Scientists see the dark side of our galaxy

11 November 2020

An international team of astronomers has challenged a popular theory that mysterious dark matter dominates gravity in the inner parts of spiral galaxies such as our Milky Way.

Senior lead researcher Professor Ken Freeman at The Australian National University (ANU) has found that stars - visible matter - dominate the gravity in the inner parts of these types of galaxies.

"One of the big questions we're trying to answer is, why does the Milky Way look the way it does?" Professor Freeman, from the ANU Research School of Astronomy and Astrophysics, said.

"We've discovered that stars and gas account for about three-quarters of the gravitational pull in the inner parts of the Milky Way and other spiral galaxies.

"But the dark matter totally dominates gravity in the outer parts of these galaxies.

"By finding out how much the stars contribute to a galaxy's total gravity, we can work out the detailed structure of the dark matter, almost as if we could see this invisible matter directly."

Professor Freeman said scientists had known about dark matter in galaxies for about 50 years, but they still don't know what it is or where it comes from.

"There are many theories about what it could be, including that it is potentially some form of exotic particle that we have not discovered yet," he said.

This type of study is difficult to conduct on the Milky Way, as astronomers can only observe it from the inside. The team needed to study a spiral galaxy front-on, which is like looking at a plug hole of a bath from above.

"We chose NGC 6946, which is more than 22 million lightyears away from Earth," Professor Freeman said.  

Professor Freeman, along with PhD scholar Ms Surashree Aniyan and Dr Anastasia Ponomareva from the ANU Research School of Astronomy and Astrophysics, led the international team, which included scientists from Germany, the Netherlands and the UK. 

The team developed a new technique to settle this question of how dark matter is distributed across spiral galaxies such as the Milky Way.

The ANU group designed and built a specialised instrument, the Planetary Nebula Spectrograph, that was mounted on the William Herschel Telescope in the Canary Islands. Dr Magda Arnaboldi of the European Southern Observatory in Munich is the leader of the Planetary Nebula Spectrograph Team.

The research is published in Monthly Notices of the Royal Astronomical Society.