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

¹ Department of Astronomy, Xiamen University Xiamen Fujian 361005, China
² Institute for Computational Cosmology, Department of Physics, University of Durham South Road Durham DH1 3LE, UK
³ Department of Astronomy, Westlake University, Hangzhou Zhejiang 310030, China

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 September 2025
Accepted: 30 January 2026

Abstract

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 ATLAS^3D surveys. For these systems, we measured key kinematic parameters widely employed in both observational and simulation-based studies, including the intrinsic spin parameter λ_R, intr (with the λ_R parameter measured for edge-on viewing), the cylindrical rotational energy fraction κ_rot, and structural mass ratios such as the spheroid mass fraction f_spheroid and the stellar halo mass fraction f_halo.

Results. This study performs a comparative kinematic analysis of ETGs using IllustrisTNG simulations and IFS data from MaNGA and ATLAS^3D. We demonstrate that standard classifiers – the λ_R(R_e) = 0.31√ε relation and ¯k₅ coefficient (the higher-order term of the Fourier decomposition of velocity fields) – fail to align with kinematic bimodality. Revised thresholds are proposed: the spin λ_R, intr ∼0.4, the ratio of rotation energy κ_rot ∼ 0.5, and the mass fraction of a spheroid component f_spheroid ∼ 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 κ_rot and f_spheroid from observations. TNG simulations exhibit a bimodality deficit, characterized by a lack of fast rotators and suppressed λ_R, 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.