Astronomers have observed a rare galaxy type as it existed 11 billion years ago, and this, they say, is likely to shake up theories about the earliest formation of galactic structures and how they evolve.
Shaped like a gigantic doughnut and described as a “cosmic ring of fire”, galaxy R5519 is roughly the mass of the Milky Way and 11 billion light-years away. Evidence suggests it is a collisional ring galaxy, which would make it the first ever located in the early Universe.
An artist’s impression of how the ring galaxy formed. Credit: James Josephides, Swinburne Astronomy Productions
“Contrary to previous predictions, this work suggests that massive collisional rings were as rare 11 Gyr ago as they are today,” the researchers write in a paper in the journal Nature Astronomy.
“Our discovery offers a unique pathway for studying density waves in young galaxies, as well as constraining the cosmic evolution of spiral disks and galaxy groups.”
R5519 is a “very curious object”, says lead researcher Tiantian Yuan from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) – it “looks strange and familiar at the same time”.
It’s also big – the doughnut’s hole has a diameter two billion times the distance between the Earth and the Sun – and productive.
“It is making stars at a rate 50 times greater than the Milky Way,” says Yuan. “Most of that activity is taking place on its ring, so it truly is a ring of fire.”
Yuan led a team of researchers from Australia, the US, Canada, Belgium and Denmark, that used spectroscopic data gathered by the WM Keck Observatory in Hawaii and images recorded by NASA’s Hubble Space Telescope to identify R5519.
There are two types of ring galaxies. The more common is the result of internal processes, while collisional galaxies, as the name suggests, form as a result of immense and violent encounters with other galaxies.
Co-author Kenneth Freeman from the Australian National University says the discovery of R5519 has implications for understanding how galaxies like the Milky Way formed.
“The collisional formation of ring galaxies requires a thin disc to be present in the victim galaxy before the collision occurs,” he says.
“The thin disc is the defining component of spiral galaxies: before it assembled, the galaxies were in a disorderly state, not yet recognisable as spiral galaxies.
“In the case of this ring galaxy, we are looking back into the early Universe by 11 billion years, into a time when thin discs were only just assembling. For comparison, the thin disc of our Milky Way began to come together only about nine billion years ago.
“This discovery is an indication that disc assembly in spiral galaxies occurred over a more extended period than previously thought.”
Nick Carne
Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.
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