THE Milky Way has a dark twin. A dimly lit massive galaxy called Dragonfly 44 is a record 99.99 per cent dark matter, and could help rewrite our theories of galaxy formation.
Dragonfly 44 is the Milky Way’s equal in mass, but its opposite in star number and structure.
“If you take the Milky Way and for every 100 stars you keep only one, then you’re getting pretty close,” says Pieter van Dokkum at Yale University. “You also have to put those remaining stars in a blender and mix them all up into a blob.” This galaxy doesn’t have the iconic spiral structure of the Milky Way, nor is it a flat disc.
And it’s not alone. Van Dokkum and his colleagues spotted it and its neighbours in 2014, when they aimed the unique Dragonfly observatory at the Coma cluster. They found 47 faint smudges: galaxies that could be at least as large as the Milky Way, but which contain so few stars that they glow as dimly as dwarf galaxies.
To find out how they became so sparse, van Dokkum and his colleagues tracked how fast the stars in Dragonfly 44 move around the galaxy, and so calculated its mass: a faster speed means a more massive galaxy.
The stars clocked in at 47 kilometres per second, making Dragonfly 44 a trillion times more massive than the sun. With so little normal matter, it must be 99.99 per cent dark matter to hold itself together (Astrophysical Journal Letters, doi.org/bpzf).
It even beats another similarly dark galaxy in the Virgo cluster discovered earlier this year, which is 99.96 per cent dark matter.
This article appeared in print under the headline “Ghostly dragonfly is 99.99 per cent dark matter”