The dark matter in small galaxies has been giving cosmologists a big headache. Now it turns out that blowing up a few stars may provide the solution.
Dark matter is thought to make up most of the matter in the universe and should be found mixed in with the regular matter of galaxies. The standard model of cosmology, called lambda-CDM, predicts that the density of this dark matter should increase as you move towards the centre of a galaxy, but we’ve seen certain dwarf galaxies that didn’t get the memo: they have a constant density of dark matter.
That might seem a minor wrinkle, but if it can’t be explained within lambda-CDM, it could deal a major blow to our understanding of dark matter.
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Galactic disagreement
“It’s a very subtle measurement, and the implications of these very subtle measurements are large so people have argued a lot over them,” says Marla Geha at Yale University in Connecticut. “If you had dark matter vision and you looked at the centre of these galaxies, you couldn’t tell the difference.”
One potential explanation is that bursts of star formation in small galaxies might blow some dark matter towards the edges, smoothing out the distribution. New stars are born when old ones die by exploding in a supernova, sending matter flying.
Justin Read at the University of Surrey in the UK and his colleagues used measurements of the mass at the centres of dwarf galaxies to test this idea.
They picked out samples of two different types of galaxy, eight that stopped forming stars long ago, and eight that are still forming stars or only stopped relatively recently. They calculated the mass in the central 500 light years of these galaxies by measuring how fast gas and stars orbit the centre, and matched that mass to the dark matter distribution using a computer model.
They found that the galaxies with more star formation had less dark matter in their centres, pointing to a more uniform distribution. That matches up with the idea that star formation will move mass towards the edges of the galaxy, which then pulls the dark matter outward. It’s a good sign for lambda-CDM.
But, partially because these dwarf galaxies are so dim and difficult to see and the measurements are so subtle, it’s not quite set in stone.
For example, it may be that galaxies that formed earlier and thus have less star formation now simply have more dark matter because of the circumstances of their formation. “It is hard to take all the physics into account, especially when we don’t understand every detail” says John Kormendy at the University of Texas.
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