Polycyclic aromatic hydrocarbons destruction in star-forming regions across 42 nearby galaxies

TL;DR

The study combines high-resolution JWST/MIRI imaging with VLT/MUSE spectroscopy to map the PAH-to-dust fraction across ~17k H II regions and ~400 SNRs in 42 galaxies, spanning $12+\log(O/H)$ from 8.0 to 8.8. It finds that PAHs are depleted in H II regions relative to diffuse ISM at fixed metallicity, with a strong anti-correlation between the PAH fraction and the ionization parameter for $12+\log(O/H) > 8.2$, pointing to ionizing UV radiation as the main destruction mechanism. At low metallicity, PAH fractions decline steeply in both environments, suggesting reduced PAH formation or enhanced destruction. Shocks from SNRs show little evidence for selective PAH destruction at ~50 pc scales, implying metallicity and radiation fields primarily govern the PAH life cycle. Overall, the results support a picture where PAHs are ionized and destroyed by hydrogen-ionizing photons near H II regions, with metallicity shaping their global abundance in galaxies.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM) of Solar metallicity galaxies, where they play a critical role in ISM heating, cooling, and reprocessing stellar radiation. The PAH fraction, the abundance of PAHs relative to total dust mass, is a key parameter in ISM physics. Using JWST and MUSE observations of 42 galaxies from the PHANGS survey, we analyze the PAH fraction in over 17 000 H II regions spanning a gas-phase oxygen abundance of 12+log(O/H) = 8.0-8.8 (Z ~ 0.2-1.3 Zsun), and ~400 isolated supernova remnants (SNRs). We find a significantly lower PAH fraction toward H II regions compared to a reference sample of diffuse ISM areas at matched metallicity. At 12+log(O/H) > 8.2, the PAH fraction toward H II regions is strongly anti-correlated with the local ionization parameter, suggesting that PAH destruction is correlated with ionized gas and/or hydrogen-ionizing UV radiation. At lower metallicities, the PAH fraction declines steeply in both H II regions and the diffuse ISM, likely reflecting less efficient PAH formation in metal-poor environments. Carefully isolating dust emission from the vicinity of optically-identified supernova remnants, we see evidence for selective PAH destruction from measurements of lower PAH fractions, which is, however, indistinguishable at ~50 pc scales. Overall, our results point to ionizing radiation as the dominant agent of PAH destruction within H II regions, with metallicity playing a key role in their global abundance in galaxies.

Figures (15)

• Figure 1: Distribution of $\rm \log({\rm R_{PAH}^*})$ vs oxygen abundance color-coded by $\rm \log([SIII]/[SII])$ (tracer of the ionization parameter) for all H ii regions in the sample. Red circles show average values in different metallicity bins (reported in Tab. \ref{['tab:Rpah_vs_z_bins']}). Vertical gray dashed line marks $\rm 12 + \log(O/H) = 8.2$ as a threshold between the low- and high-metallicity regimes. The horizontal dashed line correspond to the average ${\rm R_{PAH}}$ for diffuse ISM measured by Sutter2024. Black squares correspond to measurements for WLM and NGC6822 (from low- to high-metallicity) made by Chown2025 using JWST F770W and F2100W images. The PAH fraction drops significantly in the low-metallicity regime.
• Figure 2: Distribution of ${\rm R_{PAH}^*}$ vs oxygen abundance for the entire sample of H ii regions (gray histogram with intensity tracing the probability density), and average distribution for two bins in $\log$([S iii]/[S ii]). Dashed lines are the same as in Fig. \ref{['fig:RPAH_vs_met']}. Black squares represent measurements for WLM, NGC6822 (same as in Fig. \ref{['fig:RPAH_vs_met']}), SMC and LMC from Chown2025, listed in the order of increasing metallicity. Spitzer IRAC4 and MIPS24 filters were used to estimate ${\rm R_{PAH}^*}$ for the SMC and LMC Chown2025.
• Figure 3: ${\rm R_{PAH}^*}$ (shown by color) calculated on the regularly distributed bins with unique O/H and [S iii]/[S ii] values. Both metallicity and ionization parameter are both important for setting up the PAH fraction.
• Figure 4: Distribution of ${\rm R_{PAH}^*}$ for "resolved" (luminous) H ii regions (orange) and the regions of the similar size from diffuse ISM (blue) for several gas-phase metallicity bins. Dashed and two dotted lines on each distribution show median and 25 and 75 percentile quartiles, respectively. Both ${\rm R_{PAH}}$ and ${\rm R_{PAH}^*}$ are lower in H ii regions than in diffuse ISM. A strong decrease of ${\rm R_{PAH}^*}$ in the lowest metallicity bin is seen for both H ii regions and diffuse ISM.
• Figure 5: PAH fraction anti-correlates with $\log$([S iii]/[S ii]) (ionization parameter tracer) for thousands H ii regions in 42 PHANGS-JWST galaxies. Panel (a, b) shows all $\sim 17200$ analyzed H ii regions, while $\sim 5600$ brightest "resolved" H ii regions are shown on Panels (c, d). The dashed red line shows the best-fit linear regression to the "resolved" sample defined by Eq. \ref{['eq:RPAH_vs_ip']}. Histograms on panels (b, d) show how the logarithmic residuals after subtracting this linear trend depend on metallicity. Color on panels (a, c) denotes gas-phase oxygen abundance; red contours show the probability density of the high-metallicity ($\rm 12+\log(O/H) > 8.2$) points (levels correspond to 50, 65, 80, 95 and 99 percentile intervals). The Spearman correlation coefficient ($\rho$) and RMS scatter around the linear fit are reported on panels (a, c).