A harbor lives by the tide schedule. Water rises and falls, and with it everything changes: the level at the dock, the composition of sediment on the bottom, even the salinity in distant 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, high or low, 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 those where the motion of neutral gas is visible in sodium lines. Some galaxies eject gas outward, which are outflows, like an ebb tide. Others, conversely, pull gas in from their surroundings, which 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, and the abundance of heavy elements in their central regions is slightly elevated. Slow inflows also spur star formation, but the gas composition does not change. Fast inflows act differently: star formation increases only barely, but metallicity drops slightly, because metal-poor gas pours into the galaxy and dilutes what has already accumulated.

Previously, the connection between gas flows and how a galaxy forms stars and accumulates heavy elements remained mostly theoretical. Now it has been directly tested on the largest sample of detected flows. This will help more accurately interpret data from new surveys and better understand how galaxies mature.