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 an Atacama Large Millimeter/submillimeter Array (ALMA) survey of CO(4-3) line emitting galaxies in 17 quasar fields at z ∼4 aimed at performing the first systematic search of dusty galaxies in high-z quasar environments. Our blind search of galaxies around the quasars results in five CO emitters with S/N ≥ 5.6 within a projected radius of R ∼2 1.5 h -1 cMpc and a velocity range of δv = ±1000 km s-1 around the quasar. In blank fields, we expect to detect only 0.28 CO emitters within the same volume, implying a total overdensity of 17.6-7.6+11.9 in our fields, and indicating that quasars trace massive structures in the early universe. We quantify this overdensity by measuring the small-scale clustering of CO emitters around quasars, resulting in a cross-correlation length of r0,QG=8.37-2.04+2.42h-1 cMpc, assuming a fixed slope γ = 1.8. This contradicts the reported mild overdensities (x1.4) of Lyα emitters (LAEs) in the same fields at scales of R ∼2 7 h -1 cMpc, which are well described by a cross-correlation length 3.0-1.4+1.5 times lower than that measured for CO emitters. We discuss some possibilities to explain this discrepancy, including low star formation efficiency, and excess of dust in galaxies around quasars. Finally, we constrain, for the first time, the clustering of CO emitters at z ∼4, finding an autocorrelation length of r 0,CO = 3.14 ±1.71 h -1 cMpc (with γ = 1.8). Our work, together with the previous study of LAEs around quasars, traces simultaneously the clustering properties of both optical and dusty galaxy populations in quasars fields, stressing the importance of multiwavelength studies, and highlighting important questions about galaxy properties in high-z dense environments.