Deep inside the Coma constellation lurks a loose collection of dim stars that, at first, makes little sense.
Using some of the planet's most powerful telescopes, astronomers decided to zoom-in on the galactic oddity called Dragonfly 44 to find it wasn't one of your run-of-the-mill galaxies, it is in fact a galaxy rammed full of material we can't see.
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"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 astronomer Pieter van Dokkum, of Yale University and lead author of a new paper published in the Astrophysical Journal Letters.
Using the Keck Observatory in Hawaii, van Dokkum and his team spent six nights clocking the speeds of the few stars that could be seen in the blob of galactic material. Then, using the Gemini North telescope, also on Hawaii, they spotted a halo of spherical clusters of stars around the galaxy's core, much like the halo that surrounds our Milky Way's core.
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With this information, the astronomers deduced that there is a lot more matter inside the galaxy than meets the eye. To explain the high speeds of the stars buzzing around inside Dragonfly 44, the vast majority of mass must be held in dark matter. This galaxy is a dark matter galaxy where normal matter (i.e. stars) are not only in the minority, they are very rare.
"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," said collaborator Roberto Abraham of the University of Toronto.
It turns out that Dragonfly 44 is of the equivalent mass to our Milky Way, but only 0.01% of that mass is normal matter. It is a galaxy that is 99.99% dark matter. It is well known that 85% of the universe is dark matter, but to find such a huge galaxy so rich in the mysterious stuff is extremely rare. Although smaller galaxies rich in dark matter have been spotted before, Dragonfly 44 is by far the biggest.
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"We have no idea how galaxies like Dragonfly 44 could have formed," said Abraham. "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."
Huge efforts are currently underway to work out what dark matter is, so finding such a unique dark matter galaxy may help astrophysicists work out what it is really made of.
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