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
about the relationship between kinematically and morphologically derived
galactic structures. Comparing the morphology of kinematic structures with that
of traditional bulge+disk decomposition, we conclude: (1) the morphologically
decomposed bulges are composite structures comprised of 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.
