In a distant galaxy, light from hot gas should reveal its composition. The James Webb Telescope split that light into a spectrum, and astronomers saw: oxygen lines too weak. The conclusion seemed obvious — the galaxy contains almost no elements heavier than helium; it's one of those pristine systems that haven't had time to enrich with metals. But it turned out to be not so simple.

The weakness of oxygen lines can be explained differently. If the gas has a very high density of free electrons, they collide with oxygen ions and quench their emission. The gas remains oxygen-rich but looks poor. To distinguish these two cases, one must measure the electron density.

The traditional method didn't do this. It assumed low density, as in typical star-forming regions. The new approach, applied to four galaxies from the SPURS survey, allowed determining density and temperature simultaneously. For three galaxies, the density turned out to be hundreds of thousands of times higher than assumed. Their true metallicities increased by tens of times.

These three galaxies are 'impostors.' They pretended to be metal-poor, but actually contain from 2% to 30% of solar metallicity. Only one of the four truly turned out to be poor. If such a fraction of impostors is typical, then conclusions about the rate of chemical enrichment in the early universe will need revision.

Now that the mask is off, astronomers can more accurately search for truly pristine galaxies. The method that simultaneously determines temperature and density can be applied to any distant galaxy with accessible oxygen lines. And then the illusion will no longer deceive.