2021A&A...647A.194F

Massive starburst galaxies in the early Universe are estimated to have depletion times of ∼100Myr 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 [CII] 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 500km/s and gas velocity dispersions as low as ≃20km/s, 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 ≃10¹¹M_☉, 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.