In the local ( z \approx 0 ) Universe , collisional ring galaxies make up only \sim 0.01 % of galaxies [ 1 ] and are formed by head-on galactic collisions that trigger radially propagating density waves [ 2 , 3 , 4 ] .
These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe [ 5 , 6 , 7 , 8 , 9 ] .
However , not much is known about distant ( z > 0.1 ) collisional rings [ 10 , 11 , 12 , 13 , 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 of 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 , 18 , 19 , 20 , 21 ] .
Contrary to previous predictions [ 10 , 12 , 11 ] , 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 .