Galaxies are burnin' things and sometimes they make a fiery ring.
A violent, catastrophic collision between two galaxies has given rise to an extremely rare ring galaxy, lurking some 11 billion light years from the Earth. The monstrous, donut-shaped galaxy is making stars in its huge ring at a rate 50 times faster than our home galaxy, earning it an ominous moniker Johnny and June Cash would surely dig: The cosmic "ring of fire."
In a paper, published in the journal Nature Astronomy on Tuesday, an international team of scientists detail the ring galaxy R5519, discovered after scouring data from the Hubble Space telescope and the W.M. Keck observatory in Hawaii. Among almost 4,000 galaxies detected in the data sets, R5519 was one of the brightest and displayed a clear ring structure. So the team investigated further -- and quickly realized they'd found something unusual.
"It is very a curious object, one that we have never seen before," says Tiantian Yuan, an astronomer at Swinburne University in Australia and first author on the study. "The gigantic hole in this galaxy was caused by a head-on collision with another galaxy."
Probing the features of R5519, Yuan and her team began picking up clues as to how it formed. They ruled out gravitational lensing or a galaxy merger for its unusual structure and nearby, they detected a companion galaxy -- G5593. They suspect this cosmic neighbor is the "intruder" galaxy that may have collided with R5519 around 40 million years ago.
The two galaxies must have smashed into each other pretty much head-on -- a galactic bulls-eye -- and it's likely there was already a disk of stars present in R5519. As G5593 came swooping through the galaxy, it split the disk through the guts and a wave of stars expanded from the center, as seen in the GIF above.
"The collisional formation of ring galaxies requires a thin disk to be present in the 'victim' galaxy before the collision occurs," said Kenneth Freeman, an astronomer at Australian National University and co-author on the paper, in a statement.
If R5519 is caused by a huge collision, that would make it an extremely rare cosmic phenomena. Only one in every 10, 000 galaxies in the local universe are formed in such a way. Notably, the early universe was much more crowded so the belief was these kind of collisions may have been more common. Yuan suggests the data is telling a different story.
"Previously, people think we would find more of these collisional ring galaxies in the young universe, simply because there are more collisions back then," she says. "We find that is not the case."
There are still some "unsolved puzzles" about the ring of fire, Yuan says. "We do not know if this ring was a first ring after the collision or it was the second ring." She's obtained further data from W.M. Keck to resolve this issue.
Astronomers will have to gather more data to be certain the ring is caused by a collision, rather than through natural evolution. The authors of the paper write the imaging performed by NASA's soon-to-be-launched (and recently-assembled) James Webb Space Telescope will be able to resolve any lingering questions. Yuan says she has already discovered another ring galaxy likely formed by a head-on collision -- and this is a billion years older than the "ring of fire."