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Unique, oddball galaxy is strangely devoid of dark matter

Scott Sutherland
Meteorologist/Science Writer

Thursday, March 29, 2018, 12:46 PM - Astronomers have discovered something strange and unique out in the universe - a galaxy with virtually no dark matter - and perhaps even stranger still, this discovery offers up a validation that dark matter actually exists!

Look out into the universe, and we see an abundance of matter, in stars, planets, dust, nebulae and galaxies, but there's something even more abundant out there, that we can't see. Dark matter - matter that we know is there from its effects on gravity, but which emits no radiation that we've been able to detect - is so abundant in our universe that it far outweighs normal matter. Scientists have found that normal matter - the stuff we do see - makes up only around 15 per cent of all the matter in the universe. Mysterious "dark matter" makes up the rest.

So, when astronomers look at the galaxies, our own Milky Way included, they find that the normal matter - stars, dust, nebulae, etc - are moving so fast that they'll all just fly apart from one another. It's dark matter that exerts the extra gravitational force that binds the galaxy together. In fact, it's now thought that galaxies wouldn't even form it if wasn't for dark matter; that it's dark matter that keeps the normal matter together as the galaxy coalesces, allowing the normal matter to gather into the forms we see it in.

Abraham (far left) and van Dokkum (far right) stand with
their team of U of Toronto and Yale graduate students,
with one-half of the 48-lens Dragonfly array.
Credit: U of T

Now, while using a new telescope, known as the Dragonfly Telescope Array, astronomers looking for faint, hard-to-find objects out in the universe, have found what they call an "Ultra-Diffuse Galaxy", named NGC1052-DF2 (DF2 for short), which has them scratching their collective heads.

"We thought all galaxies were made up of stars, gas and dark matter mixed together, but with dark matter always dominating," Prof. Roberto Abraham, from the University of Toronto's Dunlap Institute for Astronomy & Astrophysics, said press release. "Now it seems that at least some galaxies exist with lots of stars and gas and hardly any dark matter. It is pretty bizarre."

Abraham is the co-author of a new study, published in the journal Nature, that describes this unusual galaxy.

In the paper, Abraham and his colleagues say that DF2 has only 1/400th of the amount dark matter they'd expect to find in it. They discovered this by watching smaller collections of stars, known as globular clusters, orbit the galaxy.

A close-up of one of the globular clusters surrounding DF2. The unusual brightness of the clusters orbiting DF2 allowed the astronomers to plot their spectrum, and thus deduce their speed. Credit: Gemini Observatory/NSF/AURA/Keck/Jen Miller/Joy Pollard

Working with the speeds at which these clusters of stars were moving, they were able to calculate the mass of DF2. Counting up all the normal matter they could see, from the stars, dust and gas, they found that the two values were much closer than they should be. The visible matter accounted for nearly all of the total mass of the galaxy, which had never been seen before.

"If there is any dark matter at all, it's very little," said Pieter van Dokkum of Yale University, who lead the study on DF2. "The stars in the galaxy can account for all of the mass, and there doesn’t seem to be any room for dark matter."

For comparison, astronomers have found that the Milky Way Galaxy has an amount of dark matter that's 30 times greater than all the matter we can see with our array of different telescopes.

So, DF2 is definitely an oddball.

"The idea that our universe is filled with a mysterious substance called ‘dark matter’ which doesn't emit or absorb light has gained traction in astronomy since the mid-20th century," says Dr. Nathalie Ouellette, an astronomer from Queens University, who was not involved in the study. "Dark matter has been rolled into galaxy formation and evolution theories for decades now, but this discovery of a galaxy that seems dark-matter-free puts a damper on many of those theories."

"This is like building a house without any mortar," she said in an email to The Weather Network. "In other words, it shows that there could be many different ways to build a galaxy."

The research team identified a few ways that DF2 may have formed. In one, a sudden explosion of massive star-birth inside the galaxy could have cleared out all of its gas and dark matter. In another, an immense galaxy that's relatively close to DF2 may have somehow influenced the smaller galaxy during its formation. They're going to need more study and more time to think on this, though, before they come up with anything concrete.

Oddly enough, this galaxy with very little dark matter actually helps to confirm the existence of dark matter.

Some have thought that what we think of as 'dark matter' isn't actually another form of matter, but may be just another expression of how normal matter affects the universe. However, if that was the case, DF2 would exhibit the exact same abundance of dark matter that all other galaxies - even other Ultra-Diffuse Galaxies - appear to have. Since it does not, that particular explanation for dark matter, and why we can't see it, becomes less likely.

What's next for this research? Will they find more of these dark matter deficient galaxies?

"We're currently undertaking a survey to find more objects like DF2," Abraham said. "Dragonfly is good at finding these sorts of objects. Maybe we're only seeing the tip of the iceberg."

Sources: Dunlap Institute for Astronomy & Astrophysics | Gemini Observatory | Hubble

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