Metal Before It Was Cool: Super-Enrichment in the Early Universe

Astronomers have long known that the universe has grown more metallic over time: in its younger, purer days, it was composed almost entirely of hydrogen and helium. Recently, however, researchers discovered a galaxy that was notably ahead of the trend and had already amassed a high metal content only a billion years after the Big Bang.

Building Starstuff

Nearly all of the atoms heavier than helium began their lives in a star, the forges of the cosmos responsible for crushing primordial materials into the rich array of elements we see today. These forges run day and night, constantly churning through the universe’s finite supply of hydrogen and helium. Consequently, the overall budget of hydrogen goes down over time, while the proportion of heavier elements (which astronomers call “metals,” regardless of their actual metallic properties) grows. When astronomers look back in time and observe the high-redshift universe, they expect to find mostly pure hydrogen and helium, unpolluted by the starstuff that makes up rocks and people and telescopes.

This prediction usually stands up to observations, and when looking at galaxies with redshifts beyond z = 4 (those born in the first roughly 1.5 billion years after the Big Bang), researchers most often observe clouds of gas with barely any metals. However, a collaboration led by Jianghao Huyan, University of South Carolina, recently discovered a surprising contradiction to this harmonious agreement: their observations of hazy galaxy at z = 4.7 revealed a metal fraction more than two orders of magnitude above the prediction for such a young source.

Mystery Metals

An 8-panel plot, each of which shows wavelength vs. flux for a different absorption line. The red model generally agrees quite well with the black data.

Zoomed-in regions of the measured spectrum centered on different metal absorption lines. The red curve represents the best-fitting model spectrum, while the black histogram is shows their data. [Huyan et al. 2023]

Huyan and colleagues made their discovery when observing a distant quasar named SDSS J002526.84-014532.5 that sits at a redshift of 5.07. Sitting between Earth and this luminous radiation source is a still distant, but slightly closer, galaxy at a redshift of 4.74. As the light from the quasar passed through the wispy gas of the intervening galaxy on the way to our telescopes, specific wavelengths were preferentially absorbed by the molecules and atoms it encountered along the way. By measuring the relative amount of this absorption across many wavelengths, the researchers could back out which elements had tried to block the light’s path, and how dense each species must have been within the gas.

They found that the galaxy possessed a substantial amount of carbon, oxygen, magnesium, and other heavy elements. In fact, just 1.2 billion years after the Big Bang, this galaxy already had a higher relative amount of carbon and oxygen than our own Sun that was born many billions of years later. This was a startling find: models of early galaxy formation expect a significantly smaller fraction of metals, even when accounting for the large uncertainties about the behavior of theorized-but-not-yet-seen first-generation stars.

A 2D plot showing a set of gently downward sloping curves, meant to indicate that metallicity should drop with larger redshifts. The point marking this galaxy lies near the upper right corner of the plot.

Previous measurements and predictions of galaxy metallicity as a function of redshift. The galaxy in question here is marked as the pink triangle that lies far above the model curve. Click to enlarge. [Huyan et al. 2023]

Like many of the most intriguing surprise discoveries, the authors currently have no explanation for what could lead to such a substantial metal enrichment. They concede that it’s possible this particular line of sight may have passed through an anomalously developed region of gas, and that on average, the galaxy as a whole may be as metal-poor as expected. Even in this scenario, however, they cannot explain how that small patch could have been processed to such an extent. Perhaps it is time to revisit the chemical evolution models of early galaxies; perhaps there is something special about this particular galaxy that remains to be uncovered. For now, though, astronomers have another mystery on their hands, and once again the universe has proved ready to challenge our attempts to explain it.


“Discovery of Super-enriched Gas ∼1 Gyr after the Big Bang,” Jianghao Huyan et al 2023 ApJL 954 L19. doi:10.3847/2041-8213/aceefe