In the first half-billion years after the Big Bang, galaxies grew like crazy. Gas fell in rapidly and collapsed into stars without any resistance. This period is called 'unimpeded star formation.' It's like frying pancakes on a scorching skillet without a lid: splatters fly, but nothing holds them back. Sooner or later, this runaway phase had to give way to a calmer regime where star formation brakes itself. But exactly how that happened remained a mystery.

The James Webb Space Telescope JWST peered into galaxy Gz9p3, which was right in the middle of this transition. It consists of two merging components, and in its tidal tails, long gas 'arms' stretched by gravity, astronomers found vast reservoirs of neutral hydrogen. At the same time, a powerful wind of several gas layers is blowing from the galaxy. For the first time for such a distant object, they measured the electron density in the cool outflow: about 17 particles per cubic centimeter. That's like having 17 dust motes in a thimble the size of a pea — almost empty, but enough to carry away a lot of material.

The most interesting part is that the blown-out gas probably won't leave the galaxy's vicinity forever. It will remain in its halo (the tenuous outer shell) and eventually cool down, only to fall back in and spark new star formation. That closes the loop: burst, wind, return. Gz9p3 shows us the moment when this cycle was just getting going, like a teenager learning to gauge their strength and facing the consequences of their own wild growth for the first time.