The Evolutionary Pathways of Disk-, Bulge-, and Halo-dominated Galaxies
  To break the degeneracy among galactic stellar components, we extract
kinematic structures using the framework described in Du et al. (2019, 2020).
For example, the concept of stellar halos is generalized to weakly-rotating
structures that are composed of loosely bound stars, which can hence be
associated to both disk and elliptical type morphologies. By applying this
method to central galaxies with stellar mass $10^{10-11.5}\ M_\odot$ from the
TNG50 simulation, we identify three broadly-defined types of galaxies: ones
dominated by disk, by bulge, or by stellar halo structures. We then use the
simulation to infer the underlying connection between the growth of structures
and physical processes over cosmic time. Tracing galaxies back in time, we
recognize three fundamental regimes: an early phase of evolution ($z\gtrsim2$),
and internal and external (mainly mergers) processes that act at later times.
We find that disk- and bulge-dominated galaxies are not significantly affected
by mergers since $z\sim2$; the difference in their present-day structures
originates from two distinct evolutionary pathways, extended vs. compact, that
are likely determined by their parent dark matter halos; i.e., nature. On the
other hand, slow rotator elliptical galaxies are typically halo-dominated,
forming by external processes (e.g. mergers) in the later phase, i.e., nurture.
This picture challenges the general idea that elliptical galaxies are the same
objects as classical bulges. In observations, both bulge- and halo-dominated
galaxies are likely to be classified as early-type galaxies with compact
morphology and quiescent star formation. However, here we find them to have
very different evolutionary histories.
