Fast rotating and low-turbulence discs at z  ≃ 4.5: Dynamical evidence of their evolution into local early-type galaxies

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- μ m 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 ≈10 11 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.