Dark matter accounts for almost 85 percent mass of the observable universe and yet, we have never actually found the stuff outside of theories that prove it must exist. Even the visible disk of the Milky Way, our home galaxy, is believed to exist within a roughly spherical halo of dark matter that accounts for about 90 percent of the galaxy.
In their search for the pervasive-yet-elusive particles of dark matter, astronomers have tried to find galaxies with much higher concentrations of the mysterious substance — it does not interact with visible matter at all, except through gravity, which is how scientists can theorize its existence. And while galaxies believed to be made up almost entirely of dark matter have been discovered before, they are usually quite small, such as VIRGOHI21 — about 50 million light-years away, it appears to contain no visible stars, has 99.9 percent dark matter and is about 10th the size of Milky Way.
But now, a team of astronomers has found a galaxy that is comparable in size to ours but with a similar proportion of dark matter as VIRGOHI21. Named Dragonfly 44, it is about 300 million light-years away in the Coma constellation with an estimated mass of about 1 trillion times that of the sun.
The discovery was made by astronomers from universities in the U.S. and Canada, who used the W.M. Keck Observatory and Gemini North telescope — both in Hawaii — for their observations. A paper, titled “A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44,” describing their findings was published in the journal Astrophysical Journal Letters on Thursday.
Pieter van Dokkum, an astronomer at Yale University and lead author of the paper, said in a statement: “Very soon after its discovery, we realized this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together.”
The researchers measured the velocities of stars in Dragonfly 44 using data from Keck over six nights. “Star velocities are an indication of the galaxy’s mass ... The faster the stars move, the more mass its galaxy will have,” according to the statement.
The stars in Dragonfly 44 were observed to be moving much faster than was expected for a galaxy of its brightness, or rather, its lack of brightness. Roberto Abraham of the University of Toronto, co-author of the paper, said: “It means that Dragonfly 44 has a huge amount of unseen mass.”
Observations from the Gemini North telescope showed that Dragonfly 44 has “a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way galaxy.”
Abraham said the team had no idea how galaxies like this could have formed. “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.”