Massive starburst galaxies in the early Universe are estimated to have depletion times of ∼100 Myr and thus be able to convert their gas very quickly into stars, possibly leading to a rapid quenching of their star formation. For these reasons, they are considered progenitors of massive early-type galaxies (ETGs). In this paper, we study two high-z starbursts, AzTEC/C159 (z ≃ 4.57) and J1000+0234 (z ≃ 4.54), observed with ALMA in the [C II] 158-μm emission line. These observations reveal two massive and regularly rotating gaseous discs. A 3D modelling of these discs returns rotation velocities of about 500 km s−1 and gas velocity dispersions as low as ≈ 20 km s−1, leading to very high ratios between regular and random motion (V/σ ≳ 20), at least in AzTEC/C159. The mass decompositions of the rotation curves show that both galaxies are highly baryon-dominated with gas masses of ≈1011 M⊙, which, for J1000+0234, is significantly higher than previous estimates. We show that these high-z galaxies overlap with z = 0 massive ETGs in the ETG analogue of the stellar-mass Tully-Fisher relation once their gas is converted into stars. This provides dynamical evidence of the connection between massive high-z starbursts and ETGs, although the transformation mechanism from fast rotating to nearly pressure-supported systems remains unclear.

High-redshift observations are often biased towards massive and bright galaxies that are not necessarily representative of 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” resolution observations of the [CII] emission line at 158μm 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 [CII] 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 [CII] ‘Halo’ has been detected to date. For a matched Sérsic profile with n = 1, we measured a [CII] effective radius of 0.50 ± 0.04” (3.07±0.25 kpc) and an average rest-frame UV effective radius of 0.2 ± 0.04” (1.48 ± 0.16 kpc). The [CII] morphology and kinematics of the system suggest a merging event resulting in a non rotating disk system. This event could be responsible for the extended [CII] emission. Alternatively, some potential obscured emission could also explain the [CII] to UV size ratio. These results contribute to the growing consensus with respect to the existence of extended [CII] emission around galaxies.

Context. Observations of the rest-frame UV emission of high-redshift galaxies suggest that the early stages of galaxy formation involve disturbed structures. Imaging the cold interstellar medium (ISM) can provide a unique view of the kinematics associated with the assembly of galaxies. Aims. In this paper, we analyze the spatial distribution and kinematics of the cold ionized gas of the normal star-forming galaxy COS-2987030247 at z = 6.8076, based on new high-resolution observations of the [C II] 158µm line emission obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Methods. The analysis of these observations allowed us to: compare the spatial distribution and extension of the [C II] and rest-frame UV emission, model the [C II] line data-cube using the 3D Barolo code, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions. Results. The system is found to be composed of a main central source, a fainter north extension, and candidate [C II] companions located 10-kpc away. We find similar rest-frame UV and [C II] spatial distributions, suggesting that the [C II] emission emerges from the star-forming regions. The agreement between the UV and [C II] surface brightness radial profiles rules out diffuse, extended [C II] emission (often called a [C II] halo) in the main galaxy component. The [C II] velocity map reveals a velocity gradient in the northsouth direction, suggesting ordered motion, as commonly found in rotating-disk galaxies. However, higher resolution observations would be needed to rule out a compact merger scenario. Our model indicates an almost face-on galaxy (i ∼ 20 • ), with a average rotational velocity of 86 ± 16 km s −1 and a low average velocity dispersion, σ < 30 km s −1 . This result implies a dispersion lower than the expected value from observations and semi-analytic models of high redshift galaxies. Furthermore, our measurements indicate that COS-2987030247 and its individual regions systematically lie within the local L [CII] −SFR relationship, yet slightly below the local Σ [CII] -Σ UV relation. Conclusions. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will possibly become perturbed at later times by accreting sources.