Unifying the dynamical classification of early-type galaxies: Kinematic deficits in IllustrisTNG versus observations

Aims. This study aims to make a comparative analysis of galaxy kinematics using IllustrisTNG simulations and integral-field spectroscopy (IFS) observations. Methods. We identified 2342 early-type galaxies (ETGs) from the TNG100 simulation and 236 ETGs from the TNG50 simulation, to compare with those from the MaNGA and ATLAS3D surveys. For these systems, we measured key kinematic parameters widely employed in both observational and simulation-based studies, including the intrinsic spin parameter λR, intr (with the λR parameter measured for edge-on viewing), the cylindrical rotational energy fraction κrot, and structural mass ratios such as the spheroid mass fraction fspheroid and the stellar halo mass fraction fhalo. Results. This study performs a comparative kinematic analysis of ETGs using IllustrisTNG simulations and IFS data from MaNGA and ATLAS3D. We demonstrate that standard classifiers – the $ \lambda_{R}(R_{\mathrm{e}}) = 0.31\sqrt{\varepsilon} $ relation and $ \overline{k_5} $ coefficient (the higher-order term of the Fourier decomposition of velocity fields) – fail to align with kinematic bimodality. Revised thresholds are proposed: the spin λR, intr ∼0.4, the ratio of rotation energy κrot ∼ 0.5, and the mass fraction of a spheroid component fspheroid ∼ 0.6. It provides a universal threshold that classifies all galaxy types into rotation-dominated (fast rotators) and random motion-dominated (slow rotators) cases. Scaling relations derived from TNG enable estimating κrot and fspheroid from observations. TNG simulations exhibit a bimodality deficit, characterized by a lack of fast rotators and suppressed λR, intr, attributable to excess galaxies with intermediate rotation and high spheroid or stellar halo mass. A novel method for estimating stellar halo mass fractions from IFS kinematics is introduced, even though significant uncertainties persist.