Structure and kinematics of a massive galaxy at z ~ 7

Abstract: 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 can provide a unique view of the kinematics associated with the assembly of galaxies. In this paper, we analyzed 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] 158um line emission obtained with the Atacama Large Millimeter/submillimeter Array. 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 3DBarolo, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions. 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 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. But higher-resolution observations would be needed to rule out a compact merger scenario. Our model indicates 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. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will be possibly perturbed at later times by accreting sources.