A harbor lives by the tide schedule. Water rises and falls, and with it everything changes: the level at the dock, the composition of the sediment on the bottom, even the salinity in the far corners. A galaxy works similarly, except instead of water it has gas, and instead of the Moon, it has its own stars and neighboring clouds. This gas flows into and out of the galaxy, and each such tide leaves a mark on how the galaxy forms stars and how rich it is in heavy elements.

Astronomers from the DESI survey looked into the spectra of more than 52,000 galaxies and found among them those where the motion of neutral gas is visible in sodium lines. Some galaxies eject gas outward, these are outflows, like an ebb tide. Others, on the contrary, pull gas in from their surroundings, these are inflows, like a flood tide. Inflows can be slow or fast, and the speed determines what happens to the galaxy next.

It turns out that galaxies with outflows form stars noticeably more actively: their specific star formation rate is higher by 0.25–0.40 dex, and the abundance of heavy elements in the central regions is elevated by 0.04–0.06 dex. Slow inflows also boost star formation, but metallicity remains unchanged. Fast inflows act differently: star formation is barely enhanced, but metallicity drops slightly, because metal-poor gas flows in and dilutes what has already been accumulated.

Previously, the link between gas flows and a galaxy's position on the star-forming main sequence or the mass–metallicity relation was mostly theoretical. Now it has been tested directly on the largest sample of detected flows. Galaxies with outflows lie about one standard deviation above the main sequence, those with slow inflows lie 0.8 sigma above, and those with fast inflows only 0.25 sigma. This means that different phases of the gas cycle shift a galaxy on these diagrams in different ways.

Tides in a harbor do more than just move water. They determine where silt settles, where mussels grow, and where clean sand remains. Likewise, gas flows do more than just add or remove matter: they change the pace of star birth and the composition of future generations. Understanding this connection will help us read data from new surveys more accurately and better model how galaxies mature.