Every galaxy has an invisible halo of cold gas, like a crown around a head. This gas is needed to birth new stars. When a galaxy stops forming stars, its halo cools. The question was: does it go out instantly, like a lightbulb, or gradually, like coals in a campfire?
Astronomers shone light through the halos of 716 galaxies, using bright distant quasars as flashlights. The quasar light passes through the gas, and magnesium atoms absorb some of it. The more gas, the darker the imprint in the spectrum. Then they compared: in galaxies that are actively forming stars, the inner part of the halo is rich in gas: it covers 65% of the lines of sight. In passive galaxies, only 22%. And transitional galaxies (the so-called 'green valley') lie right in between.
This means the halo doesn't go out all at once. The gas recedes gradually, like water from a drying well. First, the galaxy forms stars actively, then more and more slowly, and its gas halo thins out. That's how the transition from 'blue' to 'red' galaxies happens.
For now, this knowledge has no practical application, but it helps us understand how galaxies die. Next, scientists want to look at more distant galaxies and compare them with computer models to find out what exactly triggers the shutdown.
Magnesium in the halo is the ash of supernovae: it is ejected by explosions and can later fall back onto the disk, participating in new cycles of star formation.