The Milky Way has a dark twin. A dimly lit massive galaxy, called Dragonfly 44, consists of a record 99.99 per cent dark matter, and could help rewrite our theories of galaxy formation.
Dragonfly 44 is the Milky Way’s doppelgänger in mass, but its opposite in number of stars 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.
A dragonfly’s eye
And it’s not alone. Van Dokkum and his colleagues spotted it and its neighbours in 2014 with an array of telephoto lenses.
“By pulling them all together into this array, which makes them look like the eyes of an insect, we gather a lot more light,” says van Dokkum. Naturally, they called it Dragonfly.
When the team aimed Dragonfly at the Coma cluster, an immense cluster of galaxies 320 million light years away, they detected 47 faint smudges: galaxies that could be at least as large as the Milky Way, 100,000 light years from end to end, but which contain so few stars that they glow as dimly as dwarf galaxies.
There are two explanations for the appearance of these galaxies: they may be tightly bound by dark matter, the so-far-undetected substance thought to make up about 85 per cent of the mass of the universe. Either that, or they’re unstable – and the violent Coma cluster is currently ripping them to shreds.
How can it have so much dark matter?
To find out, van Dokkum and his colleagues observed Dragonfly 44, one of the largest of the galaxies, with the spectrograph on board the 10-metre Keck II telescope on Mauna Kea, Hawaii. This allowed the team to track how fast its few stars move around the galaxy and therefore calculate its mass: a faster speed means a more massive galaxy.
The team found that the stars clocked in at 47 kilometres per second, making Dragonfly 44 roughly a trillion times more massive than the sun. With so little normal matter, it must contains 99.99 per cent dark matter to hold itself together, which is much higher than the universe at large. It even beats another similarly dark galaxy in the Virgo cluster discovered earlier this year, which is 99.96 per cent dark matter.
But astronomers are baffled as to how these dark galaxies arise. “It’s hard to argue with the observations, yet the conclusion from this paper runs counter to my understanding of how galaxies are formed,” says Marla Geha at Yale University. “I’m hoping these objects are rather rare and/or only form in special environments such as a dense galaxy cluster. Otherwise we may need to rewrite galaxy formation.”