REBELS-25: Discovery of a dynamically cold disc galaxy at z = 7.31

We present high-resolution (similar to 0.14 arcsec = 710 pc) Atacama Large Millimetre/submillimetre Array [C ii] 158 mu m and dust continuum follow-up observations of REBELS-25, a [C ii]-luminous (L-[CII]=(1.7 +/- 0.2)x10(9)L(circle dot)) galaxy at redshift z=7.3065 +/- 0.0001. These high-resolution, high signal-to-noise observations allow us to study the sub-kpc morphology and kinematics of this massive (M-& lowast;=8(-2)(+4)x10(9)M(circle dot)) star-forming (SFRUV+IR=199(-63)(+101)M(circle dot)yr(-1)) galaxy in the Epoch of Reionization. By modelling the kinematics with (3D)BAROLO, we find it has a low-velocity dispersion (sigma((sic))=33(-7)(+9) km s(-1)) and a high ratio of ordered-to-random motion (V-rot,V- max/sigma((sic))=11(-5)(+6)), 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 S & eacute;rsic index, n, of 1.3 +/- 0.2, 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