The Mystery of Dark-Matter Galaxies Orbiting the Milky Way

070214_darkGalaxy_hmed5p.grid-6x2 People used to think that the heavens were a vast clockwork, with planets and moons moving in circular orbits like a vast timepiece.  Recent advances have shown us that this stellar machinery is far vaster than they ever suspected – even our galaxy has satellites, mini-galaxies orbiting the Milky Way and some of them could be very interesting indeed.

Although dark matter doesn't emit or absorb light, scientists can measure its gravitational effect on ordinary matter and believe it makes up about 85 percent of the total mass in the universe. Finding ultra-faint galaxies like Segue 1, which is so rife with dark matter, provides clues as to how galaxies form and evolve, especially at the smallest scales.


But despite its small number of visible stars, Segue 1 is nearly a thousand times more massive than it appears, meaning most of its mass must come from dark matter, according to the researchers.

"I'm excited about this object," said Marla Geha, an assistant professor of astronomy at Yale and the paper's lead author. "Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass."

Geha, along with her colleague Josh Simon at the California Institute of Technology, has observed about half of the dwarf satellite galaxies that orbit the Milky Way. These objects are so faint and contain so few stars that at first they were thought to be globular clusters – tightly bound star clusters that also orbit our host galaxy. But by analyzing the light coming from the objects using the Keck telescope in Hawaii, Geha and Simon showed that these objects are actually galaxies themselves, albeit very dim ones.

Looking only at the light emitted by these ultra-faint galaxies, Geha and her colleagues expected them to have correspondingly low masses. Instead, they discovered that they are between 100 and 1000 times more massive than they appear. Invisible dark matter, she said, must account for the difference.

We now know about two dozen of these satellite galaxies.  One of the most recent is "Segue 1", uncovered by the Sloan Digital Sky Survey (SDSS), whose extremely low light-to-mass ratio makes it a particularly significant cosmic find.  Despite having a mass of a million suns it is nowhere near as luminous as astronomers would expect, with only a couple of hundred stars visible.  They think "How can so much matter be so dark?", then they go "Dark matter!" and at this point we like to believe their monocle flies out and they dash down the street shouting Eureka.

Of course, the actual physics of arriving at this conclusion is a tiny bit more complicated, but the result is the same: Marla Geha (Yale professor of astronomy) and colleagues believe that it's a galaxy composed mainly of dark matter.  A handy thing to have around when you're trying to study the stuff or even prove that it exists.

The SDSS has made many such observations possible, picking out objects in the sky which have the bad manners not to twinkle twinkle bright enough, despite being little stars.  These mini-galaxies and other previously unobservable objects offer a wealth of data on galaxy formation, evolution, and even the composition of the universe itself.

Casey Kazan with Luke McKinney via Yale University

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