A year ago, astronomers announced they’d found the first known galaxy without dark matter. Called NGC 1052-DF2 – or just DF2 for short – this object is 6.5 million light-years away and roughly the same size as our Milky Way galaxy, but with 200 times fewer stars. Now the same team of astronomers is back with what they say is “stronger evidence” about DF2’s bizarre nature. Plus the team says it has found a second galaxy without dark matter. The astronomers have published their studies in two separate papers in the peer-reviewed Astrophysical Journal Letters. One study was published in the March 20, 2019, issue, and the other was published on March 27.
The fact that we’re seeing something that’s just completely new is what’s so fascinating. No one knew that such galaxies existed, and the best thing in the world for an astronomy student is to discover an object, whether it’s a planet, a star, or a galaxy, that no one knew about or even thought about.
When astronomers think about dark matter, the observed motions of space objects often come to mind. In the 1970s, Vera Rubin and Kent Ford of the Carnegie Institution of Washington realized that stars at the outskirts of the large spiral galaxy next to ours, the Andromeda galaxy, were moving just as fast as the stars near the center. This observation apparently violated Newton’s Laws of Motion, which explain, for example, why Mars moves more slowly in orbit than Earth (because it is farther from the sun). And so the idea was born that “something” unseen must exist beyond the visible boundaries of the Andromeda galaxy, and, by extension, all galaxies. That something is what we call dark matter.
The Yale astronomers are studying the motions of space objects, too. In the first of the two new studies, they used the W. M. Keck Observatory’s Keck Cosmic Web Imager (KCWI) to gather high-precision measurements of globular star clusters inside the galaxy DF2. They said they found that – unlike the stars in Rubin and Ford’s 1970s study – these clusters are moving at a speed consistent with the mass of the galaxy’s normal matter. Their statement explained:
If there were dark matter in DF2, the clusters would be moving much faster … [thus] the team confirmed its initial observations of NGC 1052-DF2, or DF2 for short, which show dark matter is practically absent in the galaxy.
In the second study, the team used Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS) to find the second galaxy devoid of dark matter, named NGC 1052-DF4, or DF4 for short. Astronomer Pieter van Dokkum at Yale University – lead author of the DF4 study – said:
Discovering a second galaxy with very little to no dark matter is just as exciting as the initial discovery of DF2. This means the chances of finding more of these galaxies are now higher than we previously thought. Since we have no good ideas for how these galaxies were formed, I hope these discoveries will encourage more scientists to work on this puzzle.
Both DF2 and DF4 belong to a relatively new class of galaxies called ultra-diffuse galaxies (UDGs). They are as large as the Milky Way but have between 100 to 1000 times fewer stars, making them appear fluffy and translucent, therefore difficult to observe.
Ironically, these astronomers said, the lack of dark matter in these UDGs strengthens the dark matter theory. Their statement explained:
It proves that dark matter is a substance that is not coupled to ‘normal’ matter, as both can be found separately. The discovery of these galaxies is difficult to explain in theories that change the laws of gravity on large scales as an alternative to the dark matter hypothesis.
The team acknowledge their work drew criticism from other astronomers, who did not believe their result, when they first announced their results in March of 2018. Van Dokkum commented:
It was a little stressful at times. On one hand, this is how the scientific process is supposed to work; you see something interesting, other people disagree, you obtain new data, and in the end you learn more about the universe. On the other hand, although the majority of the critiques were constructive and polite, not all of them were. Every time a new critique came out we had to scramble and figure out if we had missed something.
Now the team plans for look for more galaxies of this type. Danieli is leading a wide area survey with the Dragonfly Telephoto Array in New Mexico to look for more examples in a systematic way, then observe candidates again using the Keck telescopes. She said:
We hope to next find out how common these galaxies are and whether they exist in other areas of the universe. We want to find more evidence that will help us understand how the properties of these galaxies work with our current theories. Our hope is that this will take us one step further in understanding one of the biggest mysteries in our universe – the nature of dark matter.
Bottom line: Astronomers based at Yale say they’ve confirmed their 2018 observation of an ultra-diffuse galaxy, or UDG, that, they say, contains little or no dark matter. And they say they’ve found a second galaxy lacking in dark matter.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.