Object: Dwarf galaxy Segue 2
Brightness: Equal to 900 suns
Segue 2 didn't want to make waves. The dwarf galaxy was content to hide on the outskirts of the Milky Way, mostly being ignored by a universe filled with bigger, mightier objects. Then along came those pesky humans, who dragged Seque 2 into the limelight and crowned it the smallest galaxy known. Now astronomers astonished by its existence just won't leave the poor little runt alone…
Segue 2 was discovered in 2009 during an extension of the
Sloan Digital Sky Survey, an ambitious project to map more than a quarter of the sky in three dimensions. The survey has turned up several galaxies so small and faint that they challenge the definition of what counts as a galaxy at all.
The runt even of this litter, Segue 2, holds only a few thousand stars, which are bound by the smallest dark matter halo ever known. The whole caboodle weighs in at about 150,000 times the mass of our sun.
"Generally speaking, this is one of the tiniest bound systems of stars we know about," says Evan Kirby at the University of California, Irvine. "It's also interesting because it shouldn't exist."
Heavy hitters
It was once possible to confuse faint dwarf galaxies like Segue 2 with globular clusters – tightly bound clumps of stars that are also known to orbit larger galaxies like the Milky Way. To make a clear distinction between galaxies and globular clusters, astronomers decided that true galaxies, no matter how small, must be massive enough to hold on to heavy elements.
Elements heavier than iron are
forged when the cores of massive stars collapse and trigger supernova explosions. Material from a supernova will be thrown out of a globular cluster, but a galaxy should be massive enough for its gravity to hold on to those heavy elements. New stars that form out of this debris will then contain heavier elements too, while stars born in globular clusters will not be similarly enriched. So if a collection of stars boasts a diversity of heavy elements, as revealed by their starlight, then it is a galaxy, says Kirby.
He and colleagues observed 25 stars in Segue 2 with the Keck telescope in Hawaii. They found that, while it may be diminutive, Segue 2 has the chemical composition of a galaxy that has hosted several supernovae and kept hold of the debris.
Malevolent Milky Way
The problem is that if it was born this small, such chemical enrichment should be impossible. The tiny dwarf is just not massive enough to have held on to its heavy elements, says Kirby. In fact, current theories of star formation suggest that the galaxy was not massive enough to make new stars in the first place.
The more likely scenario, he says, is that Segue 2 is the core of a galaxy 100 times more massive that has been
broken down by repeated interactions with the Milky Way.
"I think of this as the schoolyard kid that had his lunch money taken away from him every day – the little runt that the bigger galaxy is beating up on," says Kirby. "We're just looking at the barest little nub of a galaxy that has been harassed to extinction by the Milky Way. There's almost nothing left of it."
Lost and found?
That sad tale fits with previous observations of dwarf galaxies orbiting the Milky Way and
having their matter stripped away. But Segue 2 would be an extreme case. It would have had to have lost 99.7 per cent of its stars and more than 99.9 per cent of its dark matter to appear as it does today.
This extreme scenario still doesn't explain why Segue 2 is so physically small. Other galaxies that have been stripped by their neighbours seem to maintain their original size, often with the addition of a long, tell-tale tail of material that signals matter was being pulled away.
The other possibility is that Segue 2 really was born in an unusually small dark matter halo. That would mean a rethink of how galaxies enrich their stars with heavy metals. But it could help solve the mystery of the
Milky Way's missing satellites. Simulations suggest that the Milky Way should have thousands of tiny galaxies buzzing around it, but only 27 have been observed. If Segue 2 was born small, maybe other tiny galaxies were born that way too, and they are so faint we've had trouble spotting them.
"If when the galaxy formed it had a very low mass, that would tell us that even these tiniest dark matter haloes are able to form some stars, at least in some cases," says Josh Simon of the Carnegie Observatory in Pasadena, California, who was not involved in the new research. "Since this is only one object, I wouldn't say it would help solve the entire problem, but it would point us in the direction of the solution."
The more sensitive eyes of the next generation of telescopes should help solve the puzzle. It is too faint to tell for now, but if Segue 2 has a tail of stars streaming away from it into the Milky Way, that's strong proof for the tidal stripping scenario, says Kirby.
