Resolving Emission from Small Dust Grains in the Blue Compact Dwarf II Zw 40 with JWST

We present James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) and Mid25 infrared Instrument (MIRI) integral-field spectroscopy of the nearby blue compact dwarf II Zw 40, 26 which has a low metallicity of 25% of solar. Leveraging the high spatial/spectral resolution and 27 wavelength coverage of JWST/NIRSpec, we present robust detections of the 3.3 µm polycyclic aromatic 28 hydrocarbon (PAH) emission on 10 pc scales. The strength of the Pf δ emission relative to the 3.3 29 PAH feature is significantly stronger than typical higher metallicity star-forming regions. We find that 30 3.3 µm PAH emission is concentrated near the northern super star cluster and is co-spatial with CO gas. 31 Additionally, a strong correlation exists between the 3.3/11.3 PAH ratio and radiation field hardness 32 probed by [Ne iii]/[Ne ii], providing evidence of photodestruction of PAH molecules in intense radiation 33 environments. Our analysis shows that while the overall PAH fraction is lower in II Zw 40 than in 34 higher metallicity galaxies, the contribution of the 3.3 µm PAH feature to the total PAH emission is 35 higher. We therefore propose that the PAH size distribution is fundamentally shaped by two competing 36 mechanisms: photon destruction and inhibited growth. Additionally, the high radiation field intensity 37 in II Zw 40 suggests that multi-photon heating of PAHs could also be a compelling explanation. As 38 one of the first spatially resolved studies of aromatic emission in a low-metallicity environment, our 39 spectroscopic results offer practical guidance for future observations for probing the 3.3 µm PAH feature 40 with the removal of Pf δ line using the NIRCam filters.