  The ALMA [CII] Resolved Ism in STar-forming gALaxies (CRISTAL) survey is a
Cycle 8 ALMA Large Programme that studies the cold gas component of
high-redshift galaxies. Its sub-arcsecond resolution observations are key to
disentangling physical mechanisms that shape galaxies during cosmic dawn. In
this paper, we explore the morphology and kinematics of the cold gas,
star-forming, and stellar components in the star-forming main-sequence galaxy
CRISTAL-05/HZ3, at z = 5.54. Our analysis includes 0.3" spatial resolution (~2
kpc) ALMA observations of the [CII] line. While CRISTAL-05 was previously
classified as a single source, our observations reveal that the system is a
close interacting pair surrounded by an extended component of carbon-enriched
gas. This is imprinted in the disturbed elongated [CII] morphology and the
separation of the two components in the position-velocity diagram (~100 km/s).
The central region is composed of two components, named C05-NW and C05-SE, with
the former being the dominant one. A significant fraction of the [CII] arises
beyond the close pair up to 10 kpc, while the regions forming new massive stars
and the stellar component seem compact (r_[CII] ~ 4 r_UV), as traced by
rest-frame UV and optical imaging obtained with the Hubble Space Telescope and
the James Webb Space Telescope. Our kinematic model, using the DYSMALpy
software, yields a minor contribution of dark matter of C05-NW within a radius
of ~2x Reff. Finally, we explore the resolved [CII]/FIR ratios as a proxy for
shock-heating produced by this merger. We argue that the extended [CII]
emission is mainly caused by the merger, which could not be discerned with
lower-resolution observations. Our work emphasizes the need for high-resolution
observations to fully characterize the dynamic stages of infant galaxies and
the physical mechanisms that drive the metal enrichment of the circumgalactic
medium.
