The Dark Secret of Galaxies May Not be so Dark After All

Since the discovery of dark matter, most astronomers have concluded that every galaxy held some of its mass in the form of this enigmatic — something. Now, a pair of studies show that some galaxies do not hold onto any dark matter at all.

NGC 1052-DF2, found in 2015, is the first galaxy ever seen to contain little to no dark matter. This ultra-diffuse galaxy, which only loosely holds on to its member stars, was discovered by a team including Yale University graduate student Shany Danieli.

“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,” said Pieter van Dokkum of Yale University.

The Ultra-diffuse galaxy NGC 1052-DF2. Image credit: NASA, ESA, and P. van Dokkum (Yale University)

The mysteries of dark matter are immense. Whatever it is, this — something — is invisible, gives off no radiation, and only seems to interact with normal matter through the force of gravity. Yet, it holds galaxies together, and greatly “outweighs” normal matter throughout the visible Universe.

Playing Hide and Seek with Dark Matter

Evidence for dark matter between galaxies was first discovered in 1933 by astronomer Fritz Zwicky, as he examined galaxies in the Coma Cluster. If the structure contained just the mass that he could see, Zwicky reasoned, the cluster would fly apart. There had to be something else holding the cluster together, while remaining unseen by telescopes. But, it turns out, dark matter was not found only in the space between galaxies.

Vera Rubin with pioneering astronaut John Glenn. Image credit: Jeremy Keith (CC)

In the late 1970’s, Vera Rubin and Kent Ford were studying rotational rates of the Andromeda Galaxy. What they found came as a shock. The laws of gravitation state that as galaxies rotate, the velocity of objects around the galaxy revolve around the center at a rate dependent on the mass within the family of stars. What Rubin and Ford saw, however, was objects around the edge of galaxies were spinning at the same velocity as material closer to the center. While pondering the conundrum, Rubin remembered reading about Zwicky’s work four decades previously, and realized dark matter spread out within galaxies would explain the unusual rotational rates.

Today, the amount of dark matter in the visible Universe is measured to make up 84 percent of the “stuff” out there, far more than all the stars and galaxies we see around us when we look into the sky.

Where has All the Dark Matter Gone?

When astronomers searched DF2, however, they found galaxies within the clusters moved at rates matching what would be expected if only ordinary matter was present within the system.

“For decades, we thought that galaxies start their lives as blobs of dark matter. After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way. NGC1052-DF2 challenges the standard ideas of how we think galaxies form,” said van Dokkum in 2018, following the discovery of the first such body.

NASA explains the mysteries of DF2, showing images taken of the ultra-diffuse galaxy by the Hubble Space Telescope. Credit: NASA’s Goddard Space Flight Center/Scientific Visualization Studio

Finding a single body defying the norm in science is, by definition, highly unusual. Most of the time, the discovery turns out to be in error, but when it isn’t, the finding can be revolutionary.

“If there’s one object, you always have a little voice in the back of your mind saying, ‘but what if you’re wrong?’ Even though we did all the checks we could think of, we were worried that nature had thrown us for a loop and had conspired to make something look really special whereas it was really something more mundane,” van Dokkum explains.

Two is Better than One

Images of DF 2 and DF4, as seen by the Dragonfly Telephoto Array. Image credit: P. Van Dokkum (Yale University)/STScI/ACS

Utilizing the Keck Observatory, researchers found what they were looking for in their study of NGC 1052-DF4, a second diffuse galaxy showing no signs of being influenced by the presence of dark matter.

Ultra-diffuse galaxies are a newly-discovered class of galaxies, having less than one percent as many stars as the Milky Way, spread out over the same amount of space as our home galaxy.

The discovery of galaxies without a significant quantity of dark matter ironically strengthens the theory developed by Zwicky, Rubin, and Ford. This finding shows normal and dark matter can exist apart from each other, as well as in tandem. This would be difficult to explain using alternative theories postulating differences in gravitational forces in various parts 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,” Danieli explained.