It’s hardly an exaggeration to say that dark matter is one of the biggest mysteries in the universe. Cosmologists haven’t directly observed the stuff, but based on measurements of baryonic matter—the type we can see—it is thought that about 27 percent of the universe is made up of dark matter.
The leading theories about the structure of the universe describe dark matter as a particle that acts like a sort of galaxy starter. The basic idea is that dark matter particles gravitationally attract hydrogen atoms until they are dense enough to form stars. To exert this kind of gravitational influence, however, there has to be far more dark matter in a galaxy than ‘ordinary,’ baryonic matter.
And that’s exactly why the recent discovery of NGC1052-DF2 is so puzzling. This galaxy hardly has any dark matter at all, which has lead astronomers to wonder how it could have possible formed in the first place. It’s the only galaxy of its kind that has ever been observed and may have huge implications for theories of dark matter particles and galactic formation.
“Dark matter is not something we thought was optional for a galaxy to have,” Pieter van Dokkum, an astronomer at Yale university who helped discover DF2, told me on the phone. “We really thought dark matter is the skeleton upon which everything else in the universe is built. Seeing a galaxy without it suggests maybe there's other ways of building galaxies that don't require dark matter.”
As detailed today in Nature, van Dokkum and his colleague Roberto Abraham, an astronomer at the University of Toronto, used a telescope array they designed and built in 2013 to discover this unique galaxy. The telescope array, called Dragonfly, consists of an array of 48 commercially available Canon telephoto camera lenses modified with a special type of glass that reduces scattered light.
Over the past five years, van Dokkum and Abraham have successfully used Dragonfly array, which van Dokkum described as “almost a hobby project,” to discover a number of ultra diffuse galaxies (UDG). These types of galaxies are notable for being about the size of the Milky Way, but with only a fraction of the number of stars. This makes them almost see-through, but the Dragonfly is especially adept at detecting these sorts of dim galaxies.
“We're looking for things that are so faint that with typical telescopes the light from these things are completely overwhelmed by the stars and other things that in the same location of the sky,” van Dokkum told me. “Dragonfly is extremely good at finding big, faint things in the sky because it uses these telephoto lenses with superb optics and very few reflections and other optical issues.”
Van Dokkum told me that DF2 qualifies as an ultra diffuse galaxy since it only has about 0.5 percent the number of stars as the Milky Way, but it’s still totally unique among UDGs. Unlike most ultra diffuse galaxies, which have an excess of dark matter, DF2 has hardly any: Only about 1/400th of the amount van Dokkum expected for its size.
After initially discovering DF2 with the Dragonfly array, van Dokkum and Abraham used data from the Keck Observatory to measure the velocities of the brightest stars in the galaxy. They found these stars to be moving far slower than expected and were able to calculate how much dark matter was present in DF2 based on this information.
“When we set out to measure the mass of this galaxy, we expected to find a huge amount of dark matter,” van Dokkum said. “But when we ran the numbers, it turns out to be a factor of 400 below what you'd expect for a galaxy of this brightness to have in terms of dark matter. The discovery of this object shows that the class of ultra diffuse galaxies is far more diverse than we could have ever imagined.”
DF2 also doesn’t have a dense central region of stars, spiral arms, or a galactic disk like a typical spiral galaxy. To make matters more strange, it also has no central black hole, as is typically found in elliptical galaxies. In short, DF2 doesn’t really fit into any pre-established galactic category.
DF2 is located about 60 million light years away in a galaxy cluster that is dominated by a massive elliptical galaxy called NGC 1052. According to van Dokkum, DF2’s formation in this unique cosmic neighborhood may account for its unusual properties—perhaps NGC 1052 exerted some influence on its formation, or an eruption of stars forming could have driven almost all of the dark matter from the galaxy.
The discovery of DF2 has big implications for theories of dark matter and galactic formation. On the one hand, van Dokkum said it is a significant challenge to theories that seek to explain galactic formation without dark matter, such as Erik Verlinde’s theory of emergent gravity.
“Those theories don't involve a dark matter particle to explain the motions of galaxies, they change the laws of gravity to govern those motions,” van Dokkum said. “So every galaxy should show a dark matter signature not because of dark matter, but because that signature is something inherent in the laws of gravity. Not seeing that signature, even for a single object, is a big problem for those models.”
“So not finding the dark matter in this galaxy is actually evidence for its existence, which is kind of counterintuitive,” he added.
According to van Dokkum, DF2 may provide some strong empirical evidence for certain dark matter explanations, such as the theory of “fuzzy” dark matter that characterizes dark matter as ultra-light particles that don’t form dense structures.
For now, DF2 is a unique astronomical object, although van Dokkum hopes their may be more galaxies like it just waiting to be discovered. He said that he and Abraham are currently working on a galactic survey that will use the Dragonfly array to look for other galaxies with similar properties.
“One object is nice, it tells us something about these alternative gravity theories,” said van Dokkum. “But what we'd like to know is if this is a highly unusual thing, the only galaxy of its kind, or if there is a whole population of these things. That would be far more exciting.”