Letter to the Editor

Kiloparsec view of a typical star-forming galaxy when the Universe was ∼1 Gyr old

II. Regular rotating disk and evidence for baryon dominance on galactic scales

¹ Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
e-mail: rhc@astro-udec.cl
² Max-Planck-Institut für Extraterrestische Physik (MPE), Giessenbachstr., 85748 Garching, Germany
³ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Avenue, Cambridge CB3 0HE, UK
⁴ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶ Centre for Astrophysics and Supercomputing, Swinburne Univ. of Technology, PO Box 218 Hawthorn, VIC 3122, Australia
⁷ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
⁸ Max-Planck Institute for Astrophysics, Karl Schwarzschildstrasse 1, 85748 Garching, Germany
⁹ School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978, Israel
¹⁰ Center for Computational Astrophysics, 162 5th Ave., New York, NY 10010, USA
¹¹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Received: 2 November 2021
Accepted: 16 February 2022

Abstract

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 [C II] 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^+2.9 km s⁻¹), 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 × 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.