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ABSTRACT In the era of precision cosmology and ever-improving cosmological simulations, a better understanding of different galaxy components such as bulges and discs will give us new insight into galactic formation and evolution. Based on the fact that the stellar populations of the constituent components of galaxies differ by their dynamical properties, we develop two simple models for galaxy decomposition using the TNG100 cosmological hydrodynamical simulation from the IllustrisTNG project. The first model uses a single dynamical parameter and can distinguish four components: thin disc, thick disc, counter-rotating disc, and bulge. The second model uses one more dynamical parameter, was defined in a probabilistic manner, and distinguishes two components: bulge and disc. We demonstrate the improved robustness of these models compared to a widely used method in literature involving cuts on the circularity parameter. The number fraction of disc-dominated galaxies at a given stellar mass obtained by our models agrees well with observations for masses exceeding log10(M*/M⊙) = 10. The galaxies classified as bulge-dominated by the second model are mostly red; however, the population classified as disc-dominated contains significant number of red galaxies alongside the blue population. The contributions of the different galaxy components to the total stellar mass budget exhibits similar trends with stellar mass compared to the observational data, although there is a quantitative disagreement at high and low masses. The Sérsic indices and half-mass radii for the bulge and disc components agree well with those of real galaxies.
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This content will become publicly available on March 1, 2026
The assembly of the most rotationally supported disc galaxies in the TNG100 simulations
Context.Disc-dominated galaxies can be difficult to accommodate in a hierarchical formation scenario such as Λ cold dark matter (ΛCDM), where mergers are an important growth mechanism. However, observational evidence indicates that these galaxies are common in the Universe. Aims.We seek to characterise the conditions that lead to the formation of disc-dominated galaxies within ΛCDM. Methods.We used dynamical decomposition of the stellar particles in all galaxies with stellar massM∗= [1010− 1011] M⊙within the cosmological hydrodynamical simulation Illustris TNG100. We selected a sample of 43 mostly-disc galaxies that have less than ∼10% of their mass in a bulge component. For comparison, we also studied two additional stellar-mass matched samples: 43 intermediate galaxies having ∼30% of their stellar mass in the bulge and 43 with a purely spheroidal-like morphology. Results.We find that the selection purely based on stellar dynamics is able to reproduce the expected stellar population trends of different morphological types, with higher star-formation rates and younger stars in disc-dominated galaxies. Halo spin seems to play no role in the morphology of the galaxies, in agreement with previous works. At a fixedM*, our mostly-disc and intermediate samples form in dark matter haloes that are two to ten times less massive than the spheroidal sample, highlighting a higher efficiency in disc galaxies to retain and condensate their baryons. On average, mergers are less prevalent in the buildup of discs than in spheroidal galaxies, but there is a large scatter, including the existence of mostly-disc galaxies, with 15%–30% of their stars coming from accreted origin. Discs start to form early on, settling their low vertical velocity dispersion as early as 9–10 Gyr ago, although the dominance of the disc over the spheroid was established more recently (3–4 Gyr lookback time). The most rotationally supported discs form in haloes with the lowest virial mass in the sample and the best aligned distribution of angular momentum in the gas.
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- PAR ID:
- 10587252
- Publisher / Repository:
- Rodriguez et al. 2025
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 695
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A218
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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