First Results from the TNG50 Simulation: The evolution of stellar and gaseous disks across cosmic time (1902.05553v2)

Abstract: We present a new cosmological, magnetohydrodynamical simulation for galaxy formation: TNG50, the third and final instaLLMent of the IllustrisTNG project. TNG50 evolves 2x2160^3 dark-matter particles and gas cells in a volume 50 comoving Mpc across. It hence reaches a numerical resolution typical of zoom-in simulations, with a baryonic element mass of 8.5x10^4 Msun and an average cell size of 70-140 parsecs in the star-forming regions of galaxies. Simultaneously, TNG50 samples ~700 (6,500) galaxies with stellar masses above 10^10 (10^8) Msun at z=1. Here we investigate the structural and kinematical evolution of star-forming galaxies across cosmic time (0 < z < 6). We quantify their sizes, disk heights, 3D shapes, and degree of rotational vs. dispersion-supported motions as traced by rest-frame V-band light (i.e. roughly stellar mass) and by Halpha light (i.e. star-forming and dense gas). The unprecedented resolution of TNG50 enables us to model galaxies with sub-kpc half-light radii and with <300-pc disk heights. Coupled with the large-volume statistics, we characterize a diverse, redshift- and mass-dependent structural and kinematical morphological mix of galaxies all the way to early epochs. Our model predicts that for star-forming galaxies the fraction of disk-like morphologies, based on 3D stellar shapes, increases with both cosmic time and galaxy stellar mass. Gas kinematics reveal that the vast majority of 10^9-11.5 Msun star-forming galaxies are rotationally-supported disks for most cosmic epochs (Vmax/sigma>2-3, z<5), being dynamically hotter at earlier epochs (z>1.5). Despite large velocity dispersion at high redshift, cold and dense gas in galaxies predominantly arranges in disky or elongated shapes at all times and masses; these gaseous components exhibit rotationally-dominated motions far exceeding the collisionless stellar bodies.

• The paper presents the TNG50 simulation, a high-resolution cosmological simulation revealing galaxy size evolution consistent with observations and an increase in disk-like morphologies over time.
• TNG50 demonstrates that star-forming galaxies increasingly gain rotational support over cosmic time, with gaseous components showing higher rotation-to-dispersion ratios than stellar ones.
• The simulation provides insights into the balance of rotational vs. dispersion dynamics across redshift and enables future high-resolution comparisons with observations like JWST.

Overview of the TNG50 Simulation and Its Insights Into Galaxy Evolution

The paper by Pillepich et al. presents the TNG50 simulation, the latest in the IllustrisTNG series of cosmological magnetohydrodynamical simulations. TNG50 uniquely combines high resolution with substantial cosmological volume, enabling the paper of galaxy formation and evolution across a variety of environments with unprecedented detail. This simulation harnesses advanced numerical methods to solve complex equations governing gas dynamics, gravity, and magnetism, while including processes such as star formation, chemical enrichment, and feedback from supernovae and black holes.

Methodology and Simulation Details

TNG50 evolves dark matter particles and gas cells in a periodic cube of 50 comoving megaparsecs (Mpc) on a side, achieving a spatial resolution typical of zoom-in simulations within a larger cosmological context. It employs the AREPO code, which utilizes a moving mesh approach to simulate ideal magnetohydrodynamics, alongside a suite of physics modeling galaxy formation processes. The simulation starts at high redshift (z=127) and implements a realistic model of baryonic processes that include star formation and feedback.

Key Findings

1. Structural Evolution and Morphology:

• The TNG50 simulation effectively captures the size evolution of galaxies, presenting sizes consistent with observational data across a range of redshifts (0 < z < 5).
• There is a marked trend of increasing disk-like morphologies over time. For massive galaxies, disk-like shapes become prevalent especially towards lower redshifts, aligning with observational studies that indicate a shift from turbulent, disordered structures to more settled disk configurations.

2. Rotational Support and Kinematics:

• TNG50 shows that star-forming galaxies predominantly acquire rotational support with time, with gaseous components revealing higher rotation-to-dispersion ratios compared to stars. This trend is most pronounced in massive galaxies at low redshift.
• The simulation provides insights into the balance between rotational and dispersion-supported dynamics, suggesting that less chaotic high-redshift environments allow for the formation of dynamically hotter, dispersion-supported structures.

Implications and Future Prospects

The results from TNG50 have significant implications for our understanding of galaxy evolution. The clear depiction of the transition to rotationally-supported, disk-like structures echoes the progression towards a better-organized cosmic structure with time, providing a coherent narrative consistent with observational evidence. This simulation also underscores the importance of high-resolution numerical experiments in capturing the interplay between different physical processes and accurately modeling galactic morphology and dynamics.

The insights from TNG50 pave the way for future theoretical and observational investigations. Future work could involve direct comparison with upcoming high-resolution observational data from facilities like JWST and ELT. Moreover, the TNG50 framework offers potential extensions to explore detailed galactic features, such as the formation of spiral arms or bars, and the role of minor mergers and interactions in shaping galactic structure over cosmic time.