λ_e–ε(1 R_e) diagrams for TNG and observed galaxy samples, in stellar mass bins. Top row: λ-ellipticity diagram for the sample of TNG50 and TNG100 ETGs selected by color and mass as in Fig. 3, observed along a random LOS. The galaxies are color coded according to their intermediate to major axis ratios p at 1 R_e. Middle row: λ-ellipticity diagram for the TNG galaxies after the additional selection p(1 R_e)≥0.6. Bottom row: λ-ellipticity plots for observed ETG samples as shown in the legend, including also late-type galaxies from the MANGA survey for comparison. Their λ_e and ε(1 R_e) are as described in Sect. 3. The solid black line is the threshold separating fast and slow rotators as defined in Emsellem et al. (2011): . The magenta line shows the semi-empirical prediction for edge-on axisymmetric rotators with anisotropy parameter δ = 0.70ε_intr from Cappellari et al. (2007). After the additional removal of centrally elongated systems, the colour and mass selected TNG ETG samples give a good representation of the observed ETG samples in the λ_e–ε(1 R_e)-plane, with the exception of the high-λ_e (> 0.7) S0 disks specific to the MANGA survey.