A giant galaxy in the young Universe with a massive ring

摘要

In the local (redshift iz  ≈ 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~(2,3,4). 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 (iz   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 (iz  = 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~(17,18,19,20,21). Contrary to previous predictions~(10,11,12), 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.