Astronomers have discovered a massive “ghost” galaxy roughly the same size as the Milky Way.
An international team of astronomers and physicists using a telescope array in Hawaii found a galaxy – named Dragonfly 44 – with a mass similar to our galaxy, but made almost entirely of dark matter.
Dark matter is a kind of gravitational glue and – theoretically – makes up around 27 per cent of the universe.
Astronomers had not studied the dim region in detail because it lies on the edge of a group of around 1,000 galaxies, known as the Coma Cluster.
They had missed the Milky Way-sized galaxy, which appears as a diffuse and dim “blob” because it contains few stars. It emits about one per cent of the light produced by our galaxy.
Its discovery was not the first time a galaxy of dark matter had been detected but it was the first time a dark matter galaxy of considerable size was detected. Previously, dark matter has been detected in tiny, dwarf galaxies.
The Coma Cluster is 300 million light-years from Earth and contains thousands of galaxies, some of which are dim with few stars.
Dragonfly 44 was discovered when the team scrutinised a region of the sky near to the cluster and they realised something else was going on.
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A massive galaxy with few stars would rip itself apart so there was something else holding galactic space-time together.
Astronomers measured the galaxy’s mass and used the telescopes on Hawaii, including the largest telescope in the United States at the Keck Observatory, to capture images of the galaxy and stars around its core.
Yale University astronomer Pieter van Dokkum said the team studied the motions of the galaxy’s stars to determine the amount of dark matter.
Dragonfly 44 is 99.99 per cent dark matter.
“This has big implications for the study of dark matter.
“It helps to have objects that are almost entirely made of dark matter so we don’t get confused by stars and all the other things that galaxies have. The only such galaxies we had to study before were tiny. This finding opens up a whole new class of massive objects that we can study.”
Finding a dark matter galaxy with the mass of the Milky Way was almost entirely unexpected.
“They [stars] don’t care what form the matter is, they just tell you that it’s there.
“In the Dragonfly galaxy stars move very fast. So there was a huge discrepancy. Using Keck Observatory, we found many times more mass indicated by the motions of the stars, than there is mass in the stars themselves.”
With this information, the team estimated the mass of the galaxy was a trillion times the mass of the Sun – a similar mass to the Milky Way. The Milky Way is thought to be mostly dark matter but it has many more stars then Dragonfly 44.
University of Toronto study co-author Roberto Abraham said the discovery posed more questions than answers.
“We have no idea how galaxies like Dragonfly 44 could have formed.
“The data show that a relatively large fraction of the stars are in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”
The galaxy is believed to be one of dozens of dim “shadow” galaxies – known as ultra diffuse galaxies – in the cluster. One theory suggests these dim space phenomena may be “failed” galaxies.