2022A&A...665L...8H

We present a kinematic analysis of the main-sequence galaxy HZ4 at z = 5.5. Our study is based on deep, spatially resolved observations of the [CII] 158 µm transition obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA). From the combined analysis of the disk morphology, the 2D velocity structure, and forward modeling of the 1D velocity and velocity dispersion profiles, we conclude that HZ4 has a regular rotating disk in place. The intrinsic velocity dispersion in HZ4 is high (σ₀=65.8_–3.3^+2.9km/s), and the ratio between the rotational velocity and the intrinsic velocity dispersion is V_rot/σ₀ = 2.2. These values are consistent with the expectations from the trends of increasing σ₀ and decreasing V_rot/σ₀ as a function of the redshift observed in main-sequence galaxies up to z ≃ 4. Galaxy evolution models suggest that the high level of turbulence observed in HZ4 can only be achieved if, in addition to stellar feedback, there is radial transport of gas within the disk. Finally, we find that HZ4 is baryon-dominated on galactic scales (≤2 x R_e), with a dark-matter fraction at one effective radius of f_DM(R_e) = 0.41_–0.22^+0.25. This value is comparable to the dark-matter fractions found in lower redshift galaxies that could be the descendants of HZ4: massive (M_* ≃ 10¹¹ M_☉), star-forming galaxies at z ∼ 2, and passive, early-type galaxies at z ≃ 0.