A dynamically cold disk galaxy in the early Universe

TL;DR

This paper reports the discovery of a young, high-redshift galaxy with a surprisingly cold, stable rotating disk, challenging the expectation that early galaxies are predominantly chaotic and dynamically hot.

Contribution

It provides the first detailed observation of a dynamically cold, rotating disk galaxy at redshift 4.2, showing early galaxies can resemble local spiral galaxies despite their youth.

Findings

Galaxy exhibits a high V/σ ratio of 9.7, similar to local spirals.

Rotation curve resembles that of nearby massive spiral galaxies.

Galaxy's properties align with current numerical simulations and observations.

Abstract

The extreme astrophysical processes and conditions that characterize the early Universe are expected to result in young galaxies that are dynamically different from those observed today. This is because the strong effects associated with galaxy mergers and supernova explosions would lead to most young star-forming galaxies being dynamically hot, chaotic and strongly unstable. Here we report the presence of a dynamically cold, but highly star-forming, rotating disk in a galaxy at redshift ($z$) 4.2, when the Universe was just 1.4 billion years old. Galaxy SPT-S J041839-4751.9 is strongly gravitationally lensed by a foreground galaxy at $z = 0.263$, and it is a typical dusty starburst, with global star-forming and dust properties that are in agreement with current numerical simulations and observations of its galaxy population. Interferometric imaging at a spatial resolution of about 60 pc reveals a ratio of rotational-to-random motions of $V/\sigma = 9.7\pm 0.4$, which is at least four times larger than expected from any galaxy evolution model at this epoch, but similar to the ratios of spiral galaxies in the local Universe. We derive a rotation curve with the typical shape of nearby massive spiral galaxies, which demonstrates that at least some young galaxies are dynamically akin to those observed in the local Universe, and only weakly affected by extreme physical processes.