existing evidence for dark matter may instead indicate a breakdown of Newton's Laws over large distances or, equivalently, when the force of gravity is very weak.

A general theory known as Modification of Newtonian Dynamics or MOND was developed largely by Professor Mordehai Milgrom of the Weizman Institute in Israel to explain away previous evidence without invoking dark matter, but at the expense of either Newton's universal law of gravitation or his law of inertia.

Einstein's theory of General Relativity already supplants Newton's gravitation, but the effects are tiny except near unusually dense objects like black holes, Professor Canizares said. Over most of the universe, Newton's law has been thought to work to extremely high accuracy, except for the skeptics like Milgrom. But a preliminary analysis shows that MOND cannot explain the flattened gravitational field seen in NGC 720-it too should predict a nearly spherical shape, Professor Canizares said. If this is confirmed by the detailed analysis, then dark matter would be the only viable explanation.

Professor Canizares noted that "most astronomers have continued to believe in Newton, but we've also harbored secret doubts now and then. I think this evidence permits us to rest a lot easier about Sir Isaac."

Another interesting finding is that the dark matter halo of NGC 720 itself has the shape of a flattened ellipsoid with one axis roughly twice the other. This can give clues about the processes that formed the galaxy, presumably billions of years ago when the universe was only a fraction of its present age, the MIT researchers say.

The flat halo is consistent with predictions from the currently popular Cold Dark Matter model of cosmology. Numerical simulations carried out on large supercomputers predict galaxy halos, which, on average, are flatter than the distribution of stars in elliptical galaxies. The new observations support those predictions, Mr. Buote said.

A version of this
article appeared in the
June 16, 1993

issue of MIT Tech Talk (Volume
37, Number
36).


Topics: Space, astronomy and planetary science

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