Galaxy with missing dark matter

Galaxy with missing dark matter

For a galaxy like NGC1052-DF2, you'd expect to see about one hundred times more dark matter than visible matter-but astronomers found it has nearly no dark matter. One priority will be to measure the galaxy's mass by looking at the orbits of its stars, not just the motions of the bright globular clusters.

Scientists have discovered a faraway galaxy without dark matter, which may ironically prove the existence of dark matter and its importance to the universe.

So NGC 1052-DF2 is a head-scratcher whether you lean toward dark matter or toward MOND. "But what would you get if there were no dark matter at all?"

The absence of dark matter from a small patch of sky might appear to be a non-problem, given that astronomers have never directly observed dark matter anywhere. The newly discovered galaxy is about the volume and size of the Milky Way but has 200 times fewer stars. The greater mass of dark matter is believed to hold the gas together as the galaxy forms.

To make this discovery, the team used a telescope invented by van Dokkum and built with co-author Roberto Abraham of the University of Toronto. The fact that this galaxy is thought to lack dark matter means the galaxy would have to form in a different way. Which is pretty weird.

The team's results will appear in the March 29, 2018, issue of the journal Nature. But there is no theory as to why dark matter would be missing in galaxies when it was thought to be their building block. The unusual brightness of the clusters orbiting DF2 allowed the astronomers to plot their spectrum, and thus deduce their speed. The data on the 10 globular clusters the team tracked showed them moving much more slowly than would be expected. They realized that it is about equal to the sum of masses of the stars, gas and dust that constitute it. Logic?

"Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy", Pieter van Dokkum, of Yale University, said in a statement. "In those theories, every galaxy should show a dark matter signature, as it's not due to dark matter at all but due to the laws of physics", van Dokkum explained.

So, DF2 is definitely an oddball. "What we found is that there is no dark matter at all".

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Since dark matter was (sort of) discovered, researchers assumed dark matter was essential to galaxy formation. "Not sure what to make of it, but it is definitely intriguing", she said.

DF2 was first identified by Russian astronomers conducting a photographic survey, but it's uniqueness didn't come to light until later. It is about the size of our own galaxy, but only one-hundredth of its stars, while - unlike ours - it does not have a black hole in its center.

"It challenges the standard ideas of how we think galaxies work", said Dokkum. Galaxies like the Milky Way have some 30 times more dark matter than normal matter. Some theories say that dark matter is only an illusion caused by a failure to understand the way gravity works on the grand scale of the universe.

"If there is any dark matter at all, it's very little", van Dokkum explained.

What's next for this research?

Adding up the clusters' motions and NGC 1052-DF2's mass, they realized there might be no dark matter behind this particular galactic curtain.

"We're now undertaking a survey to find more objects like DF2", says Abraham. The weirdness came courtesy of the Dragonfly Array, a collection of small telescopes created to pick up faint objects.

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