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A Fossil Galaxy Packed with Dark Matter Is a Window to the Ancient Universe

A new study finds that Segue 1 is “the least chemically evolved galaxy ever known.” Which is a nice way of saying it's super old.
The Keck I telescope at night. Image: Keck/JPL

Just a month ago, Australian scientists introduced the oldest star on the books, which is barely younger than the universe itself. But a new study led by MIT professor Anna Frebel has already one-upped that discovery. Published in The Astrophysical Journal, the MIT paper found that the ancient galaxy known as Segue 1 may actually be “a surviving first galaxy that experienced only one burst of star formation.” It turns out the sky is full of senior stars, if you know how to look for them.

The MIT team definitely knew how to look for them, though studying Segue 1 presents its fair share of challenges. The whole galaxy has a luminosity less than a single red giant, plus it's over 75,000 light years from Earth. All that adds up to making it the darkest galaxy ever found. Needless to say, some special equipment was required.

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The team collected its data from two world-class echelle spectrographs—the Magellan/MIKE in Chile's Las Campanas Observatory and the Keck/HIRES in Mauna Kea, Hawaii. Using these top-notch facilities, Frebel and her colleagues were able to obtain high resolution spectra from six red giant stars in the galaxy.

The results revealed that these stars contain virtually no heavy metals, which suggests that the galaxy is remarkably old. “Segue 1 is so ridiculously metal-poor that we suspect at least a couple of the stars are direct descendants of the first stars ever to blow up in the universe,” co-author Evan Kirby told [Scientific American](Segue 1 is so ridiculously metal-poor that we suspect at least a couple of the stars are direct descendants of the first stars ever to blow up in the universe,). Whoa. Those stars have seen some shit.

Similar to the star found by the Australian team earlier this year, the red giants analyzed by Frebel's team are particularly hard-up on iron. You don't see this iron deficiency in younger stars, because stellar neighborhoods become cumulatively enriched it as they go through successive star formation cycles. A fifth or sixth generation star will have collected all the heavy elements forged by its progenitor stars, and will bequeath its own heirloom metals to the next generation when it dies. This cyclical inheritance of new metals helps astronomers figure out the heritage of stars.

In this case, it seems clear that the red giants of Segue 1 were born from the ashes of gargantuan, short-lived parents. When high-mass stars explode, they eject loads of magnesium, carbon, and calcium, while lower mass stars cough up iron. The relative abundance of the former metals and the absence of iron suggests that no low-mass stars contributed to the formation of the galaxy's current roster of giants.

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A supernova spewing its heavy element guts everywhere. Image via NASA/ESA/JHU/R.Sanskrit & W.Blair.

What's more, the galaxy seems to have oddly halted its own development, perhaps after only one cycle of star formation. That's why it's referred to as a fossil galaxy. It's just out there, orbiting the Milky Way with no discernible evolution occurring at all. It only has a few hundred stars bouncing around its gravitational web, which is small even for a dwarf galaxy.

Why did it suddenly stop in its star-making tracks? The top theory is that the deaths of the first stellar batch re-ionized the surrounding area, making a new generation impossible. But that answer is far from conclusive.

“Segue 1 is the only example that we know of now that was never enriched by these low-mass stars, meaning it formed stars really quickly, in the blink of an eye,” Kirby told Sci Am. “If it had formed stars long enough those low-mass stars would have to contribute.”

If you can believe it, being a fossilized satellite of the Milky Way isn't even the most interesting thing about Segue 1. The galaxy is also Dark Matter Central. A 2010 study discovered that this miniature cluster has “the highest measured dark matter density of any known galaxy.” So not only can Segue 1 help us to understand early galactic evolution (and lack thereof), it's also a "prime testing ground” in the burgeoning field of dark matter physics.