A giant galaxy in the young Universe with a massive ring

In the local (redshift z ≈ 0) Universe, collisional ring galaxies make up only ~0.01% of galaxies 1 and are formed by head-on galactic collisions that trigger radially propagating density waves 2 – 4 . These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe 5 – 9 . However, not much is known about distant ( z > 0.1) collisional rings 10 – 14 . Here we present a detailed study of a ring galaxy at a look-back time of 10.8 Gyr ( z = 2.19). Compared with our Milky Way, this galaxy has a similar stellar mass, but has a stellar half-light radius that is 1.5–2.2 times larger and is forming stars 50 times faster. The extended, diffuse stellar light outside the star-forming ring, combined with a radial velocity on the ring and an intruder galaxy nearby, provides evidence for this galaxy hosting a collisional ring. If the ring is secularly evolved 15 , 16 , the implied large bar in a giant disk would be inconsistent with the current understanding of the earliest formation of barred spirals 17 – 21 . Contrary to previous predictions 10 – 12 , this work suggests that massive collisional rings were as rare 11 Gyr ago as they are today. 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.