The dark side of our galaxy

Why does the Milky Way look the way it does?

An international team of scientists has challenged a well-known hypothesis that mysterious dark matter dominates gravity in the inner parts of spiral galaxies, for example, our Milky Way.

Scientists discovered that stars – visible matter – dominates the gravity in the inner parts of these kinds of galaxies.

Senior lead researcher Professor Ken Freeman at The Australian National University (ANU) said, “One of the big questions we’re trying to answer is, why does the Milky Way look the way it does?”

“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 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.”

This study is difficult to conduct on the Milky Way, as astronomers can only observe it from within. Scientists needed to study a spiral galaxy from outside.

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

Using a newly developed technique, scientists settle how dark matter is distributed across spiral galaxies such as the Milky Way.

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

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