Variations in the 3.3 $μ$m Polycyclic Aromatic Hydrocarbon Feature Across Nearby Galaxies Driven by Metallicity and Radiation Field Spectrum

Abstract

We use JWST NIRCam imaging to investigate the 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) feature in nearby galaxies. NIRCam observations of the 3.3 $μ$m feature are emerging as a powerful tool for studying the structure of the interstellar medium (ISM) and the conditions of the dust at ~0".1 resolution. These maps require accurate subtraction of the underlying continuum emission. We present an empirical method to isolate the PAH-correlated emission in the F335M filter using the F300M and F360M filters for continuum subtraction. We find that the slope of the F335M/F300M versus F360M/F300M colors for PAH-correlated emission shows a dependence on local ISM properties, with the strongest dependence on specific star formation rate. Weaker emission features captured by these bands appear suppressed relative to the main 3.3 $μ$m feature in regions of active star formation. We find trends in the 3.3/7.7 and 3.3/11.3 $μ$m ratios that suggest changes in PAH size, charge, and heating by a varying radiation field spectrum. We find decreases in both band ratios with increasing metallicity, which we attribute to a shift to smaller PAH populations at low metallicity. Comparison to optical ionized gas line ratios and dust models show that variations in the interstellar radiation field spectrum influence the PAH feature ratios. This analysis supports inhibited growth formation scenarios for the observed PAH band ratio trends with metallicity and emphasizes the importance of considering the local radiation field characteristics and gas-phase metallicity when using these band ratios as PAH property diagnostics.