SCIENCE

Why The Discovery Of The Milky Way's 'Dark Twin' Is Such A Big Deal

Dragonfly 44 could help solve one of nature's biggest mysteries.

29/08/2016 3:47 AM AEST | Updated 30/08/2016 5:53 AM AEST
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Pieter van Dokkum Roberto Abraham Gemini Sloan Digital Sky Survey
The dark galaxy Dragonfly 44. In the image on the left, the galaxy is visible only as a faint smudge of light. In the long-exposure image on the right, the galaxy is visible as a large elongated object

Astronomers have discovered a vast galaxy made up almost entirely of dark matter, an unexpected find that could bring us one step closer to understanding what the mysterious stuff is all about.

Dubbed Dragonfly 44, the collection of stars lies about 330 million light-years from Earth in a group of galaxies known as the Coma cluster. It’s about the size of our own Milky Way galaxy but much, much fainter. It contains only about 1 billion stars, which is roughly 1 percent the number of stars in our galaxy. 

And 99.99 percent of the mass in what some are calling “the Milky Way’s dark twin” is in the form of dark matter, an invisible substance that is believed to make up 85 percent of everything in the universe. Dark matter can be “seen” in the way if affects the movement of stars within galaxies but it has never been directly observed ― at least, not with any certainty.

The team of astronomers who discovered Dragonfly 44 with the help of Hawaii’s W. M. Keck Observatory were completely surprised to find the dark galaxy.

“We knew about other galaxies that are almost entirely dark matter, but those are tiny dwarf galaxies that are about a million times less massive than Dragonfly 44,” Yale University astronomer Pieter van Dokkum, lead author of a paper about the discovery that was published last week in The Astrophysical Journal Letters, told The Huffington Post.

“It is an exciting discovery,” said Jeremiah Ostriker, an astrophysicist at Columbia University. “It gives us a chance to study [dark matter] in detail ― close-up.”

How do we know Dragonfly 44 is made of dark matter if we can’t see the stuff?

The greater a galaxy’s overall mass, the faster its stars move relative to each other. And the stars in Dragonfly 44 “move at velocities that are far greater than expected for such a dim galaxy,” Roberto Abraham, a University of Toronto astronomer and a co-author of the paper, said in a news release. “It means that Dragonfly 44 has a huge amount of unseen mass.”

In other words, oodles of dark matter.

Theory holds that dark matter particles annihilate each other and emit gamma radiation whenever they collide, van Dokkum said. Scientists have been searching for this distinctive signal in dwarf dark galaxies, he added, but the signal would be much stronger in a big dark matter galaxy like Dragonfly 44.

“The race is on to find massive dark galaxies are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle,” he said in the news release. 

Stay tuned!

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