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High-resolution JWST-MIRI images of nearby spiral galaxies reveal emissionwith complex substructures that trace dust heated both by massive young starsand the diffuse interstellar radiation field. We present high angular (0."85)and physical resolution (20-80 pc) measurements of the probability distributionfunction (PDF) of mid-infrared (mid-IR) emission (7.7-21 $\mu$m) from 19 nearbystar-forming galaxies from the PHANGS-JWST Cycle-1 Treasury. The PDFs of mid-IRemission from the disks of all 19 galaxies consistently show two distinctcomponents: an approximately log-normal distribution at lower intensities and ahigh-intensity power-law component. These two components only emerge onceindividual star-forming regions are resolved. Comparing with locations of HIIregions identified from VLT/MUSE H$\alpha$-mapping, we infer that the power-lawcomponent arises from star-forming regions and thus primarily traces dustheated by young stars. In the continuum-dominated 21 $\mu$m band, the power-lawis more prominent and contains roughly half of the total flux. At 7.7-11.3$\mu$m, the power-law is suppressed by the destruction of small grains(including PAHs) close to HII regions while the log-normal component tracingthe dust column in diffuse regions appears more prominent. The width and shapeof the log-normal diffuse emission PDFs in galactic disks remain consistentacross our sample, implying a log-normal gas column density$N$(H)$\approx10^{21}$cm$^{-2}$ shaped by supersonic turbulence with typical(isothermal) turbulent Mach numbers $\approx5-15$. Finally, we describe how thePDFs of galactic disks are assembled from dusty HII regions and diffuse gas,and discuss how the measured PDF parameters correlate with global propertiessuch as star-formation rate and gas surface density.

A Two-Component Probability Distribution Function Describes the mid-IR Emission from the Disks of Star-Forming Galaxies