NASA Webb Finds Early-Universe Analog’s Unexpected Talent for Making Dust

Using NASA’s James Webb Space Telescope, astronomers have spotted two rare types of dust in the dwarf galaxy Sextans A, one of the most chemically primitive galaxies near the Milky Way. The discovery of metallic iron and silicon carbide (SiC) dust produced by aging stars, as well as tiny clusters of carbon-based molecules, shows that even when the universe had only a fraction of today’s heavy elements, stars and the interstellar medium could still form solid dust grains. This research with Webb reshapes ideas about how early galaxies evolved and developed the building blocks of planets, as NASA explores the secrets of the universe and our place in it.

Sextans A is about 4 million light years away and contains only 3 to 7 percent of the Sun’s metallic content, or metallicity, the astrophysical term for elements heavier than hydrogen and helium. Because the galaxy is so small, unlike other nearby galaxies, its gravitational pull is too weak to retain heavy elements like iron and oxygen created by supernovae and aging stars.

Galaxies like this resemble those that filled the early universe just after the big bang, when the universe was composed mainly of hydrogen and helium, before stars had time to enrich space with “metals.” Because it is relatively close, Sextans A gives astronomers a rare chance to study individual stars and interstellar clouds in conditions similar to those following the Big Bang.

“Sextans A gives us a model for early dusty galaxies,” said Elizabeth Tarantino, a postdoctoral researcher at the Space Telescope Science Institute and lead author of the results of one of two studies presented at a news conference at the 247th meeting of the American Astronomical Society in Phoenix. “These results help us interpret the most distant galaxies photographed by Webb and understand what the universe was building with its first ingredients.”

One of these studies, published in the Astrophysical Journal, focused on half a dozen stars with the low-resolution spectrometer on board Webb’s MIRI (Mid-Infrared Instrument). The data collected shows the chemical fingerprints of stars swelling very late in their evolution, called asymptotic giant branch (AGB) stars. Stars with masses between one and eight times that of the Sun pass through this phase.

“One of these stars is at the high-mass end of the AGB range, and stars like this typically produce silicate dust. However, with such low metallicity, we expect these stars to be almost dust-free,” said Martha Boyer, associate astronomer at the Space Telescope Science Institute and lead author of this second companion study. “Instead, Webb revealed a star forging dust grains made almost entirely of iron. This is something we have never seen in stars that are analogues of the stars of the early universe.”

Silicates, the usual dust formed by oxygen-rich stars, require elements like silicon and magnesium that are almost non-existent in sextans A. It would be like trying to make cookies in a kitchen without flour, sugar and butter.

A normal cosmic cuisine, like the Milky Way, contains these crucial ingredients in the form of silicon, carbon and iron. In a primitive cuisine, like Sextans A, where almost all of these ingredients are missing, you have virtually no flour or sugar. Therefore, astronomers expected that without these key ingredients, Sextans A stars would not be able to “cook” much dust.

However, not only did they find dust, but Webb also showed that one of these stars used a completely different recipe than usual to make that dust.

The iron dust alone, along with the silicon carbide produced by the less massive AGB stars despite the galaxy’s low silicon abundance, proves that evolved stars can still build strong material even when typical ingredients are missing.

“The dust of the early universe may have been very different from the silicate grains we see today,” Boyer said. “These iron grains absorb light efficiently but leave no sharp spectral imprint and may contribute to the large dust reservoirs seen in distant galaxies detected by Webb.”

In the companion study, currently undergoing peer review, Webb imaged the interstellar medium of Sextans A and discovered polycyclic aromatic hydrocarbons (PAHs), which are complex carbon-based molecules and the smallest dust grains that glow in infrared light. This discovery means that Sextans A is now the lowest metallicity galaxy ever found containing PAHs.

But unlike the broad, rapid emission of PAHs seen in metal-rich galaxies, Webb revealed PAHs in tiny, dense pockets only a few light-years across.

“Webb shows that PAHs can form and survive even in the most metal-poor galaxies, but only in small, protected islands of dense gas,” Tarantino said.

The clusters likely represent regions where dust shielding and gas density reach just high enough to allow PAHs to form and grow, solving a decades-old mystery of why PAHs seem to disappear in metal-poor galaxies.

The team has an approved Webb Cycle 4 program to use high-resolution spectroscopy to further study the detailed chemistry of Sextans A PAH clusters.

Together, the results show that the early universe had more diverse dust production pathways than more established and proven methods, such as supernova explosions. Additionally, researchers now know that there is more dust than expected at extremely low metallicities.

“Every discovery in Sextans A reminds us that the early universe was more inventive than we imagined,” Boyer said. “Clearly, stars found a way to make the building blocks of planets long before galaxies like ours existed.”

The James Webb Space Telescope is the world’s first space science observatory. Webb solves the mysteries of our solar system, looks beyond distant worlds around other stars, and probes the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

To learn more about Webb, visit:

https://science.nasa.gov/webb

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