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-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.