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

TL;DR

This study demonstrates that polycyclic aromatic hydrocarbons (PAHs) can form and persist in extremely metal-poor environments, by detecting and resolving compact PAH clumps in the 7% solar metallicity galaxy Sextans A with JWST. Using simultaneous NIRCam and MIRI imaging and a robust continuum-subtraction framework, the authors isolate PAH flux in three key bands (3.3, 7.7, 11.3 μm) and identify small, neutral PAHs concentrated in 3–10 pc clumps, with little evidence for strong processing by the local radiation field. The observed band ratios align with small, neutral PAH populations and favor inhibited PAH grain growth over enhanced destruction as the origin of the low PAH abundance, consistent with shielding in dense ISM pockets that could support in-situ PAH growth. These results extend the metallicity dependence of PAHs to the lowest detected level, underscore the importance of dust shielding and dense environments for PAH survival, and have implications for ISM physics in early-universe analogs and high-redshift galaxies.

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 $μ$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.

Figures (9)

• Figure 1: JWST NIRCam and MIRI photometry of Sextans A reveals emission from PAHs a.) Optical image of Sextans A (Credit: KPNO/NOIRLab/NSF/AURA) with the JWST field outlined in magenta. b.) JWST color composite image of Sextans A (Blue: F115W, Cyan: F150W + F200W, Green: F335M, Yellow: F560W, Orange: F770W, Red: F1000W; Credit: A. Pagan, STScI). The region of brightest PAH emission is outlined in green. c.--e.) 3-color zoom-ins on this region, with the PAH filter in green and adjacent continuum filters in red and blue (c. Red: F360M, Green: F335M, Blue: F300M; d. Red: F1000W, Green: F770W, Blue: F560W; e. Red: F1500W, Green: F1130W, Blue: F1000W). We apply the Lupton RGB algorithm Lupton2004 to visualize the high dynamic range of the JWST data, using the same asinh stretch and scaling for all panels. The compact green clumps trace enhanced emission from the PAH 3.3, 7.7, and 11.3 $\mathrm{\mu m}$ features, with the brightest clump highlighted by a circle.
• Figure 2: SED of the brightest clump of PAH emission in Sextans A. The flux from clump 1 (encircled in Figure \ref{['fig:Optical_JWST']}c.--e) is shown by magenta and green horizontal lines, with green denoting the PAH-centered filters (F335M, F770W, F1130W). Line widths indicate the wavelength range where the filter transmission falls to 50% of its maximum. Errorbars are smaller than the linewidth. The excess flux in the PAH filters (green) relative to the continuum filters (magenta) demonstrates the detection of PAHs. PHOENIX stellar models Husser2013 and Draine2021 PAH/dust emission spectral models that fit the data within uncertainties are presented as the gray spectra, with the transparency scaled by the $\chi^2$ and the average spectrum presented in black. The spread in models illustrates how different ISM conditions (e.g., radiation field, neutral gas column density, PAH size and charge distribution) can produce equally valid fits to the photometric data (see Section \ref{['sec:D21_SED']}). The purple horizontal lines show the synthetic photometry of these models for direct comparison with the observations.
• Figure 3: PAH emission clumps and their spatial distribution in Sextans A Clump structures and labels defined with the dendrogram structure analysis (see Section \ref{['sec:dendro']}) overlaid on Sextans A JWST and H$\alpha$ data. Clump properties are described in Table \ref{['tab:clump_prop']}. We require a 3$\sigma$ detection in all three PAH bands$-$ 3.3, 7.7, and 11.3 $\mathrm{\mu m}$$-$ to define a PAH emission clump. We present the clumps on: a.) RGB 3-color images of the stellar and hot dust continuum and b.) the continuum-subtracted PAH flux in each PAH band, where the red color is 11.3 $\mathrm{\mu m}$, green is 7.7 $\mathrm{\mu m}$, and blue is 3.3 $\mathrm{\mu m}$ all scaled at the same asinh stretch to emphasize extended emission. Emission from continuum dominates in Sextans A, as there are many more features in the continuum image as compared to the PAH data. The continuum-subtracted PAH data show the importance of requiring a detection in all three PAH bands, as spurious background galaxies can be bright in a single band, resulting in the red and blue artifacts in the PAH RGB image. PAH clumps are visible as the yellow and white clumps of emission outlined from the PAH clump boundary. The white circle marks the "sf-3" region defined in Shi2014 used to calculate the total infrared luminosity and compare to the PAH flux. c.) Observations of the F1500W filter, which acts as a high resolution probe of the hot dust as the reddest available filter in our JWST dataset. PAH clumps frequently appear near regions of hot dust, though the hot dust exhibits a more extended spatial distribution, especially in the southern region near clumps 12 and 13. d.) H$\alpha$ observations of Sextans A from Kennicutt2008 that trace the ionized gas, which shows similar spatial distribution with the F1500W hot dust map. We also overlay the massive star catalog from Lorenzo2022, including O and early-type B stars, as the cyan "x" symbols. Some PAH clumps are spatially coincident with regions of H$\alpha$ emission, though they generally do not coincide with the brightest H$\alpha$ peaks, suggesting a complex geometry between the photodissociation region (PDR) and H II region structure.
• Figure 4: Continuum-subtracted images of the 3.3, 7.7, and 11.3 $\mathrm{\mu m}$ features reveal compact and clump structure The PAH emission flux for the three PAH features, as well as the hot dust from the F1500W filter, targeted through the photometric imaging in JWST GO 2391. The region is defined by the green square in Figure \ref{['fig:Optical_JWST']} and is the area with the brightest and most concentrated PAH emission. All data is PSF matched to the F1500W filter prior to continuum subtraction, ensuring each image has a resolution of $\sim$0.48$"$ (PSF in bottom left corners). PAH emission clumps are compact, comparable to the PSF, with sizes of $0.5-1.5"$ or $3-10 \rm \, pc$. Contours are at the 3$\sigma$, 5$\sigma$, 10$\sigma$, and 20$\sigma$ level.
• Figure 5: The fraction of infrared power emitted by PAH features as a function of metallicity Comparison of $\Sigma$PAH/TIR, the ratio between the integrated intensity of detected PAH features and the total infrared luminosity, across a range of galaxy samples. Sextans A is shown as the orange star, the sample of metal-poor blue compact dwarf (BCD) galaxies from Hunt2010 are displayed as blue circles, and the higher metallicity non-AGN SINGS galaxies from Smith2007 are the green squares. We also plot the empirical fit of $\Sigma$PAH/TIR as a function of the metallicity gradients for the galaxies M101, NGC 628, and NGC 2403 from Whitcomb2024 in the black line, with the uncertainty presented as gray bars and the extrapolation shown as the dashed line. Sextans A is the lowest metallicity galaxy with a robust detection of PAH emission and generally $\Sigma$PAH/TIR follows the trends with metallicity seen in the other samples. Figure adapted from Hunt2010.