Dust Formation in a Primitive Environment

Observations suggest that Asymptotic Giant Branch (AGB) stars contribute significantly to the dust budgets of galaxies, from our own Presolar Nebula to the Magellanic Clouds. However, models suggest their contribution should decrease with metallicity, possibly becoming overshadowed by supernova and interstellar grain growth in primitive environments. Recent observations of nearby dwarf galaxies contradict this prediction, showing that AGB dust forms easily at low metallicity, which allows AGB stars to contribute dust as early as 30 Myr after they form in the early Universe. Because of limitations in sensitivity, absolutely nothing is known about the mineralogy, the grain properties, or the even the quantity of this extremely metal-poor dust. We propose observations of the nearby dwarf galaxy Sextans A, which is the Goldilocks galaxy for a comprehensive assessment of the effect of primitive abundances on dust formation because it 1) is nearby enough to escape crowding and sensitivity limits at the longest wavelengths, 2) is extremely metal-poor (just ~7% solar or 2.5x lower than the SMC), 3) is known to harbor a large AGB population that spans the full AGB mass range, 4) many of these AGB stars are known to be dusty, and 5) dust has been detected in its interstellar medium. No other galaxy comes close to providing the opportunities afforded by Sextans A. We will image the star-forming disk with NIRCam and MIRI from 0.9-25.5 microns, with careful filter selection to fully sample key molecular and dust features. We will also obtain LRS spectra of 6 known dusty stars to guide our interpretation of the photometry around the most prominent infrared features from 5-12 microns.