The development of kinematic structures of the galaxies in the New Horizon simulation

Disk galaxies are believed to consist of structures with different kinematics and stellar populations: young disk stars rotating on the galactic plane and old stellar bulge and halo having non-ordered motions. In the favored LCDM paradigm, several internal and external processes work together in settling the kinematic components. Investigating the significance of processes responsible for each component of a galaxy is, therefore, essential to understanding the evolution of kinematics and morphologies of galaxies. Using the New Horizon simulation, which is a high-resolution cosmological zoom-in simulation, we can resolve the detailed structures for a statistically significant number of galaxies. We first decompose stellar particles in a galaxy into a rotating disk and a dispersion-dominated spheroidal component based on their orbits and then see how the stars in these components are formed and assembled into the galaxy. We find that disk stars are mostly young stars formed in situ, and spheroidal components consist of old stars born with misaligned orbits with respect to the galactic rotating planes, disrupted disk stars, and accreted stars (mostly distributed at large radii). In the star formation history, stars mainly form with chaotic orbits at high redshift z~3, and thus galaxies are mostly spheroid-dominated. As redshift decreases, massive galaxies start to form disk stars predominantly (z~1-2), while less massive galaxies form these stars much later. This study highlights the relative importance of different growth channels for galaxies with different kinematics (i.e., disk-dominated galaxies, spheroid-dominated galaxies, and galaxies with second disks) and the role of mergers in these processes.