Relation between star formation rate surface density and gas surface density for the field sample (grey points and contour) and massive field sample (black stars) and SMGs (red circles). We indicate a constant star formation timescale of 0.1 Gyr with the black line. Adopting the typical rotation velocity and velocity dispersion from Birkin et al. (2024) derived for a sample of AS2UDS SMGs, we define the boundary between stable (Q > 1) and unstable (Q < 1) gas discs. The majority of the SMGs indicate unstable gas discs with a median Toomre Q parameter of Q = 0.47 ± 0.06 while most of the field samples are stable or marginally stable.