Large-scale gas flows and streaming motions in simulated spiral galaxies

From a galactic perspective, star formation occurs on the smallest scales within molecular clouds, but it is likely initiated from the large-scale flows driven by galactic dynamics. To understand the conditions for star formation, it is important to first discern the mechanisms that drive gas from large scales into dense structures on the smallest scales of a galaxy. We present high-resolution smoothed particle hydrodynamics simulations of two model spiral galaxies: one with a live stellar disc (N-body) and one with a spiral potential. We investigate the large-scale flows and streaming motions driven by the simulated spiral structure. We find that the strength of the motions in the radial direction tends to be higher than in the azimuthal component. In the N-body model, the amplitude of these motions decreases with galactocentric radius whereas for the spiral potential, it decreases to a minimum at the corotation radius, and increases again after this point. The results show that in both simulations, the arms induce local shocks, an increase in kinetic energy that can drive turbulence and a means of compressing and expanding the gas. These are all crucial elements in forming molecular clouds and driving the necessary conditions for star formation.