Unveiling the Aromatic and Aliphatic Universe at Redshifts <i>z</i> ∼ 0.2–0.5 with JWST NIRCam/WFSS

Abstract Utilizing deep NIRCam/WFSS data from JWST’s FRESCO program, we spectroscopically survey the 3.3 μ m aromatic and 3.4 μ m aliphatic C–H stretching emission bands of polycyclic aromatic hydrocarbon (PAH) molecules in galaxies at redshifts z ∼ 0.2–0.5. Unlike pre-JWST studies, largely limited to infrared (IR)-bright galaxies ( L IR ≳ 10 11 L ⊙ ) at z ≲ 0.1, we probe 200 galaxies down to L IR ∼ 10 8.5 –10 10 L ⊙ , well beyond the local Universe. The 3.3 μ m emission is detected at ≥3 σ in 88 out of 187 galaxies, correlating tightly with galaxy IR luminosity and star formation rate (SFR) and confirming the 3.3 μ m PAH as a viable SFR tracer. Despite a large scatter, the 3.3 μ m–to–IR luminosity ratio ( L 3.3 / L IR ) exhibits a strong metallicity dependence with a drop of L 3.3 / L IR by a factor of ≳10 at 12+log(O/H) ∼ 8.4–8.5 toward lower metallicities. The 3.4 μ m emission is detected in 37 out of 159 galaxies, with the 3.4 μ m–to–3.3 μ m luminosity ratio ( L 3.4 / L 3.3 ) spanning from ∼0.05 to ∼0.58 (median ∼0.19), corresponding to PAH aliphatic fractions of ∼0.78%–8.3% (median ∼2.9%) in terms of fractional carbon atoms in aliphatic units. While L 3.4 / L 3.3 does not depend significantly on redshift, stellar mass, metallicity, or galaxy morphology, it does decrease with various SFR tracers, suggesting that ultraviolet photons in active star-forming regions may strip aliphatic side groups from PAH molecules. Our study showcases the unique power of JWST’s NIRCam/WFSS to systematically map PAH aromatic and aliphatic content in statistically significant, less biased galaxy samples, providing critical insights into PAH chemistry and its connection to galaxy properties.