A recently-discovered, blob-like galaxy is approximately the same size as the Milky Way, but is almost entirely made up of dark matter, according to new research led by astronomers from Yale University and published in September’s edition of the Astrophysical Journal Letters.
Using the W.M. Keck Observatory and the Gemini North telescope in Hawaii, lead investigator Pieter van Dokkum and his colleagues analyzed the galaxy, which was dubbed Dragonfly 44 and is located in the nearby Coma constellation, and made some surprising discoveries.
For one thing, Dragonfly 44 was found to contain far fewer stars than the Milky Way, but similar to our home galaxy, it contained a ring of spherical star clusters close to its core. The researchers then measured the velocities of the galaxy’s stars to determine its mass, as the more quickly stars travel in a galaxy, the greater amount of mass that said galaxy will contain.
“Amazingly, the stars move at velocities that are far greater than expected for such a dim galaxy. It means that Dragonfly 44 has a huge amount of unseen mass,” University of Toronto professor and study co-author Roberto Abraham explained Thursday in a statement.
In fact, the galaxy was found to have a mass of 2 tredecillion kilograms – similar to that of the Milky Way. However, only one-hundredth of 1% of that mass is in the form of stars and normal matter. The other 99.99% of the galaxy’s mass, the authors concluded, must be dark matter, the hypothetical substance researchers believe makes up the majority of the universe.
Astronomers puzzled as to how such a galaxy originally formed
“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,” said van Dokkum, an astronomy and physics professor at the Connecticut university.
While Dragonfly 44 is hardly the first galaxy made up primarily of dark matter that researchers have discovered, it is unusual in that most of those ultra-faint galaxies had similar compositions and were roughly 10,000 times less massive than the newly discovered one.
“We have no idea how galaxies like Dragonfly 44 could have formed. The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters,” said Abraham, “and that is probably an important clue. But at the moment we’re just guessing.”
“Ultimately what we really want to learn is what dark matter is. 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,” added van Dokkum, who was also assisted by researchers at the University of California Observatories, Harvard and San Jose State University.
Image credit: Sloan Digital Sky Survey