By Ron Cowen
Darth Vader and other rulers of the dark side have reason to celebrate. Observations confirm that a faint group of stars in the Milky Way’s backyard has the highest density of dark matter — the invisible material thought to account for 83 percent of the mass of the universe — of any galaxy known.
The findings, reported online July 28 at arXiv.org by Joshua Simon of the Carnegie Observatories in Pasadena, Calif., along with Marla Geha of Yale University and their colleagues, provide a bonanza for astronomers trying to unveil the nature of dark matter.
When astronomers discovered the galaxy Segue 1 in 2007, they weren’t sure if it was anything more than a cluster of stars, perhaps stripped from the nearby Sagittarius dwarf galaxy. But observations with the Keck II telescope atop Hawaii’s Mauna Kea now confirm the status of Segue 1 as a galaxy by showing that its stars have a diverse chemical composition, Simon says.
Observations by the Anglo-Australian Telescope near Coonabarabran, Australia, have also found a diversity of stellar composition in Segue 1, a team including Rosemary Wyse of Johns Hopkins University in Baltimore, Md., report in an article scheduled to be posted at arXiv.org in early August.
After examining the stars’ compositions, Simon’s team calculated the total amount of mass in Segue 1 — both the unseen dark matter and the small number of faint, visible stars — by measuring how fast the stars move. The faster the stars orbit about the center of Segue 1, the heavier the galaxy.
The team found that although the stars in Segue 1 have a combined mass of no more than about 1,000 suns, the mass of the whole galaxy is about 500 times larger. “That tells us that Segue 1 is made almost entirely of dark matter,” Simon says.
Segue 1 is both dark matter–dominated and compact, yielding a dark matter density higher than any known galaxy. The galaxy’s high density and proximity to Earth — about 80,000 light-years distant — make it an ideal place to look for proposed signatures of dark matter.
“It’s extremely important to figure out the dark matter properties of galaxies,” says Wyse. Galaxies such as Segue 1, which have such a tiny amount of visible material to gravitationally disturb the dark matter, are the best places to reveal the true distribution and nature of the unseen material.
In addition, Wyse says, the primitive chemical composition of several of the stars in Segue 1 may shed light on the formation and evolution of some of the oldest stars in the universe.
Just as particles of matter and antimatter can annihilate each other upon contact to produce large numbers of gamma rays, so may particles of dark matter annihilate one another, depending on exactly what the unseen material is made of. The higher the density of dark matter, the higher the rate of these annihilations, theory suggests.
The Fermi Gamma-ray Space Telescope, as well as ground-based telescopes that record energetic radiation that could be due to annihilation, should therefore make the galaxy a prime target, says Simon. “A detection of dark matter annihilation would be a breakthrough for both astronomy and particle physics, and the first step is figuring out where to look for it,” he says.
