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

TL;DR

This study compares galaxy kinematics in IllustrisTNG simulations with observations, revealing discrepancies in rotational properties and proposing revised classification thresholds to better distinguish galaxy types.

Contribution

It introduces new kinematic thresholds and a method to estimate halo mass from integral-field spectroscopy, improving galaxy classification accuracy.

Findings

Simulations lack a clear bimodal distribution of galaxy rotation.

Revised thresholds better classify galaxies into fast and slow rotators.

A new method estimates halo mass from kinematic data.

Abstract

We conduct a comparative analysis of galaxy kinematics using IllustrisTNG simulations and integral-field spectroscopy (IFS) observations. We identify 2,342 early-type galaxies (ETGs) from the TNG100 simulation and 236 ETGs from the TNG50 simulation, comparing them with observations from MaNGA and ATLAS$^{3D}$. For these systems, we measure key kinematic parameters: the intrinsic spin parameter $\lambda_{R,\mathrm{intr}}$ (measured edge-on), the cylindrical rotational energy fraction $\kappa_{\mathrm{rot}}$, and structural mass ratios including the spheroid mass fraction $f_{\mathrm{spheroid}}$ and stellar halo mass fraction $f_{\mathrm{halo}}$. Our study reveals that standard classifiers--the $\lambda_{R}(R_e)=0.31\sqrt{\varepsilon}$ relation and $\overline{k_5}$ coefficient (higher-order Fourier term of velocity fields)--fail to align with observed kinematic bimodality. We propose revised thresholds: $\lambda_{R,\mathrm{intr}} \sim 0.4$, $\kappa_{\mathrm{rot}} \sim 0.5$, and $f_{\mathrm{spheroid}} \sim 0.6$, which classify galaxies into rotation-dominated (fast rotators) and random motion-dominated (slow rotators). Scaling relations from TNG enable observational estimates of $\kappa_{\mathrm{rot}}$ and $f_{\mathrm{spheroid}}$. The simulations exhibit a bimodality deficit, characterized by a lack of fast rotators and suppressed $\lambda_{R,\mathrm{intr}}$, attributed to excess galaxies with intermediate rotation and high spheroid/stellar halo mass. We introduce a novel method to estimate $f_{\mathrm{halo}}$ from IFS kinematics, though uncertainties remain.