The late Carl Sagan got lots of praise for his brilliance in explaining science to the masses, especially on his PBS TV series Cosmos back in the 1980s. But he also got teased unmercifully for the super-geeky way he said “billions” — and given the subject matter, he said it a lot. The universe is billions of years old, and billions of light-years across, and contains a hundred billion galaxies, each containing a hundred billion stars…and Sagan got his teeth into every “billion” he could find.
Which is why he would be at something of a loss talking about a new report in The Astrophysical Journal describing the least massive galaxy every found. Known as Segue 2, it contains just 1,000 or so stars, and while this puny object, like most galaxies, is somewhat bulked up with invisible dark matter, that only adds another 100,000 stars’ worth. There’s not a billion to be found, unless you start weighing the stars and dark matter in pounds. “Finding a galaxy as tiny as Segue 2,” says co-author James Bullock, of the University of California, Irvine, “is like discovering an elephant smaller than a mouse.”
But it’s also very different. A mouse-sized elephant is the last thing you’d ever go looking for; a galaxy this tiny, by contrast, is something astronomers have long sought. The reason: it’s firmly established by now that most of the mass in the universe comes in the form of dark matter, not stars. The leading candidate for what makes up the dark matter is some sort of still-undiscovered subatomic particle. And computer models suggest that while these particles should coalesce into gigantic blobs or haloes that surround normal galaxies, they should also form into thousands of much smaller clumps, buzzing around galactic fringes.
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Those smaller clumps would have their own tiny retinue of stars as well — and in 2006, astronomers finally found an example. Called Segue 1 (it was discovered by the Sloan Extension for Galactic Understanding and Exploration, or SEGUE, an offshoot of the Sloan Digital Sky Survey) it had a mere 300 or so stars, but its dark matter component was equivalent to about 600,000 stars — small, but not as small as what astronomers were hoping to find.
Then, in 2009, the SEGUE survey snagged a true pipsqueak: Segue 2 has three times the star count as its brother, but only one-sixth the dark matter — right in the ballpark of what theory says should be there. “It’s quite encouraging,” says Kirby.
Maybe, but it’s reasonable to wonder why you’d give a tiny thing like this a grandiose name such as “galaxy.” Why not just call it a star cluster? The answer, says lead author Evan Kirby, also at Irvine: a galaxy is defined as a system of stars that can enrich its own chemical composition. What that means is that the earliest stars were made almost entirely of hydrogen and helium. Those atoms were forged into heavier elements in the stars’ cores, and eventually blown out into space at high velocity in supernova explosions. The heavier elements, known as “metals,” then became incorporated into the next generation of stars (in astronomy jargon, iron and aluminum are metals, but so are oxygen and carbon).
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In a star cluster like the Pleiades, there isn’t enough gravity to keep those metals from flying away; in a galaxy, there is — and that gravity can come from either stars or dark matter, doesn’t matter which. Kirby and his team used the powerful Keck II telescope, in Hawaii, to measure the metal content of the stars in Segue 2, and found that they vary a lot. That’s evidence that some stars are younger, some older, and that the metals have stayed put. Puny or not, Segue 2 is a galaxy. It’s also likely not to be the only one of its kind. “We think there are lots more,” says Kirby.
Unfortunately, those junior members of the galactic corps might not be easy to find. Spotting very dim collections of stars on the fringe of the Milky Way isn’t incredibly hard, but probing their light for evidence of metal composition, and measuring their orbital speeds as an indirect method of weighing their dark matter, is. The Keck II armed with an instrument called the DEIMOS spectrograph — among the world’s most powerful systems for doing this kind of work — can just manage it.
At this point, says Bullock, there’s no set of instruments, either in existence or on the drawing board, that could do a lot better. “Right now,” he says, “we’re thinking of what it would take.” Once the astronomers come up with something though, they’re betting that the little galaxies will be out there waiting.
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