How did youthful galaxies in the early universe fatten up to become the behemoths we see today? One explanation, put forth more than a decade ago, is that galaxies in the early universe supped on cold gas to fuel their prodigious star formation. Theoretical astrophysicist Avishai Dekel of the Hebrew University of Jerusalem found that narrow streams of intergalactic gas could act as supply lines, penetrating a budding galaxy's hot halo of gas and feeding that galaxy's growth. Yet the faint streams of cold gas have proved difficult to detect.

A chance cosmic alignment has now brought a galactic gas line to light. Neil Crighton of the Max Planck Institute for Astronomy in Heidelberg and his colleagues examined a brilliant, distant quasar whose light, en route to Earth, pierced an intervening galaxy when the universe was only about three billion years old. The chemical constituents of the galaxy absorbed specific wavelengths of the quasar's light, imprinting a signature of the gas supplying the galaxy.

The gas surrounding the young galaxy “has all the characteristics we'd expect of a cold accretion stream,” says Crighton, lead author of a recent study in Astrophysical Journal Letters. The telltale traits include low temperature, high density, and a low abundance of elements other than hydrogen and helium forged in the big bang.

Dekel is not ready to claim victory from a single detection, however. “We will have to see many of those to make it compelling,” he says.