This paper uses new James Webb Space Telescope (JWST) mid‑infrared images to study what lights up in the nearby Whirlpool Galaxy, M51. The authors compare emission seen in eight mid‑infrared filters (5.6–21 microns) to maps of molecular gas (12CO(1–0)), atomic gas (HI), and ionized gas and young stars (Pa‑alpha). Their goal is to understand whether mid‑infrared light mostly traces the amount of gas or the presence of recent star formation, and how that depends on wavelength.

The team measured correlations between each mid‑infrared band and the gas tracers at high spatial resolution. At the adopted distance of 7.59 megaparsecs, 1 arcsecond equals 46 parsecs, so many comparisons probe scales of order 40 parsecs where molecular clouds and ionized regions start to separate on the sky. The mid‑infrared data come from JWST’s MIRI instrument and were reduced with the pjpipe processing pipeline. The gas and star‑formation tracers used were CO(1–0) for molecular hydrogen (H2, via an assumed CO‑to‑H2 conversion), HI for atomic hydrogen, and the Pa‑alpha hydrogen recombination line for ionized gas and recent star formation.

Their main finding is a clear, wavelength‑dependent split in what the mid‑infrared traces. Filters dominated by polycyclic aromatic hydrocarbons (PAHs) — F560W, F770W, F1130W and F1280W — show nearly linear correlations with the CO(1–0) map at ~40 pc scales. The authors interpret this as PAHs being well mixed with molecular gas and exposed to fairly constant background radiation fields. By contrast, the longer‑wavelength, dust continuum‑dominated filters (F1500W, F1800W, F2100W) show shallower relations with CO and stronger links to Pa‑alpha; for example, F2100W scales nearly linearly with Pa‑alpha. One intermediate band, F1000W, behaves more like the PAH bands.

The authors also modeled the mid‑infrared emission as a mix of a gas‑associated component and a star‑formation‑associated component using the Pa‑alpha map as a template for the latter. From that decomposition they find that the PAH‑dominated filters get comparable contributions from gas and from star‑forming regions, while the fraction tied to the Pa‑alpha (star‑formation) template rises toward longer wavelengths, reaching about 75% in F2100W. In short, mid‑infrared light traces both gas column density and local star formation, but which of those dominates depends on the wavelength: PAHs are better at tracing the gas column and the warm dust continuum is more sensitive to recent star formation.