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 and are formed by head-on galactic collisions that trigger
radially propagating density waves. These striking systems provide key
snapshots for dissecting galactic disks and are studied extensively in the
local Universe. However, not much is known about distant (z>0.1) collisional
rings. 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 large, 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, the implied large bar in a giant disk
would be inconsistent with the current understanding of the earliest formation
of barred spirals. Contrary to previous predictions, 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.
