Astronomers Discover Milky Way-Sized Galaxy That’s Almost Entirely Dark Matter

Friday, 26 August 2016 - 3:39PM
Astronomy
Friday, 26 August 2016 - 3:39PM
Astronomers Discover Milky Way-Sized Galaxy That’s Almost Entirely Dark Matter

Dark matter has been elusive to astronomers for decades; it composes approximately 27% of the matter in the observable universe, and yet we still have no idea what it is. Now, we've discovered an entire galaxy filled with dark matter-one that's the same size as the Milky Way, but contains less than 1% of its stars.

Dark matter refers to a mysterious type of matter that doesn't interact with any type of electromagnetic radiation, and so is invisible to the entire electromagnetic spectrum. This means it's almost impossible to detect, but scientists can deduce its presence based on other factors.

Galaxies, for example, rotate at extremely high speeds, to the point that they would not be able to hold together as one entity if they didn't contain more matter than we detect. As a result, scientists estimate that they contain high concentrations of dark matter.

Now, scientists have performed the same analysis on a galaxy called Dragonfly 44, which was discovered back in 2014. It was one of several galaxies that were discovered in the Coma Cluster, which were considered "fluffy," since they had much less matter than one would expect considering their size. As a result, the researchers theorized that the extra space was filled with dark matter. 

In order to test this theory, the researchers measured the stars' velocity for 33.5 hours over six nights. They found that it contains about 1 trillion times the mass of our Sun, which is far too massive to be held together by the normal matter we've detected in the galaxy. 

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"Motions of the stars tell you how much matter there is," co-author Pieter van Dokkum told Popular Mechanics. "They don't care what form the matter is, they just tell you that it's there. Using the Keck Observatory, we found many times more mass indicated by the motions of the stars than there is mass in the stars themselves."
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The researchers calculated that the galaxy would need to comprise 99.99% dark matter, which officially makes Dragonfly 44 the darkest galaxy in the Universe thus far. The discovery of these ultra-diffuse or "fluffy" galaxies are fascinating on their own, but could also help us discover other hard-to-detect objects in our Universe.

Opening quote
"It helps to have objects that are almost entirely made of dark matter so we don't get confused by stars and all the other things that galaxies have," said van Dokkum. "The only such galaxies we had to study before were tiny. This finding opens up a whole new class of massive objects that we can study."
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Via Science Alert

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