2025A&A...699A...5T

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.