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

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.