The dark galaxy Dragonfly 44 appears to have about as much mass as our own Milky Way galaxy, but only 0.01 percent of that mass is in the form of stars and normal matter. The rest is dark matter, scientists say. (Credit: Pieter van Dokkum and Roberto Abraham / Gemini Observatory / AURA)
About 85 percent of the mass of the universe consists of mysterious stuff known as dark matter, but a galaxy called Dragonfly 44 appears to be even darker: 99.99 percent dark, according to newly published findings.
Dragonfly 44, which lies about 300 million light-years away in the Coma galaxy cluster, is the subject of a studypublished in the Astrophysical Journal Letters.
“It has so few stars that it would quickly be ripped apart unless something was holding it together,” Yale astronomer Pieter van Dokkum, the study’s lead author, said in a news release.
Van Dokkum and his colleagues tracked the motions of the stars in the galaxy using the Keck Observatory and the Gemini North telescope in Hawaii. The stars’ motions told the astronomers about the gravitational field surrounding Dragonfly 44.
The stars moved much faster than they should have, if that’s all there was in the galaxy. Calculations showed that the stars and visible matter would account for only 0.01 percent of the galaxy’s gravitational mass. The researchers concluded that the rest must be dark matter.
Including the dark matter, Dragonfly 44 has the mass of 1 trillion suns, which is similar to the mass of our own Milky Way galaxy.
“We have no idea how galaxies like Dragonfly 44 could have formed,” said the University of Toronto’s Roberto Abraham, a co-author of the study. “The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”
Dark matter has loomed as a cosmological mystery for decades. The latest view is that ordinary matter makes up about 5 percent of the universe’s mass-energy content, dark matter accounts for 26 percent, and another factor known as dark energy accounts for the other 69 percent.
Scientists have gone back and forth on the nature of dark matter: It could take the form of massive astrophysical compact halo objects, or MACHOs. It could exist as weakly interacting massive particles, or WIMPS. While particle physicists are hunting for clues using powerful accelerators such as the Large Hadron Collider, astronomers are looking for evidence from deep space.
“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,” van Dokkum said.
In addition to van Dokkum and Abraham, the authors of “A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44” include Jean Brodie, Charlie Conroy, Shany Danieli, Allison Merritt, Lamiya Mowla, Aaron Romanowsky and Jielai Zhang.