Light from distant quasars—early galaxies that shine with tremendous brightness—has given researchers a new clue to the origin of vast magnetic fields studding today's galaxies: They were running strong when the universe was only a third of its present age.

Astronomers had observed that radio emissions from quasars tend to be angled, or polarized, in such a way that powerful magnetic fields must have twisted them. The greater their distance from Earth, the more polarized their light. But researchers didn't know whether the magnetic fields were part of the quasar or were present in galaxies encountered by quasar light as it made its journey to our telescopes.

So a team led by astronomers from the Swiss Federal Institute of Technology (ETH Zurich) scanned more than 70 of those quasars using the European Southern Observatory's Very Large Telescope in Chile to look for signs of galaxies hiding in front of the quasars. Specifically, they checked for a feature called the magnesium(II) absorption line, a reduction in the intensity of light of a certain wavelength, which is a commonly used indicator that gas from a star-forming galaxy has soaked up that light.

The researchers report today in Nature that light from quasars showing the magnesium(II) line was more strongly polarized than light from other quasars in the sample. The interpretation: light did indeed pass through regular galaxies and that it likely acquired its polarization in the process.

They estimated the age of the magnetic galaxies by measuring the red shift of the absorption line—the observed reddening of light that occurs when galaxies move rapidly apart. The typical red shift of the inferred galaxies corresponded to an age of 5.2 billion years, study author Francesco Miniati says. Precision measurements of the cosmic microwave background peg today's universe at 13.7 billion years old.

Miniati says the earliest galaxies did not have magnetic fields. But somehow they became commonplace, perhaps as a result of violent events in quasars that seeded other galaxies with tiny but strong magnetic fields.

Researchers speculate that turbulent matter expanded and strengthened the magnetic fields by pulling them like taffy. "What these observations tell us is that this stretching process took place very rapidly," says Ellen Zweibel, an astrophysicist at the University of Wisconsin–Madison.

She says that incorporating those fields into computer models of star formation might help solve the puzzle of why stars that formed in the past five billion years or so were typically about the size of the sun—smaller than earlier ones. Galactic magnetic fields are thought to influence the size of stars by making them spin slower.

The observations "tell us that magnetic fields really were there in these early times," she says, "and you have to include them in the models."