The invisible dark matter at the centre of galaxies is not as dense as we would expect - and now we know why. It seems that the explosions of exhausted old stars are to blame for puffing out dark matter. These supernovae can also account for a mysterious shortfall in the universe's quota of dwarf galaxies.
More than 80 per cent of the matter in the cosmos is in some strange undetected form. We can see the gravitational effect of this dark matter, but cannot identify it. Computer simulations have reconstructed how dark matter clumped together with normal gas in the early universe to form small galaxies and how these small galaxies merged over billions of years to create huge star systems like the Milky Way.
The simulations raised one mystery, however. They showed that the density of dark matter should spike sharply at the centre of galaxies, while observations of the motions of stars reveal that the dark matter cores of galaxies today are much more puffed out, with densities that are constant over thousands of light years. "We have known about this problem for more than 10 years," says Sergey Mashchenko from McMaster University in Hamilton, Ontario, Canada.
Astronomers have suggested several possible fixes for this anomaly. For instance, they thought that dark matter might puff outwards because some kind of exotic force between the particles - other than gravity - makes them collide and scatter off each other like snooker balls. Now Mashchenko and his colleagues have shown that the smoothing of the density of dark matter is down to the explosions of massive stars at the end of their lives. At their peak, these supernovae can outshine their host galaxies.
Mashchenko thought that shock waves from supernovae should churn up interstellar gas in a galaxy, and that the gravitational disturbances created by this sloshing gas should in turn smooth out the spike of dark matter at the centre. To test this, his team used a supercomputer to simulate the evolution of a small, primordial galaxy that started off with a central spike in the density of dark matter. Sure enough, just 80 supernovae explosions per million years - typical of values expected in dwarf galaxies today - were enough to smooth out the dark matter spike to match observations if they continued for at least 100 million years or so (Nature, vol 443, p 539).
The same model can also solve another conundrum. The cosmos has far fewer dwarf galaxies - which contain several billion stars compared with the hundreds of billions in larger galaxies - than cosmological simulations predict. In Mashchenko's simulations, because supernovae smoothed out the spike in dark matter density at the centre, dwarf galaxies were less tightly bound together by their own gravity. Any encounters with bigger galaxies would therefore have easily torn many dwarfs apart, and this may explain their paucity.
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