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Do Some Galaxies Lack Shrouds of Dark Matter?

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Science  11 Apr 2003:
Vol. 300, Issue 5617, pp. 233
DOI: 10.1126/science.300.5617.233b

In a challenge to the idea that all galaxies contain far more mass than meets the eye, a novel survey has turned up three galaxies that seem barren of cocoons of dark matter. “This is surprising, and we're a little puzzled about it,” says astronomer Aaron Romanowsky of the University of Nottingham, U.K. But other researchers say it will take stronger evidence to change their minds about how massive galaxies form.

For decades, astronomers have gauged the heft of galaxies by examining how fast they spin. In spiral galaxies like our Milky Way, gas clouds far from the galactic center orbit at the same rapid pace as those in the inner sections. That points to a strong gravitational pull in the outskirts—far stronger than stars and gas alone can produce. Astronomers explained the motions by invoking massive shrouds of dark matter, containing perhaps 10 times more mass than we can see.

That technique fails for the giant, featureless blobs of stars called elliptical galaxies. Ellipticals have little gas, so astronomers must try to track the motions of their stars. That's hard to do when starlight grows faint in a galaxy's outskirts. An alternate method, as Romanowsky reported this week at the U.K./Ireland National Astronomy Meeting in Dublin, Ireland, relies on locating planetary nebulae. These puffs of gas, which middleweight stars like our sun eject at the ends of their lives, are full of excited oxygen atoms that make them shine like beacons at a single green wavelength. By measuring the motions of the nebulae, astronomers can trace the overall distributions of mass far from the galaxies' centers.

Slowpokes.

Slow-moving planetary nebulae (smaller dots) on the outskirts of M105 suggest the galaxy has no dark matter.

CREDIT: 2MASS/UMASS/IPAC-CALTECH/NASA/NSF

Using a specialized spectrograph at the 4.2-meter William Herschel Telescope in La Palma, Canary Islands, Romanowsky and colleagues have discovered three galaxies that contain scores of surprisingly slow-moving nebulae in their remote outer regions. That dawdling pace suggests that no hidden mass tugs the nebulae along. “There's nowhere near as much dark matter as one would expect, and the motions are consistent with no dark matter at all,” says team member Michael Merrifield, also at the University of Nottingham.

Other scientists are intrigued but skeptical. Only ironclad data will make astrophysicists believe that ellipticals are naked, says Joshua Barnes of the University of Hawaii, Manoa. Many planetary nebulae, he notes, may swoop through space on highly elongated orbits that only make it appear as if their galaxies lack dark-matter shrouds.

Still, astronomers are taking note—especially those who control access to the Canary Islands telescopes. “They were sufficiently shocked and horrified by what we were finding,” says Merrifield, that they gave his group's proposal the highest ranking for more observing time. The team expects to study 22 more ellipticals within 2 years.

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