Around every galaxy lies an invisible corona of cold gas. It extends far beyond the stellar disk and serves both as a fuel depot for new stars and as a graveyard for ejected material. Astronomers have long debated how this corona behaves when a galaxy stops forming stars: does it flicker out instantly, like a burnt-out lightbulb, or cool gradually, like embers in a campfire?

To see this corona, scientists used the light of quasars — distant beacons that shine through the gas. When a quasar's beam passes through a halo, magnesium atoms absorb part of the light, leaving dark lines in the spectrum. The depth of these lines reveals how much cold gas lies in the path. In the new study, they gathered observations of 716 galaxies at various distances from us — from nearby ones to those whose light has traveled for 10 billion years. For each, they measured magnesium absorption and compared it with how actively the galaxy is forming stars.

The key move was introducing the σSFO metric (deviation from the main sequence of star formation). The main sequence is the typical star formation rate for a galaxy of a given mass at a given epoch. If a galaxy lags behind, it moves toward the "green valley"—the transition zone between active and passive. Previously, this zone was often lumped into binary classes of "blue" and "red," losing details. Now, they have managed to align all galaxies along a single scale and observe how their gas coronae change.

The result was clear: magnesium absorption declines smoothly from star-forming to passive galaxies. In active galaxies, the covering fraction, the fraction of sightlines with strong absorption, reaches 0.65 in the inner part of the halo (within a quarter of its size), while in passive galaxies it drops to 0.22. The green valley occupies an intermediate position. This means that cold gas does not disappear instantly but is depleted gradually, like water in a drying well.

The kinematics of the gas indicate that it is mostly gravitationally bound to the galaxy, not escaping. This is consistent with models where the inner halo is replenished by outflows from star formation episodes, while in massive passive galaxies, the hot accretion mode suppresses the formation of cold gas. Thus, a galaxy's corona cools as its heart, star formation, fades.

For now, this is pure knowledge with no obvious applications. But it is important for understanding why galaxies die: not from a sudden blow, but from the slow depletion of their reserves. The next step is to expand the sample to more distant galaxies and compare with simulations that track the metal cycle. Then it will become clear what triggers the transition through the green valley: internal processes, mergers, or environment. Meanwhile, the corona continues to cool, and each new galaxy in the sample is another ember in this fire.