Kinematic decomposition of IllustrisTNG disk galaxies: morphology and relation with morphological structures
We recently developed an automated method, auto-GMM, to kinematically decompose simulated galaxies. It extracts the kinematic structures in an accurate, efficient, and unsupervised way. Here we use auto-GMM to study the stellar kinematic structures of disk galaxies from the TNG100 run of the cosmological suite IllustrisTNG. We identify four to five kinematic 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 defined as structures that have moderate rotation but compact morphology. TNG100 returns multiple structures that have reasonable properties, qualitatively consistent with our general expectations. 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 $\sim 45\%$ of the total stellar mass, while the disky structures constitute $\sim 55\%$. 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.
- Pub Date:
- February 2020
- Astrophysics - Astrophysics of Galaxies
- 19 pages, 13 figures. Submitted to ApJ, comments welcome. The mass fraction catalogue and images of the kinematically derived galactic structures are publicly available (https://www.tng-project.org/data/docs/specifications/#sec5m)