Astronomers have discovered an extremely rare “ring galaxy” in the early universe where stars are forming terrifically fast at the rim, but with nothing at the center. Similar-looking galaxies exist today, but models of galaxy formation need reworking to accommodate one just 3 billion years after the Big Bang.
Among nearby galaxies we find many with a ring of stars surrounding central darkness. The center may be black, but it is vastly larger than any black hole. Instead it represents a relative absence of stars, usually resulting from the galaxy's internal processes. One time in a thousand, however the shape arises when a small galaxy punches its way through the middle of a larger one, sending a ripple outwards like a pebble in a still pond.
Dr Tiantian Yuan Australia's Swinburne University led a team that went looking for similar shapes billions of years ago. After examining a vast assortment of young galaxies they found only one that appears to fit, named R5519, although naturally determining the shapes of such extraordinarily distant objects presents major challenges.
Yuan told IFLScience it was initially expected collision ring galaxies would be more common when galaxies were more closely packed. However, considering R5519's unique status, the team concluded in Nature Astronomy even one galaxy of this sort took some unexpected circumstances.
For a collision like this to happen, Yuan explained, the impacting galaxy had to hit R5519’s bullseye at a 90-degree angle. Unlikely as that was, a ring could only form if R5519 was very thin. “If the Milky Way was the size of your hand, it would be as thin as a CD,” Yuan noted to IFLScience. Despite the recent discovery of an even earlier thin galactic disk, Yuan said models of galactic formation will need some adjustment to allow R5519 to gain the necessary shape so soon after formation.
On the other hand, Yuan noted, the models would have even more trouble explaining an internally produced ring galaxy in that time. Yuan added there are several other reasons to be confident R5519's shape results from a collision. “We have captured the 'criminal galaxy' in action,” she told IFLScience. The two remain close enough they are still gravitationally interacting. “[R5519] is making stars at a rate 50 times greater than the Milky Way” Yuan said in a statement, despite the two having similar mass.
The team have also found what appears to be the galactic nucleus thrown into the ring through the impact.
Finding the R5519's supermassive black hole within the nucleus would confirm the theory, which Yuan says will take the capacities of the James Webb Space Telescope. If her theory is right, Yuan anticipates R5519 will evolve into something like the giant galaxy M87 with its ultrapowerful black hole, although she does not expect humanity to survive to see this come to pass.