Nearly 11 billion light years away lurks a super-rare type of galaxy that bears the scars of a galactic battle. Rather than the more common disc of stars the galaxy instead has a gigantic hole straight through the centre. And with the outskirts making stars 50 times faster than the Milky Way, it’s been dubbed the “Ring of Fire”.
The donut-shaped galaxy named R5519, which has roughly the mass of the Milky Way, was discovered in data from the Hubble Space Telescope and the WM Keck Observatory in Hawaii by an international team of researchers, including from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D).
“It is a very curious object that we’ve never seen before,” says ASTRO 3D’s Dr Tiantian Yuan, who led the research.
“It looks strange and familiar at the same time.”
Epic collision punched a hole through the galaxy
There are two kinds of ring galaxies. The more common type forms because of internal processes. However, R5519 is the rarer type which form as a result of immensely violent collisions with other galaxies. These collisional ring galaxies make up only 0.01% of galaxies in our local universe – this one was found in the early universe, nearly 11 billion light years from Earth.
Examining the data, the researchers identified a nearby companion galaxy, dubbed G5593. Around 40 million years ago, the “intruder” galaxy G5593 collided head-on with R5519, say the researchers, punching a hole through the centre and creating the donut shape seen today.
This collision sent density waves outwards through the galaxy, condensing gas and dust and triggering star formation. That burst of star formation, in turn, makes the ring appear bright.
“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.”
The hole at its centre is truly massive, with a diameter two billion times longer than the distance between the Earth and the Sun. That also makes it three million times bigger than the diameter of the supermassive black hole in the galaxy Messier 87, which in 2019 became the first ever to be directly imaged.
Changing our impression of thin disc galaxies
R5519’s tumultuous history can help astronomers understand the history of other galaxies.
“The collisional formation of ring galaxies requires a thin disk to be present in the ‘victim’ galaxy before the collision occurs,” says Professor Kenneth Freeman from the Australian National University. Freeman was also involved in the research.
“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.”
It was thought that galaxies in the early universe existed in those disordered states, and that they didn’t form into more organised thin discs until later after the Big Bang. However the discovery of this galaxy, which would have been a thin discs before the collision, suggests that organisation may have been occurring earlier than we expected.
“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,” says Freeman.
The discovery of the ring galaxy also helps change the impression we had of the early universe.
While other collisional ring galaxies have been found previously in the local universe, R5519’s extreme distance away provides a glimpse into these galaxies in the early universe just 3 billion years after the Big Bang.
Previous predictions had suggested that as the early universe was far more compact than it was today, these types of collisions and subsequent ring galaxies would be more common. However, the researchers write that the scarcity of galaxies like R5519 shows that massive collisional rings were probably as rare 11 billion years ago as they are today.
But that doesn’t mean we’ll have to wait to find another. CNET’s Jackson Ryan reports that Yuan may have found yet another ring galaxy formed by a collision – and it’s even older.
This article was first published on Australia’s Science Channel, the original news platform of The Royal Institution of Australia.