Vertical Gas Circulation in Star-Forming Galaxies: Near-Equilibrium between Star Formation and Gravity

Gas accretion and feedback are fundamental processes in galaxy evolution models and simulations. In particular, feedback from massive stars is thought to reshape the structure of the interstellar medium (ISM) and self-regulate star formation, leading to the tight observed relationship between star formation rate (SFR) and galaxy mass. However, direct observational constraints on this feedback loop remain limited. Most studies either focus on small samples of galaxies or predominantly probe the warm and hot ionized gas phases, leaving the impact of feedback on the cold neutral gas -- a critical phase of the ISM -- largely unexplored. To address this, we use single-dish HI 21cm observations to statistically detect bulk vertical motions of neutral hydrogen in a sample of nearby, star-forming galaxies. These motions trace gas flows moving perpendicular to the galactic disk. Our analysis reveals that the vertical gas flows exist in a near-equilibrium state: they are driven outwards as galactic winds by momentum injection from star formation, while being simultaneously pulled back by gravitational forces, forming a fountain-like cycle. Such galactic fountains are prevalent in the local Universe. Their mass, momentum, and kinetic energy scale with SFR and can be well explained by a simple dynamical model balancing stellar feedback and gravity. This supports a scenario in which gas recycling, rather than complete ejection, is the dominant outcome of stellar feedback in the local Universe, highlighting the importance of vertical gas circulation in regulating star formation.