The Evolutionary Pathways of Disk-, Bulge-, and Halo-dominated Galaxies

Abstract

We have recently developed a method to kinematically decompose simulated galaxies that helps to break the degeneracy among galactic stellar structures. 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 about 500 central galaxies with stellar mass 10 10−11.5 M from the TNG50 simulation at z = 0, 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 2), 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 ∼ 2; the difference in their presentday structures originates from two distinct evolutionary pathways, extended vs. compact, that are likely determined by their parent dark matter halos in the early phase; i.e., nature. On the other hand, normal elliptical galaxies are typically halo-dominated, forming by external processes (e.g. major 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 disk galaxies. 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.

Relic galaxy analogues in Illustris TNG50: the formation pathways of surviving red nuggets in a cosmological simulation

“…A recent work using the TNG50 simulation (Du et al 2021) speculates that the initial spin of parent dark matter halos may play an important role, controlling the size evolution of galaxies. The primordial spin thus could have an impact in the formation of relic analogues as those found here, however, this kinematic analysis is considerably more complex, lying beyond the scope of this work.…”