First results from the TNG50 simulation: the evolution of stellar and gaseous discs across cosmic time

We present a new cosmological, magnetohydrodynamical simulation for galaxy formation: TNG50, the third and final instalment of the Illustris TNG project. TNG50 evolves 2 x 2160(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 8.5 x 10(4) M-circle dot and an average cell size of 70-140 pc in the star-forming regions of galaxies. Simultaneously, TNG50 samples similar to 700 (6500) galaxies with stellar masses above 10(10) (10(8)) M-circle dot at z = 1. Here we investigate the structural and kinematical evolution of star-forming galaxies across cosmic time (0 less than or similar to z less than or similar to 6). We quantify their sizes, disc heights, 3D shapes, and degree of rotational versus dispersion-supported motions as traced by rest-frame V-band light (i.e. roughly stellar mass) and by H alpha 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 less than or similar to 300-pc disc 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 disc-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) M-circle dot star-forming galaxies are rotationally supported discs for most cosmic epochs (V-rot/sigma > 2-3, z less than or similar to 5), being dynamically hotter at earlier epochs (z greater than or similar to 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.