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

TL;DR

TNG50 provides an unprecedented combination of volume and sub-kiloparsec resolution to study the 3D shapes and inner kinematics of star-forming galaxies from z~6 to 0. It reveals that dense gas disks are broadly rotation-dominated across time and mass, while stars remain dynamically hotter, with diskiness and rotational support increasing with cosmic time and mass. The study highlights systematic differences between stellar and gaseous tracers in both structure and kinematics, and demonstrates strong resolution-driven convergence for disk heights and sizes. These findings offer robust, testable predictions for upcoming JWST/ELT observations and advance our understanding of how disk galaxies settle over cosmic history.

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.

Figures (22)

• Figure 1: Spatial resolution of the baryonic elements in TNG50. Left panel: distribution of the Voronoi cell sizes in TNG50 galaxies, taken as the spherical volume equivalent radii and representing the spatial resolution over which the equations of magnetohydrodynamics are solved. The black solid curve represents the average across the galaxy population of the median sizes of star-forming gas cells within galaxies at $z=1$ (within twice the stellar-half mass radius); the hatched region denotes the 5th to 95th percentiles. The smallest cells within galaxies can have sizes that are several times smaller than the median: the galaxy-population average at $z=1$ is shown with the black dashed curve, and compared to the $z=4$ measurement (thin gray dashed curve). For reference, the smallest gas cell across the whole volume measures 6.5 phys pc. These sizes are compared to the gravitational softening lengths of both stars and gas, in red and orange respectively. Analog measurements at the lower-resolution levels of TNG50-2 and TNG50-3 are also included (dark and light gray solid curves). Right stamps: spatial distribution of the Voronoi cell sizes in three random galaxies, showing the smallest cell size along the line of sight, binned in pixels of size 1 ckpc. While central regions typically reach resolutions of a few tens of parsecs, the majority of the star-forming disk is resolved at $\sim\,$100 parsec scales.
• Figure 2: The star formation rate vs. galaxy stellar mass plane in the TNG50 simulation at different redshifts, for all galaxies in the box (centrals and satellites). In this analysis we study exclusively star-forming galaxies, here denoted as blue filled circles. The distinction between star-forming (blue filled circles) and green-valley or quiescent galaxies (black open circles) is made based on a recursive refinement of the star-forming main sequence (SFMS; see Section \ref{['sec:sample']} for details). A SFMS naturally emerges in the simulations and is present down to at least $10^{7}{\rm M}_{\odot}$ in stellar mass, as well as already at high redshift.
• Figure 3: Median galaxy sizes of TNG50 star-forming galaxies as a function of galaxy stellar mass, from low (top) to high (bottom) redshift. Different colors denote 3D or 2D face-on circularized half-mass or half-light radii of different galaxy components. We include both central and satellite galaxies. Shaded areas denote the $\pm1\sigma$ dispersions of the size distributions at fixed stellar mass (omitted for several curves to avoid overcrowding the plot). Gray annotations mark the locus of the typical gravitational softening of the stellar and gaseous resolution elements, for reference (see Appendix \ref{['sec:app_res']} for a quantification of the level of convergence across masses, redshift, and matter components).
• Figure 4: Distributions of disk heights in physical parsecs for TNG50 star-forming galaxies. We show three discrete redshifts, from $z=0.5$ (top) to $z=4$ (bottom), and in several bins of increasing galaxy stellar mass (left to right). Galaxies' thicknesses are estimated as described in Section \ref{['sec:props']} from both the V-band (blue) and $\rm H\alpha$ (orange) light distributions in edge-on projections. Vertical dashed lines denote medians of the distributions. The TNG50 calculation can return (star-forming) galaxies with averages "disk" heights of 100-400 physical parsecs in general, and as thin as 50-80 parsecs. In Appendix \ref{['sec:app_res']}, we quantify the level of convergence of TNG50 heights across masses, redshift, and matter components.
• Figure 5: Time evolution of the distributions of normalized disk heights, for TNG50 star-forming galaxies. The distributions represent the relative thickness or flatness of galaxies, in bins of galaxy stellar mass (from top to bottom), derived from either V-band (left) or $\rm H\alpha$ (right) light. In each panel, thicker and brighter curves depict lower redshifts. Galaxy populations exhibit flatter or "diskier" morphologies at more recent times.