BESIDES worrying about dark matter, the unseen mass thought to make up nearly 90 per cent of most galaxies, astronomers have also been unable to find nearly a quarter of the normal mass of galaxies, made of stars and gas, which can be estimated from studying the radiation left over from the big bang.
Now baby galaxies born from the collisions of bigger ones have revealed that this missing mass could exist as cold gas. Frederic Bournaud of France鈥檚 Atomic Energy Commission in Paris and his team base their conclusion on so-called tidal dwarf galaxies near the galaxy NGC 5291, about 200 million light years from Earth.
The team used radio telescopes to measure the rotation speeds of these tidal dwarfs. The faster a galaxy spins, the more gravity, and hence more mass, it must have to avoid flying apart. The team found that the galaxies are about three times as heavy as can be accounted for by adding up the mass of their visible stars and gas.
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Because these dwarfs are formed from matter sprayed out in collisions, they should in theory contain very little dark matter, because dark matter does not clump back together after it has been disturbed in a collision. The researchers think that the excess mass is mostly extremely cold hydrogen gas (Science, DOI: 10.1126/science.1142114).
Since this gas came from the bigger galaxies, it suggests that all galaxies contain cold gas. 鈥淭he disc of our galaxy might be two or three times more massive then we think it is,鈥� says Elias Brinks of the University of Hertfordshire, UK.
