Radial transport in high-redshift disk galaxies dominated by inflowing streams

Aims. We study the radial transport of cold gas within simulated disk galaxies at cosmic noon. Our aim is to determine whether disk instability or accretion along cold streams from the cosmic web is the driving mechanism behind the transport. Methods. Disks were selected based on kinematics and flattening from the VELA zoom-in hydro-cosmological simulations. We mapped the radial velocity fields in the disks, computed their averages as a function of radius and over the whole disk, and obtained the radial mass flux in each disk as a function of radius. The transport directly associated with fresh incoming streams was identified by selecting cold gas cells that are either on incoming streamlines or have a low metallicity. Results. We find the radial velocity fields in VELA disks to be highly non-axisymmetric, showing both inflows and outflows. However, in most cases, the average radial velocities, both as a function of radius and over the whole disk, were directed inward, with the disk-averaged radial velocities typically amounting to a few percent of the disk-averaged rotational velocities. This is significantly lower than the expectations from various models that analytically predict the inward mass transport to be driven by torques associated with disk instability. Under certain simplifying assumptions, such models typically predict average inflows of more than 10% of the rotational velocities. Analyzing the radial motions of streams and off-stream material, we find that the radial inflow in VELA disks is dominated by the stream inflows themselves, especially in the outer disks. Conclusions. The high inward radial velocities inferred in observed disks at cosmic noon at the level of ∼20% of the rotational velocities may reflect motions along inflowing streams from the cosmic web rather than being generated by disk instability.