Dust in the very metal-poor galaxy Sextans A with JWST. I. Characterizing the evolved stellar population of Sextans A based on JWST observations and stellar evolution models

C. Gavetti (Univ. Roma Tre, INAF-Roma), F. Dell'Agli (INAF-Roma), E. Tarantino (STScI), M.L. Boyer (STScI), I. McDonald (Univ. Manchester, Open Univ.), J. Th. van Loon (Keele Univ.), D.A. Garcia-Hernandez (Inst. Astrof. Canarias, Univ. La Laguna), M.A.T. Groenewegen (Roy. Obs. Belgium), A. Nanni (Nat. Centre Nuclear Research, Warsaw, INAF-d'Abruzzo), J.A.D.L. Blommaert (Vrije Univ.), R.D. Gehrz (Univ. Minnesota), L.M. Gerlach (Keele Univ), S. Goldman (STScI), M. Marengo (Florida State), K.B.W. McQuinn (STScI, Rugers), J.M. Oliveira (Keele Univ.), J. Roman-Duval (STScI), R. Sahai (JPL), E.D. Skillman (Univ. Minnesota), B.F. Williams (Univ. Washington), A. Javadi (IPM, Tehran) O.C. Jones (Royal Obs. Edinburgh) F. Kemper (ICE, ICREA, IEEC, Barcelona) F. La Franca (Univ. Roma Tre), G.C. Sloan (STScI, UNC)

2026, ApJ, 1006, 131

Full manuscript available locally (PDF), from the arXiv (2606.07421). or from the Astrophysical Journal.

The nearby star-forming dwarf galaxy Sextans A offers a unique window into galaxy evolution in the early Universe, owing to its extremely low metallicity (~0.01-0.07 Zsun). Recent JWST imaging of Sextans A spanning 1-21 µm; enables a detailed characterization of its dusty stellar populations and interstellar medium. In this work, we compare the observed JWST color-magnitude distributions of evolved stars with stellar evolution and dust-formation models to characterize the properties of the asymptotic giant branch (AGB) population, including progenitor mass, formation epoch, metallicity, and dust production. Evolutionary tracks for 0.8-7 Msun stars with metallicity Z = 10-3 provide good agreement with the overall distribution of AGB stars in Sextans A. More than 90% of the AGB population occupies a nearly vertical sequence in the color-magnitude diagrams, corresponding to stars spanning a wide range of masses and ages but exhibiting little or no circumstellar dust. This sequence appears to be dominated by oxygen-rich (M-type) AGB stars and reveals that the F444W flux is a robust luminosity diagnostic. A small subset of sources displays strong IR excesses and is dominated by carbon stars descending from 1.25 to 1.5 Msun progenitors that formed ∼2-3 Gyr ago and are currently in the final AGB phases. Their MIRI colors imply very low metallicities, consistent with estimates from the red giant branch morphology (~0.01-0.02 Zsun). Finally, we show that the JWST/NIRCam F277W - F444W color serves as an effective proxy for the dust production rate, with models predicting rates up to ~10-7 Msun yr-1 for the reddest sources in Sextans A.

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