Abstract

--- - Context. Observations of the rest-frame UV emission of high-redshift galaxies suggest that the early stages of galaxy formation involve disturbed structures. Imaging the cold interstellar medium (ISM) can provide a unique view of the kinematics associated with the assembly of galaxies. - Aims. In this paper, we analyze the spatial distribution and kinematics of the cold ionized gas of the normal star-forming galaxy COS-2987030247 at z = 6.8076, based on new high-resolution observations of the [C II] 158 mu m line emission obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). - "Methods. The analysis of these observations allowed us to: compare the spatial distribution and extension of the [C II] and rest-frame UV emission, model the [C II] line data-cube using the (3D)BAROLO code, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions." - Results. The system is found to be composed of a main central source, a fainter north extension, and candidate [C II] companions located 10-kpc away. We find similar rest-frame UV and [C II] spatial distributions, suggesting that the [C II] emission emerges from the star-forming regions. The agreement between the UV and [C II] surface brightness radial profiles rules out diffuse, extended [C II] emission (often called a [C II] halo) in the main galaxy component. The [C II] velocity map reveals a velocity gradient in the north-south direction, suggesting ordered motion, as commonly found in rotating-disk galaxies. However, higher resolution observations would be needed to rule out a compact merger scenario. Our model indicates an almost face-on galaxy (i similar to 20 degrees), with a average rotational velocity of 86 +/- 16 km s(-1) and a low average velocity dispersion, sigma < 30 km s(-1). This result implies a dispersion lower than the expected value from observations and semi-analytic models of high redshift galaxies. Furthermore, our measurements indicate that COS-2987030247 and its individual regions systematically lie within the local L-[CII]-SFR relationship, yet slightly below the local Sigma([CII])-Sigma(UV) relation. - Conclusions. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will possibly become perturbed at later times by accreting sources.