Dynamically cold disks in the early Universe: Myth or reality?

Context. Theoretical models struggle to reproduce dynamically cold disks with significant rotation-to-dispersion support (Vrot/σ) observed in star-forming galaxies in the early Universe at redshift z > 4. Aims. We aim to explore the possible emergence of dynamically cold disks in cosmological simulations and to understand whether different kinematic tracers can help reconcile the tension between theory and observations. Methods. We used 3218 galaxies from the Serra suite of zoom-in simulations, with 8 ≤ log (M?/M ) ≤ 10.3 and star formation rates SFR ≤ 128 M /yr, within the 4 ≤ z ≤ 9 range. We generated hyperspectral data cubes for 2 × 3218 synthetic observations of Hα and [CII]. Results. We find that the choice of kinematic tracer strongly influences gas velocity dispersion (σ) estimates. In Hα ([C ii]) synthetic observations, we observe a strong (mild) correlation between σ and M?. This difference mostly arises for M? > 109 M galaxies, for which σHα > 2σ[CII] for a significant fraction of the sample. Regardless of the tracer, our predictions suggest the existence of massive (M? > 1010 M ) galaxies with Vrot/σ > 10 at z > 4, maintaining cold disks for >10 orbital periods (∼200 Myr). Furthermore, we find no significant redshift dependence for the Vrot/σ ratio in our sample. Conclusions. Our simulations predict the existence of dynamically cold disks in the early Universe. However, different tracers are sensitive to different kinematic properties. While [C ii] effectively traces the thin gaseous disk of galaxies, Hα includes the contribution from ionized gas beyond the disk region, characterized by prevalent vertical or radial motions that may be associated with outflows. We show that Hα halos could be a signature of these galactic outflows. This result emphasizes the importance of combining ALMA and JWST/NIRspec studies of high-z galaxies.