In the cores of some distant galaxies, nitrogen glows too brightly. It is as if one voice in a choir sounded louder than all the rest. For a long time astronomers could not tell whether the whole choir had grown louder or whether that one voice had been boosted on its own. Now, by comparing 121 such quasars with ordinary ones, they have heard the answer: both.

To separate the two effects, the researchers used two ways of measuring. One looked at nitrogen lines, the other at lines of silicon, aluminum and oxygen, which do not depend on how much nitrogen is present. It turned out that in the nitrogen quasars both indicators are higher than in ordinary ones. But the nitrogen indicator exceeds the nitrogen-free one by a factor of two and a half, and that gap grows with overall metallicity. This means nitrogen was added not only through general metal enrichment but also through some separate process.

Nitrogen is usually thought to form from carbon and oxygen inside stars, and its share should be tied to overall metallicity. But these quasars hold more nitrogen than such a simple relationship predicts. Perhaps gas processed by certain stars is being retained in dense regions near the black hole, or material from the disk around the black hole contributes nitrogen of its own. Now that the two effects have been separated, astronomers can read the chemical history of distant galaxies from their spectra more accurately.