In the vast choir of galactic nuclei, some voices ring out louder than the rest. In quasars whose spectral nitrogen lines are anomalously strong, the chemical makeup of the gas in the broad-line region (the gas whipping around the black hole at enormous speeds) long remained a puzzle. Astronomers argued: is all the gas there simply rich in metals, or has nitrogen in particular been added beyond measure? Now, by comparing 121 such quasars with ordinary ones, both motifs have been made out.

To separate overall metallicity from a nitrogen excess, the researchers used two kinds of tuning forks: spectral ratios. Some relied on nitrogen lines, others on lines of silicon, aluminum and oxygen, which do not care how much nitrogen was added separately. It turned out that in the nitrogen quasars both indicators run higher than in the control group matched by black hole mass and brightness. But the nitrogen-based estimate exceeds the nitrogen-free one by a factor of two and a half on average.

What is more, in the region where the metallicities overlap, the slope of the relation between the two estimates is significantly steeper for the nitrogen quasars: 0.96 versus 0.56. It is as if in an orchestra, when the overall volume rises, the nitrogen instruments grow not only louder but also larger in their share of the total sound. So these clouds really do hold more nitrogen than high metallicity alone can explain.

The standard scenario of secondary nitrogen production assumes that nitrogen forms from carbon and oxygen already forged in stars. But if the relative nitrogen excess grows with overall metallicity, that process cannot be the only one. Perhaps dense nuclear environments retain gas reprocessed by stars of a particular type, or material from the accretion disk contributes its share.

Now that the two effects have been pulled apart, astronomers can use nitrogen lines as a sharper indicator not only of overall metallicity but also of the history of gas enrichment in galactic nuclei. That will help interpret the spectra of the most distant quasars, where nitrogen lines remain visible even when other elements have already faded from view. The quasar's orchestra sounds clearer, and in it one hears not just the massed choir but individual parts as well.