March 2026 - Picture of the Month
Dwarf galaxy Sextans A
This month's colourful image is of the irregular dwarf galaxy Sextans A. It's box shape, which has been sculpted by a succession of supernovae, is filled with bright young and massive stars and H-II regions around its periphery for that splash of colour.
As the name suggests, it's a small galaxy with a diameter of about 2.45 kpc (8,000 light-years). It lies only around 1.4 Mpc (4.6 million light-years) away at the edge of our Local Group. Sextans A is also chemically primitive: it displays very low metallicity being comprised mostly of hydrogen and helium. This is largely due to its low mass as it lacks the gravitational strength to retain the the heavier elements that are energetically ejected in supernovae and which would then contribute to the generation of complex chemical species during the late evolution of massive stars.
However, its low metallicity makes Sextans A a good analogue for early galaxies, and a convenient target for detailed observation due to its proximity.
A recent survey of asymptotic giant branch (AGB) stars in Sextans A with the James Webb Space Telescope (JWST) revealed the presence of Silicon Carbide (SiC) dust. These dwarf galaxies were expected to lack the heavier elements that are required to create SiC dust, in fact they didn't expect to find any dust at all, yet it turned out to be abundant and composed almost entirely of iron. The authors found that one of the stars surveyed was producing dust via an unusual mechanism, demonstrating that solid matter can be created even when many of the usual components are absent.1
The discovery of SiC dust in such a metal poor galaxy is evidence that solid particles can be created even at this apparently early stage of galactic formation.
Polycyclic aromatic hydrocarbons (PAHs) are interpreted as signifying star formation, due to their catalytic effects on the formation of molecular hydrogen and moderation of ionising radiation. They were also thought to be absent in low mass galaxies, since the lack of dust to shield these compounds from the intense UV radiation from young stars would permit rapid decomposition.
Another study used JWST data of Sextans A's interstellar medium to reveal dense pockets of PAHs. In large galaxies PAHs are distributed throughout the galaxy inside large dust and gas clouds, many of which have featured in this column. This result suggests PAHs might not be so rare in such low metal environments, rather it's the sparse distribution of these clumps within small distant galaxies that makes them hard to measure.2
The low surface brightness of Sextans A makes it a better imaging target than one for the visual observer. We have an observation image of this dwarf galaxy on the website already, provided by David Davies, and there's another nice amateur image from the Gemsbock Observatory, both from 2025.
James Whinfrey - Website Administrator.
References
- M. L. Boyer et al, “Discovery of SiC and Iron Dust around AGB Stars in the Very Metal-poor Sextans a Dwarf Galaxy with JWST: Implications for Dust Production at High Redshift”, The Astrophysical Journal, Volume 991, Number 1. DOI 10.3847/1538-4357/adf06a
- Tarantino, E. J., “JWST Captures Growth of Aromatic Hydrocarbon Dust Particles in the Extremely Metal-poor Galaxy Sextans A”, arXiv e-prints, Art. no. arXiv:2512.04060, 2025. doi:10.48550/arXiv.2512.04060
- “NASA Webb Finds Early-Universe Analog’s Unexpected Talent for Making Dust”, NASA Webb Mission Team, 9 Jan 2026, https://science.nasa.gov/missions/webb/nasa-webb-finds-early-universe-analogs-unexpected-talent-for-making-dust/ (Accessed on 23 Feb 2026).
