Astronomers discover a new way to “see” the elusive halos of dark matter

Artist’s impression of a galaxy surrounded by gravitational distortions due to dark matter. Galaxies live within larger concentrations of invisible dark matter (colored purple in this image), however the effects of dark matter can be seen by observing the deformations of the background galaxies. Credit: Swinburne Astronomy Productions – James Josephides

A small team of astronomers have found a new way to “see” the elusive dark matter halos surrounding galaxies, with a new technique that is 10 times more accurate than the previous best method. The work is published in Royal Astronomical Society Monthly Notices.

Scientists currently estimate that up to 85% of the mass in the universe is actually invisible. This “dark matter” cannot be observed directly, because it does not interact with light in the same way as ordinary matter that makes up the stars, planets and life on Earth.

So how do we measure what cannot be seen? The key is to measure the effect of gravity produced by dark matter.

Processed image of a spiral galaxy, as seen after lens effects distorted the galaxy’s true shape. By measuring the orbital motion of gas within a distant galaxy (pink here), gravitational distortions can be measured much more precisely than was previously possible. Credit: original image by ESA / Hubble and NASA / Flickr user Det58, image modification by Pol Gurri

Pol Gurri, a PhD student at Swinburne University of Technology who led the new research, explains: “It’s like looking at a flag to try to know how windy it is. You can’t see the wind, but the movement of the flag tells you how hard the wind blows. “

The new research focuses on an effect called a weak gravitational lens, which is a feature of Einstein’s theory of general relativity. “Dark matter slightly distorts the image of anything behind it,” says associate professor Edward Taylor, who was also involved in the research. “The effect is a bit like reading a newspaper through the base of a wine glass.”

The weak gravitational lens is already one of the most effective ways to map the dark matter content of the Universe. Now, Swinburne’s team has used the ANU 2.3m telescope in Australia to map how gravitationally lensed galaxies rotate. “Because we know how stars and gas are supposed to move inside galaxies, we know pretty much what that galaxy should be like,” Gurri says. “By measuring how distorted real images of the galaxy are, we can figure out how much dark matter it would take to explain what we see.”

Photograph of the Australian National University (ANU) 2.3m telescope at Siding Spring Observatory. Credit: Australian National University

The new research shows how this speed information allows for a much more precise measurement of the lens effect than is possible using the shape alone. “With our new way of seeing dark matter,” says Gurri, “we hope to get a clearer picture of where dark matter is and what role it plays in how galaxies are formed.”

Future space missions like NASAThe Roman Nancy Grace Space Telescope and the European Space Agency’s Euclid Space Telescope are designed, in part, to make these kinds of measurements 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, simply by thinking about the problem in a different way,” adds Taylor.

Reference: “The first precision weak lens cut measurements” by Pol Gurri, Edward N Taylor and Christopher J Fluke, 21 September 2020, Royal Astronomical Society Monthly Notices.
DOI: 10.1093 / mnras / staa2893