The Metallicity Dependence of PAH Emission in Galaxies II: Insights from JWST/NIRCam Imaging of the Smallest Dust Grains in M101

Abstract

We explore the physical origins of the observed deficit of polycyclic aromatic hydrocarbons (PAHs) at sub-solar metallicity using JWST/NIRCam imaging of the nearby galaxy M101, covering regions from solar metallicity (Z$_{\odot}$) down to 0.4 Z$_{\odot}$. These maps are used to trace the radial evolution of the shortest-wavelength PAH feature at 3.3 $\mu$m, which is emitted preferentially by the smallest PAHs ($<100$ carbon atoms). The fractional contribution of PAH 3.3 $\mu$m to the total PAH luminosity ($\Sigma$PAH) increases by 3x as metallicity declines, rising from $\sim$1$\%$ to $\sim$3$\%$ over the observed range, consistent with prior predictions from the inhibited grain growth model based on Spitzer spectroscopy. We explore model refinements including photon effects and alternative size evolution prescriptions, and find that a modest amount of small grain photo-destruction remains possible, provided the grain size cutoff does not exceed $\sim55$ carbon atoms. The best-fit models predict 3.3 $\mu$m/$\Sigma$PAH will rise to $\sim5.6-7.7\%$ at 10$\%$ Z$_{\odot}$. Surprisingly, even as $\Sigma$PAH drops significantly relative to the total infrared luminosity (TIR) as metallicity declines, 3.3 $\mu$m/TIR alone rises, potentially indicating the mass fraction of the smallest PAH grains increases as the total dust content in galaxies drops. The current model cannot fully reproduce this trend even if the unusually strong effects of changing radiation field hardness on 3.3 $\mu$m/TIR are included. This may be evidence that the smallest PAHs are uniquely robust against destruction and inhibited growth effects. These results highlight the pivotal role that short-wavelength PAH emission can play in studies of low-metallicity and high-redshift galaxies.