2025ApJ...986..156L

Utilizing deep NIRCam/WFSS data from JWST's FRESCO program, we spectroscopically survey the 3.3 μm aromatic and 3.4 μm 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 ≳ 10¹¹L_⊙) at z ≲ 0.1, we probe 200 galaxies down to L_IR ∼ 10^8.5-10¹⁰L_⊙, well beyond the local Universe. The 3.3 μm emission is detected at ≥3σ in 88 out of 187 galaxies, correlating tightly with galaxy IR luminosity and star formation rate (SFR) and confirming the 3.3 μm PAH as a viable SFR tracer. Despite a large scatter, the 3.3 μm-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 3.4μm emission is detected in 37 out of 159 galaxies, with the 3.4 μm-to-3.3μm 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.