The JWST spectrograph caught a barely perceptible dip in the spectrum of a distant galaxy — two lines, like shadows cast by cold gas. These are the sodium lines, the very yellow light of streetlamps. Here, at the edge of the visible Universe, they betray the slow exhalation of a galaxy returning matter to space. Just 2 billion years after the Big Bang, an era of furious star birth, and the cosmic “lungs” are already working at full capacity.

To see this exhalation, astronomers led by Matteo Sapori turned to the prism spectrograph NIRSpec aboard the JWST. In the telescope’s archive, they found 811 galaxies with redshifts between 3 and 7 and spectra of acceptable quality. Each spectrum, a mixture of starlight and nebular emission, was first cleansed of stellar contribution using a Bayesian model. In the residue, like a cool breath on a fogged-up window, Na I D absorption emerged — a signature of neutral gas that stellar photospheres cannot produce. The low resolution made it impossible to measure velocities, but it preserved the equivalent width of the line — the total amount of absorbed light. And that, as past studies have shown, almost always means the gas is moving away from the galaxy.

The result is puzzling. Out of 811, only 20 galaxies showed a reliable Na I D signal, but tied to mass and dustiness. In dusty, actively star-forming ones, and in giants that have almost stopped their star formation, the exhalation is felt most clearly. Equivalent widths reach 16 Å. Estimates of the outflow rate, though conservative, paint a striking picture: between 4 and 12 solar masses per year. For one-third of this “golden” sample, gas is carried away faster than it turns into new stars. The neutral wind can empty the cold-gas reservoir and halt star formation — possibly in two acts. First, a powerful blast from massive stars, then a weaker but persistent one, perhaps from an active galactic nucleus, finishes the cleansing.

The outlook is mesmerizing. High-resolution spectroscopy with JWST will for the first time let us measure the speeds of these winds directly and map the outflow regions. Numerical models will trace the expelled gas’s journey to the farthest reaches — into the circumgalactic medium and intergalactic space. Then we may finally learn just how early galaxies exhaled metals, fertilizing future generations of stars. Two tiny sodium lines tell the story of galactic exhalations — the cold breath of the young Universe, which we are only beginning to hear.