Seeing dark matter in a new light

A galaxy surrounded by a coloured haze, with lenses due to gravitational distortion
Artist's impression of a galaxy surrounded by gravitational distortions due to dark matter. Galaxies live inside larger concentrations of invisible dark matter (coloured purple in this image), however the dark matter's effects can be seen by looking at the deformations of background galaxies.
Credit
Swinburne Astronomy Productions - James Josephides

A small team of astronomers have found a new way to ‘see’ the elusive dark matter haloes that surround galaxies, with a new technique 10 times more precise than the previous-best method. The work is published in Monthly Notices of the Royal Astronomical Society.

Scientists currently estimate that up to 85% 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 how do we measure what cannot be seen? The key is to measure the effect of gravity that the dark matter produces.

Pol Gurri, the 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 much wind there is. You cannot see the wind, but the flag’s motion tells you how strongly the wind is blowing.”

The new research focuses on an effect called weak gravitational lensing, which is a feature of Einstein’s general theory of relativity. “The dark matter will very slightly distort 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.”

Weak gravitational lensing is already one of the most successful ways to map the dark matter content of the Universe. Now, the Swinburne team has 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,” says Gurri. “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. “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.”

Future space missions such as NASA’s Nancy Grace Roman 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, just by thinking about the problem in a different way,” adds Taylor.

 

Media contacts

Dr Morgan Hollis
Royal Astronomical Society
Mob: +44 (0)7802 877 700
press@ras.ac.uk

Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7292 3979
Mob: +44 (0)7802 877 699
press@ras.ac.uk

 

Science contacts

Mr Pol Gurri
Swinburne University of Technology
Centre for Astrophysics and Supercomputing
Australia
Tel: +61 466 289 019
pgurriperez@swin.edu.au

Assoc. Prof. Edward Taylor
Swinburne University of Technology
Centre for Astrophysics and Supercomputing
Australia
Tel: +61 406 818 266
entaylor@swin.edu.au

Prof. Chris Fluke
Swinburne University of Technology
Centre for Astrophysics and Supercomputing
Australia
cfluke@swin.edu.au

 

Images and captions

A galaxy surrounded by a coloured haze, with lenses due to gravitational distortion
Artist's impression of a galaxy surrounded by gravitational distortions due to dark matter. Galaxies live inside larger concentrations of invisible dark matter (coloured purple in this image), however the dark matter's effects can be seen by looking at the deformations of background galaxies.
Swinburne Astronomy Productions - James Josephides

 

Stretched and distorted image of a spiral galaxy
Processed image of a spiral galaxy, as might be observed after lensing effects have distorted the galaxy's true shape. By measuring the orbital motion of gas within a distant galaxy (seen here in pink), gravitational distortions can be measured much more precisely than was previously possible.
Original image by ESA / Hubble & NASA / Flickr user Det58, image modification by Pol Gurri

 

Photograph of two telescope domes, with the Milky Way observed overhead
Photograph of the Australian National University (ANU) 2.3m telescope at Siding Spring Observatory.
Australian National University

 

Further information

The new work appears in, “The first shear measurements from precision weak lensing”, P. Gurri, E.N. Taylor, C.J. Fluke, Monthly Notices of the Royal Astronomical Society (2020), in press (DOI: 10.1093/mnras/staa2893).

The paper is available from: https://doi.org/10.1093/mnras/staa2893

This research was partially funded by the Australian Government through an Australian Research Council Future Fellowship (FT150100269) awarded to E.N. Taylor.

 

Notes for editors

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Submitted by Morgan Hollis on Fri, 06/11/2020 - 00:01