The ALMA-CRISTAL survey

Complex kinematics of galaxies at the end of the reionization era

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Ejército Libertador 441, 8370191 Santiago, Chile
² Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
³ Department of Physics and Astronomy and George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, USA
⁴ Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
⁵ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁸ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁹ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281 – S9, B-9000 Gent, Belgium
¹⁰ Institute of Astrophysics, Foundation for Research and Technology-Hellas (FORTH), Heraklion 70013, Greece
¹¹ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
¹² Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹³ Department of Astronomy, School of Science, SOKENDAI (The Graduate University for Advanced Studies), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁵ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto F.no (Firenze), Italy
¹⁶ INAF - Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50127 Firenze, Italy
¹⁷ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
¹⁸ Dept. Física Teórica y del Cosmos, Universidad de Granada, 18017 Granada, Spain
¹⁹ Instituto Universitario Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
²⁰ Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
²¹ Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan

^⋆ Corresponding author: k.telikova@gmail.com

Received: 13 November 2024
Accepted: 23 April 2025

Abstract

The history of gas assembly in the early galaxies is reflected in their complex kinematics. While a considerable fraction of galaxies at z ∼ 5 are consistent with rotating disks, recent studies indicate that the dominant galaxy assembly mechanism corresponds to minor and major mergers. Despite important progress, the dynamical classification of galaxies at these epochs is still severely limited by observations’ angular and spectral resolution. We present a detailed morphological and kinematic analysis of the far-infrared-bright main-sequence galaxy HZ10 (CRISTAL-22) at z = 5.65, making use of new sensitive high-resolution (≲0.3″) [C II] 158μm ALMA and rest-frame optical JWST/NIRSpec integral field spectroscopy observations. These observations reveal a previously unresolved complex morphology and kinematics of the HZ10 system. Using position-velocity diagrams, we confirm that HZ10 is not a single massive galaxy but instead consists of at least three components in close projected separation along the east-to-west direction. We find a [C II]-bright central component (C) separated by 1.5 kpc and 4 kpc from the east (E) and west (W) components, respectively. Our [C II] observations resolve the HZ10-C component, and we find a velocity gradient that could be produced by either rotation or a close-in merger. We tested the rotating disk possibility using DysmalPy kinematic modeling and the PVsplit tool. Based on this, we propose the most plausible dynamical scenario for HZ10: a double merger, where the companion galaxy HZ10-W merges with the disturbed rotating disk formed by the HZ10-C and HZ10-E components. Additionally, from the comparison between ALMA [C II] 158μm and JWST/NIRSpec data, we find that [C II] 158μm emission closely resembles the broad [O III] 5007Å emission both spatially and kinematically. The kinematic similarity reflects the interacting nature of the system and suggests that ionized and neutral gas phases in HZ10 are well mixed.