Galaxy-Sized Lenses Light Up Dark Matter

Just as most of an iceberg is unseen below the surface of the ocean, so to the Universe keeps a large part of itself shrouded in darkness and undetected by our telescopes. Finding this unseen dark matter has become a priority for cosmologists around the world, and astronomers at Swinburne University of Technology have pioneered a new way to see the dark matter halos that surround galaxies.

Although dark matter contributes more than 80% of the matter in the Universe, we can’t see it and are only sure of its existence because of the way it interacts gravitationally with ordinary matter. The stars in the outer reaches of galaxies orbit so quickly that they would simply fly off into space if it didn’t exist – like a rock thrown from a sling.

This also happens on even larger scales, where entire galaxy groups or clusters would break apart if not for a spherical halo of dark matter surrounding the visible matter. Galaxy clusters are the largest gravitationally bound structures in the Universe, with about 80% of the cluster content in the form of dark matter.

But how are astronomers able to see these halos? One of the predictions of Einstein’s theory of general relativity was that the light from distant galaxies would be deflected when it passed close enough to these massive objects. What we see on Earth is that the background galaxies are distorted and appear out of position, an effect known as gravitational lensing.

If the distortions caused by the gravitational lensing are very small, it is not possible to detect them on individual galaxies. A statistical method known as weak lensing, where distortions are averaged over a large number of galaxies, is used instead.

The new research focuses on weak lensing, but the team, led by Swinburne PhD candidate Pol Gurri, has found a novel way to measure the lensing effect of a foreground galaxy by analysing the motion of the stars in the distorted galaxies rather than their shapes. This new method overcomes some of the limitations of traditional methods of weak lensing.

The result is that astronomers are able to more easily measure the mass profiles of galaxies, including the dark matter haloes that extend out far beyond where the light from stars and gas can be seen.

The idea that underpins weak lensing experiments is that the observed image can be used to measure the gravitational properties of the lens provided that the true, or unlensed, scene is known. In conventional weak lensing, the shapes of galaxies are assumed to be circular, meaning that any elongation in the image is a result of lensing.

However, galaxies are intrinsically elliptical, and this leads to a source of error that must be accounted for. Sampling very large numbers of galaxies reduces, but does not eliminate, this error.

The new method of analysis devised by Pol Gurri and his team, labelled Precision Weak Lensing by the researchers, looks instead at distortions in the velocity fields of the galaxies. Using this information, also known as the kinematics of the galaxy, enables a much more precise measurement of the lensing effect than is possible using shape alone.

When they applied their technique to 18 galaxies that were observed with the Australian National University’s 2.3m telescope located in Siding Spring, NSW, Pol Gurri’s team managed to obtain a signal strength for each galaxy that would have required 50 galaxies to be observed using conventional techniques. Measuring the properties of the dark matter in the lensing galaxies follows from there.

“Because we know how stars and gas are supposed to move inside galaxies, we know roughly what that galaxy should look like,” says Pol 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.”

“With our new way of seeing the dark matter we hope to get a clearer picture of where the dark matter is, and what role it plays in how galaxies form.”

Weak gravitational lensing is already one of the most successful ways to map the dark matter content of the Universe, with massive global investments of time and resources. NASA’s Nancy Grace Roman Space Telescope and the European Space Agency’s Euclid Space Telescopes (both scheduled to launch in 2022) have been designed, in part, to make similar kinds of measurements. The larger, more sensitive telescopes will make it possible to extend the technique of precision weak lensing to galaxies that are even more distant.

With the hunt for the elusive dark matter particle intensifying in Australia and around the world, understanding its distribution in space will be like fitting another piece into one of the greatest unsolved puzzles in cosmology.

Research co-author Associate Professor Edward Taylor commented on the significance of this research. “We have shown that we can make a real contribution to these global efforts, even with a relatively small telescope built in the 1980s, just by thinking about the problem in a different way.”