The Formation of Star-forming Disks in the TNG50 Simulation

Abstract: We investigate the disk formation process in the TNG50 simulation, examining the profiles of SFR surface density ($\Sigma_{\rm SFR}$), gas inflow and outflow, and the evolution of the angular momentum of inflowing gas particles. The TNG50 galaxies tend to have larger star-forming disks, and also show larger deviations from exponential profiles in $\Sigma_{\rm SFR}$ when compared to real galaxies in the MaNGA (Mapping Nearby Galaxies at APO) survey. The stellar surface density of TNG50 galaxies show good exponential profiles, which is found to be the result of strong radial migration of stars over time. However, this strong radial migration of stars in the simulation produces flatter age profiles in TNG50 disks compared to observed galaxies. The star formation in the simulated galaxies is sustained by a net gas inflow and this gas inflow is the primary driver for the cosmic evolution of star formation, as expected from simple gas-regulator models of galaxies. There is no evidence for any significant loss of angular momentum for the gas particles after they are accreted on to the galaxy, which may account for the large disk sizes in the TNG50 simulation. Adding viscous processes to the disks, such as the magnetic stresses from magneto-rotational instability proposed by Wang & Lilly 2022, will likely reduce the sizes of the simulated disks and the tension with the sizes of real galaxies, and may produce more realistic exponential profiles.