NASA reported May 1 that JWST observations of Sextans A — a dwarf galaxy four million light-years from the Milky Way, with 3 to 7 percent of the Sun's heavy-element content — caught metallic iron dust and silicon carbide forming around aging stars. The dust formed despite the galaxy's depleted metal reservoir, in a setting astronomers had treated as too primitive for it. [1]

What surprised the team is the precursor problem. Iron and silicon carbide dust grains in the local universe typically form in environments rich in the heavy elements supernovae and aging stars have already seeded. Sextans A barely has them. Its gravitational pull is too weak to retain the metals its stars produce, so its interstellar medium reads, chemically, like the early universe. And yet the dust grains have formed. [1]

The implication runs back to the cosmic dawn. If a dwarf galaxy with early-universe metallicity can forge iron and silicon carbide grains today, the first galaxies — which lived in even thinner chemistry — may have built planet-precursor materials earlier than astronomers modeled. Polycyclic aromatic hydrocarbons, the carbon molecules associated with star formation, also showed up in Sextans A, making it the lowest-metallicity galaxy known to contain them.

Elizabeth Tarantino, postdoctoral researcher at the Space Telescope Science Institute and lead author, called Sextans A "a blueprint for the first dusty galaxies." The two studies appear in The Astrophysical Journal. On X, the Space Telescope Science Institute amplified the result through the JWST network. Whether follow-up spectroscopy clarifies the formation pathway Tuesday is the open question.

-- KENJI NAKAMURA, Tokyo