Rotation may solve cosmic mystery

Researchers propose a new explanation for why some tiny galaxies have more than their fair share of dark matter

Literally cloaked in darkness, the faintest galaxies in the universe have remained a mystery since their discovery more than two decades ago. Now a team of theorists has come up with a new explanation for the origin of these dim bodies. Known as dwarf spheroidal galaxies, these ancient stellar groupings not only serve as fossil remains of the early universe but have the highest known ratio of dark matter to ordinary, visible matter.

DARK GALAXY This false-color image shows stars in the dwarf spheroidal galaxy Leo II, just 760,000 light-years from the Milky Way galaxy. The dwarf contains only one-twenty-thousandth the amount of visible material in the Milky Way. copyright 2007 National Astronomical Observatory of Japan

In the most widely accepted model of galaxy formation, an exotic type of invisible material, known as cold dark matter, provides the gravitational glue that draws together stars and gas and keeps galaxies, along with galaxy clusters, from flying apart. It would seem that all galaxies ought to have about the same ratio of dark matter to visible matter, because gravity builds all galaxies in the same way. Yet dwarf spheroidals are the most dark matter–dominated galaxies known, with 10 to 30 times the ratio of dark to visible matter as large galaxies including the Milky Way.

That’s the puzzle that Elena D’Onghia of the University of Zurich and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her colleagues set out to solve in a study posted online July 16 ( and in an upcoming Nature.

Like other researchers, she and her collaborators assume that dwarf spheroidals were born with a lower, more typical ratio of dark to visible matter, but that much of the visible stuff somehow got pulled out.

Previous models suggest a complex, two-step process to explain the high ratio. But these models require a dwarf spheroidal to lie close to a galaxy as large as the Milky Way. In reality, some spheroidals lie far from such galaxies. Also, these models don’t easily explain the spherical shape of these galaxies or the diversity of their dark matter ratios.

In contrast, the new model proposed by D’Onghia’s team relies on the assumption that stars and gas rotate in fledgling galaxies, a property which the underlying dark matter model of formation requires.

If the rotation and orbit of stars in a dwarf spheroidal are in sync with the rotation of a slightly larger, nearby galaxy — possibly even just another dwarf spheroidal — the gravitational influences of the two galaxies on each other are enhanced, D’Onghia says.

Within 2 to 3 billion years, the gravitational pull would remove many stars from the lower-mass dwarf, D’Onghia says. Because dark matter does not rotate, it would be left behind in the dwarf galaxy. Depending on how closely the rotation of stars and gas aligns in neighboring galaxies, the dwarf spheroidals would end up with varying, but always high, ratios of dark to visible matter.

The proposed interaction could account for dwarf spheroidals, such as the recently discovered galactic duo Leo IV and Leo V, that don’t reside close to a large galaxy like the Milky Way, D’Onghia asserts.

“Certainly this is an idea that needs to be taken very seriously,” comments theorist James Bullock of the University of California, Irvine. “I bet some of the [dwarf spheroidals] formed this way, but I’m not sure if the numbers work out to explain all of them,” he adds.

D’Onghia and her collaborators simulate only the interaction of stars, not gas, cautions Rosemary Wyse of Johns Hopkins University in Baltimore, Md. But D’Onghia says that the rotating gas in a dwarf spheroidal, although more difficult to model than the stars, ought to be removed in a similar manner.

Jorge Pe±arrubia of the University of Cambridge in England takes a contrarian view. “In my opinion, the whole problem may be a theoretical misconception,” due to uncertainties about star formation in galaxies, he says. Although dark matter models require that stars form in rotating disks, star-forming regions in the Milky Way indicate that most stars form in clusters instead. If stars in dwarf spheroidals don’t form in rotating disks, the scenario proposed by D’Onghia and her collaborators wouldn’t provide an explanation, he says. 

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