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.

Unobscured quasars (QSOs) are predicted to be the final stage in the evolutionary sequence from gas-rich mergers to gas-depleted, quenched galaxies. Studies of this population, however, find a high incidence of far-infrared-luminous sources-suggesting significant dust-obscured star formation-but direct observations of the cold molecular gas fuelling this star formation are still necessary. We present a NOEMA study of CO(2-1) emission, tracing the cold molecular gas, in ten lensed z = 1-1.5 unobscured QSOs. We detected CO(2-1) in seven of our targets, four of which also show continuum emission (λ_rest = 1.3 mm). After subtracting the foreground galaxy contribution to the photometry, spectral energy distribution fitting yielded stellar masses of 10^9-11 M_⊙, with star formation rates of 25-160 M_⊙ yr^-1 for the host galaxies. These QSOs have lower L′_CO than star-forming galaxies with the same LIR, and show depletion times spanning a large range (50-900 Myr), but with a median of just 90(α_CO/4) Myr. We find molecular gas masses in the range ≤2-40 × 10⁹(α_CO/4) M_⊙, which suggest gas fractions above ~50% for most of the targets. Despite the presence of an unobscured QSO, the host galaxies are able to retain significant amounts of cold gas. However, with a median depletion time of ~90 Myr, the intense burst of star formation taking place in these targets will quickly deplete their molecular gas reservoirs in the absence of gas replenishment, resulting in a quiescent host galaxy. The non-detected QSOs are three of the four radio-loud QSOs in the sample, and their properties indicate that they are likely already transitioning into quiescence. Recent cosmological simulations tend to overestimate the depletion times expected for these z ~ 1 QSO-host galaxies, which is likely linked to their difficulty producing starbursts across the general high-redshift galaxy population.

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.