This ‘See-Through Galaxy’ Far, Far Away Contains Virtually No 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.

Astronomers have found a perplexing galaxy that appears to be nearly devoid of dark matter. Since the prevailing notion is that dark matter is essential for galaxy formation, this new galaxy — called NGC1052-DF2, or DF2 for short — is a bit of a mystery.

“This galaxy is befuddling us,” astronomer Roberto Abraham told Seeker. “It’s exactly the opposite of what we expected. DF2 challenges us to explore that there may be more than one way to form a galaxy.”

Abraham is a professor at the University of Toronto and co-author on a new paper in the research journal Nature describing DF2.

In addition to the lack of dark matter, DF2 has several other peculiarities. While it is roughly the size of our Milky Way galaxy, it contains only one two-hundredth the number of stars. It has no dense, central region and no black hole. It doesn’t have spiral arms or a disk, and is more like a “blob” of stars.

DF2 also lacks the amount of gas and dust prevalent in other galaxies. An image of DF2 taken by the Hubble Space Telescope shows the wispy galaxy, where other bright objects including a distant spiral galaxy located behind it are clearly visible.

“I spent an hour just staring at the Hubble image,” lead author Pieter van Dokkum from Yale University said in a statement. “This thing is astonishing: a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.”

Van Dokkum, Abraham, and their colleagues determined the lack of dark matter by measuring the velocity of clusters of stars called globular clusters within DF2 and found they were moving slower than expected. They then calculated the galaxy’s mass and determined that the visible stars, gas and dust in DF2 accounted for most of the mass and that there was only one four-hundredth the amount of dark matter expected. Follow-up observations with Hubble confirmed their findings.

DF2 is one of a recently discovered class of galaxies called Ultra Diffuse Galaxies (UDGs). These galaxies have been described as fluffy or wispy, almost like globular clusters but without the density of stars.

Abraham and van Dokkum published the first paper about UDGs in 2015 after finding a number of these faint, wispy galaxies in the Coma Cluster, which is a dense, active region of space about 300 million light-years from Earth. It is filled bright galaxies of all shapes and sizes and lots dark matter, but yet these faint galaxies were sprinkled throughout.

Abraham explained he and van Dokkum had built a special telescope called the Dragonfly Array that looks specifically for very faint astronomical objects. It is a multi-lens array that uses commercially available telephoto lenses with specially-coated optical glass that reduces scattered light. It turns out, it was the perfect instrument for finding UDGs. 

“Astronomers have been finding isolated example of objects like this for over 30 years,” Abraham said. “But they’ve all been one-offs, or an obscure galaxy in a larger survey and they would be really rare. But then when Pieter and I built the Dragonfly Array which optimizes the detection of these low surface brightness objects, suddenly these things have been popping up like firecrackers everywhere.”

Other astronomers started looking at archival data for UDGs and have been finding several of these objects which had previously been dismissed as image artifacts or “ghost” images.

In the only Dragonfly Array image of the Coma Cluster from 2015, the team found about 50 UDGs — Milky Way-size objects that were extremely faint. Abraham said the various ideas for what UDGs are range from “failed galaxies” that ran out of gas to normal galaxies that got “knocked around so much inside the Coma cluster that they puffed up” to pieces of other galaxies that were pulled off during galaxy mergers.

“What is really emerging now is the race to understand what UDGs are,” Abraham told Seeker. “There have been two camps of trying to model these things, where one idea is it would take a lot of dark matter to create them and the other where there is just a regular amount of dark matter. But what wasn’t considered was that something would pop up that didn’t have any dark matter.”

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