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

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
e-mail: mahsa.kohandel@sns.it
² INAF – Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
³ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark

Received: 9 October 2023
Accepted: 9 February 2024

Abstract

Context. Theoretical models struggle to reproduce dynamically cold disks with significant rotation-to-dispersion support (V_rot/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 σ_Hα > 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 V_rot/σ > 10 at z > 4, maintaining cold disks for > 10 orbital periods (∼ 200 Myr). Furthermore, we find no significant redshift dependence for the V_rot/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.