Exploring Dragonfly 44: a Mysterious Dark Galaxy

Coma Cluster
 By NASA/Donald R. Pettit [Public domain], via Wikimedia Commons

A dark-matter galaxy? Could it really happen? According to astronomers who are mapping the distribution of this mysterious stuff in the universe, it actually exists. This blobby-looking wisp of light lies in a collection of galaxies called the Coma Cluster, which is about 321 light-years away from us. Astronomers have dubbed it "Dragonfly 44".

We know that galaxies are made of stars and clouds of gas and dust and are built up through a long process of collision and cannibalism.

But, here's this galaxy that is 99.99 percent dark matter. How can this be? And, how did astronomers find it? This is a puzzling find that also gives astronomers another look at how dark matter is arrayed throughout the universe.

Dark Matter: It's Everywhere

You've probably heard of the concept of dark matter before—it is made up of "stuff" that is not at all well understood. What that really means is that it's a substance in the universe that can't be detected by ordinary means (like, through telescopes). Yet, it can be indirectly measured by its gravitational effect on the matter we CAN see, the so-called "baryonic matter". So, astronomers look for the effect of dark matter by watching for ways that it affects matter as well as light. 

It turns out that only about 5 percent of the universe is made of matter we CAN detect—such as stars, clouds of gas and dust, planets, comets, etc. Everything else is dark matter or is made up of the completely mysterious "dark energy".

Dark matter was first discovered by Dr. Vera Rubin and a team of astronomers. They measured the motions of stars as they orbit in their galaxies. If there were NO dark matter, the stars nearest a galaxy's core would orbit many times faster than the stars along the outer regions. This is similar to riding a merry-go-round: if you're in the middle, you spin faster than you would if ride on the outside edge.

However, what Rubin and her team found was that the stars in the outer regions of the galaxies were moving faster than they should have been. Star velocities are an indication of how much mass the galaxy has. Rubin's finding implied that there was still MORE mass out in the outer reaches of the galaxies. But they didn't see more stars or other visible matter. All they knew was that the stars weren't moving at the right speed, and the additional matter was affecting their speeds. That matter wasn't emitting or reflecting light, but it was still there. That "invisibility" is why they nicknamed this mysterious substance "dark matter".

A Dark Matter Galaxy?

Astronomers know that each galaxy is surrounded by dark matter. It helps hold the galaxy together. This is an important thing to know because Dragonfly 44 has so few stars and clouds of gas and dust that it should have flown apart long ago. But, this diffuse "blob" of stars that is around the same size as the Milky Way Galaxy is still in one piece. Dark matter is holding it together.

Astronomers looked at the Dragonfly with the W.M. Keck Observatory and the Gemini Observatory, both located on Mauna Kea on the Big Island of Hawai'i. These powerful telescopes let them see the few stars that do exist in Dragonfly 44 and measure their velocities as they orbit the central part of the galaxy.

Just as Vera Rubin and her team found in the 1970s, the stars in the Dragonfly galaxy are not moving at the velocities they should be if they existed without of the presence of dark matter. That is, they're surrounded by more dark matter mass, and this affects their orbital speeds.

The mass of Dragonfly 44 is about a trillion times the mass of the Sun. Yet, only about 1 percent of the galaxy's mass appears to be in stars and clouds of gas and dust. The rest is dark matter. No one is quite sure how Dragonfly 44 formed with so much dark matter, but repeated observations show that it's really there. And, it's not the only galaxy of its kind. There are a few galaxies called "ultra-faint dwarfs" that also appear to be mostly dark matter. So, they're not flukes. But, nobody's quite sure why they exist and what will happen to them.

Ultimately astronomers will need to figure out what dark matter actually is and the role it plays throughout the history of the universe. At that point, they might then get a good handle on why there are dark matter galaxies out there, lurking in the depths of space.