A stellar bar hidden in an extreme gas-rich disk galaxy at z=4.055

TL;DR

This paper reports the discovery of a stellar bar in a highly gas-rich galaxy at z=4.055, challenging existing theories that gas-richness inhibits early bar formation and suggesting new pathways for early galaxy evolution.

Contribution

It presents the first detection of a stellar bar in a gas-rich, baryon-dominated galaxy at high redshift, indicating rapid bar formation in early, gas-rich disks.

Findings

GN20 is baryon-dominated with 70% dark matter and 75% gas.

The galaxy's stellar bar exists only 1.5 billion years after the Big Bang.

Gas-rich disks can support rapid stellar bar formation in the early Universe.

Abstract

The classical picture for the formation of stellar bars -- key dynamical drivers of the evolution of galaxies -- is through secular evolution of instability in gas poor, stellar-dominated disks. The detection with the James Webb Space Telescope (JWST) of stellar bars and spiral arms in galaxies at early cosmic times has thus challenged LambdaCDM-based expectations, which recent studies reconcile by suggesting that these galaxies are baryon-dominated and have already consumed most of their gas. Yet, a paradox arises, as early galaxies are expected to be increasingly rich in gas, which is generally considered to prevent or slow down stellar bar formation. Here, we show the detection of a stellar bar in GN20, a gas-rich star-forming disk galaxy at a redshift of z=4.055, only 1.5 billion years after the Big Bang. Simultaneous observations of the stars, gas, and dust reveal that GN20 is indeed baryon-dominated (over dark matter; 70+/-30%), but the baryonic mass is largely in the form of gas (75+/-25%). This discovery demonstrates that gas-rich disks do support rapid stellar bar formation in the early Universe, motivating a new theoretical perspective on bar formation in gas-rich systems, and providing a potential new mechanism for very early galaxy assembly and quenching.