A giant galaxy in the young Universe with a massive ring

TL;DR

This paper reports the discovery of a massive collisional ring galaxy at redshift 2.19, providing insights into galaxy evolution, density waves, and the rarity of such structures in the early universe.

Contribution

It presents the first detailed study of a high-redshift collisional ring galaxy, challenging previous assumptions about their evolution and frequency.

Findings

The galaxy hosts a large, diffuse stellar light outside the ring.

The galaxy's properties suggest a recent collisional event.

Massive collisional rings were as rare 11 Gyr ago as today.

Abstract

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.