The Dark Galaxy Dragonfly 44: A Galaxy Almost Entirely Dark Matter

Relatively nearby at 300 million light years from Earth, the dark galaxy Dragonfly 44 had been missed by astronomers for decades because it was dim. This galaxy had too few stars. Astronomers suspected it was mostly dark matter.

Published on 20th Mar, 2018

Using some of the world's largest, and smallest telescopes, an international team of astronomers have discovered a massive galaxy that consists almost entirely of Dark Matter. Even though it is relatively nearby at 300 million light years from Earth, the galaxy, named Dragonfly 44, had been missed by astronomers for decades because it is very dim. The discovery of this dim object heralds in a new class of galaxies that have become known as ‘dark galaxies’ or ‘ultra diffuse galaxies’ because they contain relatively few stars, yet have a mass equal to our own Milky Way with hundreds of billions of stars. It was only discovered recently, in 2015, when the Dragonfly Telephoto Array, then a small collection of 24 commercially available 400mm telephoto lenses, looked towards a region of sky in the constellation Coma. But as astronomers looked closer using larger telescopes with specialized instruments, they realized this dim, tiny galaxy was more than meets the eye - literally. It contained so few stars. There were so few stars in this galaxy that there was no way they could all stay together as a gravitationally bound object - they would simply fly apart. So what was keeping the stars in Dragonfly 44 together? Astronomers had pretty good idea: dark matter. Observations using the DEIMOS instrument mounted on the gigantic Keck Telescope on Mauna Kea in Hawai’i allowed astronomers to determine the amount of dark matter in the Dragonfly 44 galaxy. This was accomplished by staring at it for thirty three and a half hours over a period of six nights and measuring the motions and velocities of the few stars that could be seen in Dragonfly 44. Additionally, the Gemini Multi-object Spectograph on the very large 8-meter Gemini North Telescope, revealed a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy. Armed with this information, the motions of the stars told the story of how much matter is in the galaxy. The moving stars don’t tell you what kind of matter is there, only that it is there. The discrepancy between the visible, normal matter and dark matter was huge. Both the halo of star clusters seen by Gemini North and the motions of the stars measured by Keck all added up to only a tiny fraction of the mass that must be there inferred by the looking at the velocities of the stars. This discrepancy between the amount of visible and dark matter was the largest anyone had ever seen. As its name implies, dark matter is rather mysterious. No one knows exactly what it is, we can’t see it or detect it in any meaningful way. Because of this, the amount of dark matter contained in a galaxy is inferred by measuring things we can see, like the motions of the stars inside it. The only way we can detect dark matter is by looking at the effect it has on things we can see. In the case of Dragonfly 44, it’s the stars moving around in the galaxy we can see that were being affected by the gravitational pull of the dark matter inside that we couldn’t. In the dark Dragonfly galaxy, the stars were see to move very fast - too fast to account for just the pull exerted by the visible stars. Something else - something invisible was accelerating them and affecting their motions. Based on the observed motions of its stars made by Keck, the mass of the Dragonfly 44 Galaxy is estimated to be the same as a trillion suns – this is very similar to the mass of our own Milky Way. Strangely however, when closely examined, there are nowhere near a trillion stars in this galaxy. Only one hundredth of one percent of the material in this galaxy is in the form of stars, gas and "normal" matter; the other 99.99 percent of the mass of this galaxy can’t be seen. It isn’t there. Astronomers infer that the rest must be dark matter. The Milky Way has more than a hundred times more stars than Dragonfly 44. And yet they have the same mass. Finding a galaxy with the mass of the Milky Way that is almost entirely dark was unexpected. Astronomers have no idea how dark galaxies like Dragonfly 44 could have formed, The observations from Gemini North also showed that a relatively large fraction of the stars that can been seen is in the form of very compact clusters, and that is probably an important clue. Another clue is that the stars in this galaxy while few, are also very, very old. It’s possible all star formation ended long ago and the remaining material was dispersed by the prevalent dark matter. But at the moment astronomers are only guessing - Dragonfly 44 is a galactic enigma and astronomers would love to find more dark matter galaxies like it. This discovery has big implications for the study of Dark Matter. Finding objects like this that are made up of almost entirely Dark Matter helps in our understanding of what it is because, well, these galaxies are simpler: we don’t get confused by stars, gas, black holes, dust and all the other things that galaxies have. The only other galaxies like this we’ve had to study so far were tiny. This finding opens up a whole new class of massive objects for further study. Ultimately what we really want is to learn what Dark Matter is, it accounts for 27 percent of the mass and energy of the universe, yet it doesn’t interact with us in any direct way. The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a Dark Matter particle.