We present high-resolution (arcsec = 710 pc) Atacama Large Millimetre/submillimetre Array [C ii] 158 m and dust continuum follow-up observations of REBELS-25, a [C ii]-luminous () galaxy at redshift. These high-resolution, high signal-To-noise observations allow us to study the sub-kpc morphology and kinematics of this massive () star-forming (SFR) galaxy in the Epoch of Reionization. By modelling the kinematics with BAROLO, we find it has a low-velocity dispersion (km s) and a high ratio of ordered-To-random motion (), indicating that REBELS-25 is a dynamically cold disc. Additionally, we find that the [C ii] distribution is well fit by a near-exponential disc model, with a Sersic index, n, of, and we see tentative evidence of more complex non-Axisymmetric structures suggestive of a bar in the [C ii] and dust continuum emission. By comparing to other high spatial resolution cold gas kinematic studies, we find that dynamically cold discs seem to be more common in the high-redshift Universe than expected based on prevailing galaxy formation theories, which typically predict more turbulent and dispersion-dominated galaxies in the early Universe as an outcome of merger activity, gas accretion, and more intense feedback. This higher degree of rotational support seems instead to be consistent with recent cosmological simulations that have highlighted the contrast between cold and warm ionized gas tracers, particularly for massive galaxies. We therefore show that dynamically settled disc galaxies can form as early as 700 Myr after the big bang.

We present Atacama Large Millimeter/Submillimeter Array (ALMA) [C ii] and ∼158 continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; LIR = L⊙) at z = 7.31, spectroscopically confirmed by the Reionization Era Bright Emission Line Survey (REBELS) ALMA Large Programme. REBELS-25 has a significant stellar mass of. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR. This is about four times the SFR estimated from an extrapolated main sequence. We also infer a [C ii]-based molecular gas mass of, implying a molecular gas depletion time of Gyr. We observe a [C ii] velocity gradient consistent with disc rotation, but given the current resolution we cannot rule out a more complex velocity structure such as a merger. The spectrum exhibits excess [C ii] emission at large positive velocities (∼500 km s-1), which we interpret as either a merging companion or an outflow. In the outflow scenario, we derive a lower limit of the mass outflow rate of 200, which is consistent with expectations for a star-formation-driven outflow. Given its large stellar mass, SFR, and molecular gas reservoir ∼700 Myr after the big bang, we explore the future evolution of REBELS-25. Considering a simple, conservative model assuming an exponentially declining star formation history, constant star formation efficiency, and no additional gas inflow, we find that REBELS-25 has the potential to evolve into a galaxy consistent with the properties of high-mass quiescent galaxies recently observed at z ∼4.