The Fraction of Dust Mass in the Form of Polycyclic Aromatic Hydrocarbons on 10–50 pc Scales in Nearby Galaxies

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous component of the interstellar medium (ISM) in z ∼ 0 massive, star-forming galaxies and play key roles in ISM energy balance, chemistry, and shielding. Wide field-of-view, high-resolution mid-infrared (MIR) images from JWST provide the ability to map the fraction of dust in the form of PAHs and the properties of these key dust grains at 10–50 pc resolution in galaxies outside the Local Group. We use MIR JWST photometric observations of a sample of 19 nearby galaxies from the Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) survey to investigate the variations of the PAH fraction. By comparison to lower-resolution far-infrared mapping, we show that a combination of the MIRI filters (R PAH = [F770W+F1130W]/F2100W) traces the fraction of dust by mass in the form of PAHs (i.e., the PAH fraction, or q PAH). Mapping R PAH across the 19 PHANGS galaxies, we find that the PAH fraction steeply decreases in H ii regions, revealing the destruction of these small grains in regions of ionized gas. Outside H ii regions, we find R PAH is constant across the PHANGS sample with an average value of 3.43 ± 0.98, which, for an illuminating radiation field of intensity 2–5 times that of the radiation field in the solar neighborhood, corresponds to q PAH values of 3%–6%.