On the origin of surprisingly cold gas discs in galaxies at high redshift

TL;DR

This study uses high-resolution simulations to explain the existence of unexpectedly cold, thin gas discs in high-redshift galaxies, highlighting the role of cold cosmic-web streams and the importance of the gas tracer used in kinematic analysis.

Contribution

It demonstrates that cold, thin gas discs at high redshift can form from co-planar accretion and survive for several orbital periods before disruption, emphasizing the impact of gas phase and observational tracers.

Findings

Cold gas discs form from co-planar cosmic-web streams.

The ratio v_phi/σ_r varies significantly with gas tracer.

Discs survive for about 5 orbital periods before merging disrupt them.

Abstract

We address the puzzling observational indications for very "cold" galactic discs at redshifts $z \gtrsim 3$, an epoch when discs are expected to be highly perturbed. Using a high-resolution cosmological zoom-in simulation, we identify such a cold disc at $z\sim 3.5$, with a rotation velocity to velocity dispersion ratio of $v_\phi/\sigma_r \simeq 5$ for the total gas. It forms as a result of a period of intense accretion of co-planar, co-rotating gas via cold cosmic-web streams. This thin disc survives for $\sim 5$ orbital periods, after which it is disrupted by mergers and counter-rotating streams, longer but consistent with our estimate that a galaxy of this mass ($M_\star\sim10^{10}\mathrm{M_\odot}$) typically survives merger-driven spin flips for $\sim 2-3$ orbital periods. We find that $v_\phi/\sigma_r$ is highly sensitive to the tracer used to perform the kinematic analysis. While it is $v_\phi/\sigma_r \simeq 3.5$ for atomic HI gas, it is $v_\phi/\sigma_r \simeq 8$ for molecular CO and H$_2$. This reflects the confinement of molecular gas to cold, dense clouds that reside near the disc mid-plane, while the atomic gas is spread into a turbulent and more extended thicker disc. The proposed mechanisms is a theoretical proposal that has not been validated yet with proper statistical measurements and it remains unclear whether it occurs frequently enough to explain the multiple discoveries of cold gas disks in high-z galaxies.