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Dark Matter Halos Look a Bit Like a Football

Dark matter is distributed throughout the universe in giant, football-shaped clumps, according to new, indirect images of the mysterious substance that holds galaxies and galaxy clusters together. The work provides more evidence that dark matter, even on the largest scale, strongly affects the visible cosmos with its gravity and remains virtually unaffected in return.

For decades, astronomers have known that galaxies don't seem to contain enough mass, and therefore gravity, to prevent their constituent stars from flying off into space. Ditto the vast galactic clouds of gas and dust that are necessary to form new stars and solar systems. Even the supermassive black holes were found to be quite puny, in terms of gravitationally binding a galaxy together. Then in the 1970s, astronomer Vera Rubin and colleagues published papers that posited evidence of something called dark matter, which seemed to permeate galaxies and explain why Newton's laws of motion concerning the orbits of stars seemed to be breaking down. Dark matter, it turned out, was allowing stars like our sun, which are located far from galactic centers, to zip along in their orbits much faster than Newtonian calculations permitted.

But no one has actually seen direct evidence of the particles that make up dark matter. Instead, scientists must probe its nature based on its gravitational effects. That's what astronomers led by Masamune Oguri of the National Astronomical Observatory of Japan have done, using a technique called gravitational lensing. First predicted by Einstein, gravitational lensing relies on the tendency of very massive objects to bend beams of light—in this case the light from very distant galaxies—that pass within their sphere of gravitational attraction.

Oguri and colleagues employed the 8.2-meter Subaru Telescope in Hawaii to gauge the lensing effect of dark matter on 18 galactic clusters located in various parts of the sky on average about 3 billion light-years away. As the team reports in an upcoming issue of Monthly Notices of the Royal Astronomical Society, all 18 clusters, each containing thousands of galaxies, host giant clumps of dark matter. And all the clumps, called halos, are flattened a bit like footballs, with their horizontal axes in most cases about twice as long as the vertical—regardless of the shape and distribution of their constituent galaxy clusters.

That's a significant point, Oguri says. It means that dark matter conforms to models that trace its large-scale structure back to the physics of the big bang. The models predict the elongated shape and a lack of interaction with visible matter. If dark matter was affected, either gravitationally or otherwise, by ordinary matter, then the shape of the halos would vary depending on the distribution of galaxies.

The paper's results "come from a good analysis of some especially nice data and so are especially valuable," says astronomer Rachel Mandelbaum of Princeton University. It's a significant paper and an improvement over previous work, which could not distinguish the dark matter halos of individual clusters, adds astronomer Ian Dell'Antonio of Brown University. He cautions that the book may not be closed on how the mass of galaxy clusters can affect dark matter. Detecting such influence will require a great deal more precise measurements than can be made at present, he says.