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In the local (redshift z~0) Universe, collisional ring galaxies make up only~0.01% of galaxies and are formed by head-on galactic collisions that triggerradially propagating density waves. These striking systems provide keysnapshots for dissecting galactic disks and are studied extensively in thelocal Universe. However, not much is known about distant (z>0.1) collisionalrings. Here we present a detailed study of a ring galaxy at a look-back time of10.8 Gyr (z=2.19). Compared with our Milky Way, this galaxy has a similarstellar mass, but has a stellar half-light radius that is 1.5-2.2 times largerand is forming stars 50 times faster. The large, diffuse stellar light outsidethe star-forming ring, combined with a radial velocity on the ring and anintruder galaxy nearby, provides evidence for this galaxy hosting a collisionalring. If the ring is secularly evolved, the implied large bar in a giant diskwould be inconsistent with the current understanding of the earliest formationof barred spirals. Contrary to previous predictions, this work suggests thatmassive collisional rings were as rare 11 Gyr ago as they are today. Ourdiscovery offers a unique pathway for studying density waves in young galaxies,as well as constraining the cosmic evolution of spiral disks and galaxy groups.

A giant galaxy in the young Universe with a massive ring