A Galaxy Without Dark Matter

Update: The study’s authors have provided us with a link to a free .pdf version of the full paper!

In a revolutionary development, a team of astronomers has discovered that a faint smudge of a galaxy called NGC1052-DF2 (or DF2, for short) may have no dark matter at all; the group’s results show that DF2 has less dark matter than predicted by a factor of at least 400. That’s a big deal. Astronomers have never seen a galaxy like this before, and it raises intriguing questions about galaxies and dark matter.

A Hubble Space Telescope image of the galaxy NGC1052-DF2. Distant galaxies are visible through DF2 due to its lack of stars and “ghostly” nature.
Image Credit: P. van Dokkum; R. Abraham; STScI, Space Telescope Science Institute.

Our galaxy, the Milky Way, has about 30 times more dark matter than ordinary matter. Ordinary matter is relatively easy to define: It’s all of the “stuff” in the universe made of atoms—animals, plants, planets, stars, gas, dust, etc. Dark matter is much more elusive. No one knows exactly what it is, but there is strong evidence that undetected matter of some kind exists, has mass, and interacts with ordinary matter through gravity, playing a crucial role in holding galaxies like ours together.

Enter this discovery. Yale University’s Pieter van Dokkum is the lead author of the new research paper, which appears in this week’s issue of Nature. He is also one of the leaders of the Dragonfly Telescope Array. Dragonfly is sensitive to objects that appear very faint in the visible region of the spectrum. When comparing Dragonfly images to data from the Sloan Digital Sky Survey, his team realized that DF2 was peculiar. In Dragonfly images, the object had size and structure, but in data from the Survey, the same area looked more like a collection of pinpoints of light than a typical galaxy.

The Dragonfly team of Roberto Abraham (far left), Pieter van Dokkum (far right), and University of Toronto and Yale graduate students pose with one half of the Dragonfly array at its home site in New Mexico.
Image Credit: Dunlap Institute for Astronomy & Astrophysics.

After conducting follow-up observations, van Dokkum and colleagues at San Jose State University, University of Toronto, and the Harvard-Smithsonian Center for Astrophysics concluded that DF2 is a strange galaxy. It’s about the size of the Milky Way, but has only 1/200th the number of stars. That gives it a kind of ghost-like appearance—you can see other galaxies through it. DF2 doesn’t have a dense central region like most spiral galaxies, or a central black hole like most elliptical galaxies. There are ten bright clusters of stars in the galaxy, but they orbit the center of the galaxy much slower than expected.

The researchers calculated the mass of the galaxy using two different methods. The methods agreed with one another—but not with predictions based on our current understanding of galaxies. The ordinary matter (also called baryonic matter) in DF2 accounted for the entire mass of the galaxy. “We conclude that NGC1052–DF2 is extremely deficient in dark matter, and a good candidate for a ‘baryonic galaxy’ with no dark matter at all,” write the researchers in the Nature paper.

Wow. So what does that mean? In a statement, van Dokkum put it this way, “It challenges the standard ideas of how we think galaxies work. This result also suggests that there may be more than one way to form a galaxy.” Previously, most astronomers assumed that galaxies were an intertwined mixture of ordinary matter and dark matter. Now it seems that isn’t necessarily true.

This finding also raises serious questions about the formation of galaxies. Research suggests that dark matter is key to the formation of galaxies. Is there another mechanism for galaxy formation that we don’t know about? Did DF2 once contain dark matter, and then somehow lose it in a violent cosmic event? Is DF2 a mutant due to its proximity to a much more dominant galaxy, NGC 1042? Or is DF2 not that unusual at all, just the first of many dark matter-deficient galaxies to be discovered? Dragonfly has seen other objects that look similar to DF2, says van Dokkum, but they haven’t been investigated yet.

There’s another interesting aspect of this discovery. Although it calls into question some of what we know about galaxies and dark matter, this new discovery could end up strengthening the case for dark matter in the end. Most astronomers—but not all—agree that there is some kind of dark matter in the universe. Some have offered competing theories to explain the observations often considered evidence of dark matter, such as modified Newtonian dynamics (MOND). However, MOND and some other alternative theories don’t offer predictions of galaxies that appear not to have dark matter.

Some discoveries resolve a lot of existing questions. Not this one…but that just makes it more exciting! Dark matter has always been a mysterious creature. Wherever this finding leads, you can be sure that it will be interesting—and that Physics Buzz will here to bring you the story.

—Kendra Redmond

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