Kinematic Decomposition of IllustrisTNG Disk Galaxies: Morphology and Relation with Morphological Structures

We recently developed an automated method, auto-GMM, to decompose simulated galaxies. It extracts kinematic structures in an accurate, efficient, and unsupervised way. We use auto-GMM to study the stellar kinematic structures of disk galaxies from the TNG100 run of IllustrisTNG. We identify four to five structures that are commonly present among the diverse galaxy population. Structures having strong to moderate rotation are defined as cold and warm disks, respectively. Spheroidal structures dominated by random motions are classified as bulges or stellar halos, depending on how tightly bound they are. Disky bulges are structures that have moderate rotation but compact morphology. Across all disky galaxies and accounting for the stellar mass within 3 half-mass radii, the kinematic spheroidal structures, obtained by summing up stars of bulges and halos, contribute ∼45% of the total stellar mass, while the disky structures constitute ∼55%. This study also provides important insights into the relationship between kinematically and morphologically derived galactic structures. Comparing the morphology of kinematic structures with that of traditional bulge+disk decomposition, we conclude that (1) the morphologically decomposed bulges are composite structures comprising a slowly rotating bulge, an inner halo, and a disky bulge; (2) kinematically disky bulges, akin to what are commonly called pseudo-bulges in observations, are compact disk-like components that have rotation similar to warm disks; (3) halos contribute almost 30% of the surface density of the outer part of morphological disks when viewed face on; and (4) both cold and warm disks are often truncated in central regions.