Canadian astronomer discovers Milky Way-size galaxy that's 99.99% dark matter
Dragonfly 44 is almost entirely invisible, University of Toronto prof and UBC astronomy grad tells Metro, but it has the mass of our own galaxy.
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Many a discovery or idea was hatched over a pint of ale: DNA, the existence of carbon dioxide, Pet Rocks among them.
But a “beery” conversation four years ago has led two astronomers, including a Canadian observational cosmologist, to stumble upon a gigantic galaxy that’s as heavy as our own — but has almost no visible stars.
It’s named “Dragonfly 44,” and University of Toronto’s Roberto Abraham estimates it is made of 99.99 per cent dark matter, a mysterious cosmic material only observable through its gravitational effects.
“This is a really massive galaxy, about same mass as our Milky Way, but it’s almost pure dark matter,” Abraham, who completed an astronomy degree at the University of B.C., told Metro in a phone interview. “Dragonfly 44 is a different type of galaxy.
“You could think of almost like a cloud that’s enormous but hardly has any stars in it. Such things shouldn’t really exist.”
Scientists have known about dark matter since the 1970s, he explained, but despite many “wildly speculative” theories amongst scholars no one knows what it actually is yet.
Even our own Milky Way is believed to contain some dark matter at its core. But Dragonfly 44 is the largest galaxy ever discovered made up that’s mostly dark matter, he said.
“Galaxies like this should not be found, and they shouldn’t last,” he said. “Think of it as just seeing the frosting on the cake but not seeing the cake.”
As for the name “Dragonfly 44,” Abraham explained that it’s because he and his colleague, Yale astronomer Pieter van Dokkum, found it using the Dragonfly Telephoto Array — a telescope they invented using 48 commercial Canon telephoto lenses joined together, which they hoped to use to study how galaxies form.
“We came up with the idea of using lenses for a telescope over a beery conversation about four years ago,” he said. “It’s an idea that went out of fashion a century ago, but is a pretty novel concept that’s enabling us to go after objects that are otherwise undetectable.
“This (discovery) emerged more or less by accident … We thought, ‘Hang on, there’s nowhere near enough stars for this thing to have all this mass coming from them.’”
After spotting Dragonfly 44, they were able to take a closer look at Hawaii’s Keck Observatory, using the powerful Gemini North telescope. The pair’s findings were published in the September edition of The Astrophysical Journal Letters.