Growth of aromatic hydrocarbon dust particles in the extremely metal-poor galaxy Sextans A

The mid-infrared spectrum of star-forming, high-metallicity galaxies is dominated by emission features from aromatic and aliphatic bonds in small carbonaceous dust grains, often referred to as polycyclic aromatic hydrocarbons (PAHs). In metal-poor galaxies, the abundance of PAHs relative to the total dust sharply declines, but the origin of this deficit is unknown. Here we present observations by the James Webb Space Telescope that detect and resolve emission from PAHs in the 7% solar metallicity galaxy Sextans A, representing, to our knowledge, the lowest metallicity detections of PAH emission known so far. In contrast to higher metallicity galaxies, the clumps of PAH emission are compact (0.5–1.5" or 3–10 pc), which explains why PAH emission has evaded detection by lower resolution instruments. Ratios of the 3.3-μm, 7.7-μm and 11.3-μm PAH features indicate that the PAH grains in Sextans A are small and neutral, with no evidence of substantial processing from radiation fields. These results favour inhibited grain growth over enhanced destruction as the origin of the low PAH abundance in Sextans A. The compact clumps of PAH emission are likely active sites of in situ PAH growth within a dense, well-shielded phase of the interstellar medium. Our results show that PAHs can form and survive in the extremely metal-poor environments common early in the evolution of the Universe. The formation process of polycyclic aromatic hydrocarbons (PAHs) at very low metallicity is uncertain. The detection of PAHs in a 7% solar metallicity galaxy, along with emission from compact clumps and small grain sizes, helps elucidate the mechanisms at play.