Nov 6 2020
A new technique developed by a small group of astronomers offers a new means to “see” the elusive dark matter haloes surrounding galaxies.
Artist’s impression of a galaxy surrounded by gravitational distortions due to dark matter. Galaxies live inside larger concentrations of invisible dark matter (colored purple in this image); however, the dark matter’s effects can be seen by looking at the deformations of background galaxies. Image Credit: Swinburne Astronomy Productions – James Josephides.
The technique, which is 10 times more accurate compared to earlier optimal methods, has been described in Monthly Notices of the Royal Astronomical Society.
At present, researchers predict that nearly 85% of the mass in the universe is practically invisible. It is impossible to observe this “dark matter” directly since it does not interact with light similar to the ordinary matter that constitutes planets, stars, and life on Earth.
So what is the way to quantify that which is unseen? The essential aspect is to measure the effect of gravity generated by the dark matter.
According to Pol Gurri, a Ph.D. student from the Swinburne University of Technology, who led the new study: “It’s like looking at a flag to try to know how much wind there is. You cannot see the wind, but the flag’s motion tells you how strongly the wind is blowing.”
The focus of the new study is on an effect known as weak gravitational lensing—a feature of Einstein’s general theory of relativity.
“The dark matter will very slightly distort the image of anything behind it,” noted Associate Professor Edward Taylor, who also contributed to the study. “The effect is a bit like reading a newspaper through the base of a wine glass.”
Weak gravitational lensing has already proved to be one of the most effective ways to map the dark matter content of the Universe. The Swinburne group has now used the ANU 2.3m Telescope in Australia to map the way gravitationally lensed galaxies rotate.
Because we know how stars and gas are supposed to move inside galaxies, we know roughly what that galaxy should look like. 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.
Pol Gurri, PhD Student, Swinburne University of Technology
The new study demonstrates how the velocity information can be used to achieve a considerably more accurate measurement of the lensing effect than using just the shape.
With our new way of seeing the dark matter,” Gurri says, “we hope to get a clearer picture of where the dark matter is, and what role it plays in how galaxies form.
Pol Gurri, PhD Student, Swinburne University of Technology
Forthcoming space missions like the European Space Agency’s Euclid Space Telescope and NASA’s Nancy Grace Roman Space Telescope have been engineered, in part, to perform measurements of this type based on the shapes of hundreds of millions 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.
Edward Taylor, Associate Professor, Swinburne University of Technology
Journal Reference:
Gurri, P., et al. (2020) The first shear measurements from precision weak lensing. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/staa2893.