JWST Captures Growth of Aromatic Hydrocarbon Dust Particles in the Extremely Metal-poor Galaxy Sextans A

TL;DR

This study uses JWST to detect and analyze PAH dust particles in the extremely metal-poor galaxy Sextans A, revealing their small size, neutral state, and likely in-situ growth, challenging previous assumptions about PAH destruction.

Contribution

First detection of PAH emission in a galaxy with only 7% solar metallicity, showing PAHs can form and survive in such environments.

Findings

PAH emission detected in Sextans A at 7% solar metallicity

PAH grains are small, neutral, and form in dense, shielded regions

PAH abundance decline is due to inhibited growth, not destruction

Abstract

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. We present JWST observations that detect and resolve emission from PAHs in the 7% Solar metallicity galaxy Sextans A, representing the lowest metallicity detection of PAH emission to date. 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 evaded detection by lower resolution instruments like Spitzer. Ratios between the 3.3, 7.7, and 11.3 $\mu$m PAH features indicate that the PAH grains in Sextans A are small and neutral, with no evidence of significant processing from the hard radiation fields within the galaxy. These results favor 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 extremely metal-poor environments common early in the evolution of the Universe.