Astronomers in Australia have discovered a new way to see the elusive dark matter haloes that surround galaxies.
The new technique, created by researchers at the Swinburne University of Technology in Melbourne, draws on the effect called weak gravitational lensing – a feature of Einstein’s general theory of relativity.
Scientists estimate that up to 85 per cent of the mass in the universe is effectively invisible. This dark matter cannot be observed directly because it does not interact with light in the same way as the ordinary matter that makes up stars, planets, and life on Earth. So to measure what cannot be seen, the scientists analysed the dark matter’s effect on gravity.
“It’s like looking at a flag to try to know how much wind there is,” said Pol Gurri, a PhD student at the university. “You cannot see the wind but the flag’s motion tells you how strongly the wind is blowing.”
Looking at the effect of weak gravitational lensing as the main part of the research, associate professor Edward Taylor, said: “The dark matter will very slightly distort the image of anything behind it. The effect is a bit like reading a newspaper through the base of a wine glass.”
While weak gravitational lensing is already one of the most successful ways to map the dark matter content of the universe, the Swinburne team has now used the ANU 2.3m Telescope in Australia to map how gravitationally lensed galaxies are rotating.
“Because we know how stars and gas are supposed to move inside galaxies, we know roughly what that galaxy should look like,” Gurri explained. “By measuring how distorted the real galaxy images are, then we can figure out how much dark matter it would take to explain what we see.”
The new research shows how this velocity information enables a much more precise measurement of the lensing effect than is possible using shape alone.
The researchers hope their new way of seeing dark matter will help to create a clearer picture of what role it plays in the formation of galaxies. “We have shown that we can make a real contribution to these global efforts with a relatively small telescope built in the 1980s, just by thinking about the problem in a different way,” Taylor said.