An extended [C II] halo around a massive star-forming galaxy at z = 5.3

High-redshift observations are often biased towards massive and bright galaxies that do not necessarily represent the full population. In order to accurately study galaxy evolution and mass assembly at these redshifts, observations of ‘normal’ main sequence galaxies are required. Here we present Atacama Large Millimeter/Submillimeter Array (ALMA) 0.3 arcsec resolution observations of the [C II] emission line at 158 μm of HZ7, a main sequence galaxy at z = 5.25. Comparing to archival rest-frame UV observations taken by the Hubble Space Telescope (HST), we find strong evidence of the existence of extended [C II] emission, which we estimate to be twice the size of the rest-frame UV emission, yielding one of the first high-redshift objects where a clear signature of a [C II] ‘Halo’ has been detected to date. For a matched Sérsic profile with n = 1, we measured a [C II] effective radius of 0.50 ± 0.04 arcsec (3.07 ± 0.25 kpc) and an average rest-frame UV effective radius of 0.2 ± 0.04 arcsec (1.48 ± 0.16 kpc). The [C II] morphology and kinematics of the system suggest a merging event resulting in a non-rotating disc system. This event could be responsible for the extended [CII] emission. Alternatively, some potential obscured emission could also explain the [C II] to UV size ratio. These results contribute to the growing consensus with respect to the existence of extended [C II] emission around galaxies.