Astronomers used the James Webb Space Telescope (JWST) to measure how tiny dust molecules move inside two nearby starburst galaxies, M82 (NGC 3034) and NGC 253. They looked at light from Polycyclic Aromatic Hydrocarbons (PAHs) — very small carbon-based dust molecules that glow in the mid‑infrared after being hit by ultraviolet light. Using the Mid‑Infrared Instrument (MIRI) with its medium‑resolution spectrograph, the team made velocity maps of PAH emission at 5.2, 6.2 and 11.3 micrometers and compared them to gas traced by warm molecular hydrogen and ionized gas lines such as [Ne II] and hydrogen recombination lines.

Measuring motions from PAH signals is hard because the PAH features are broad and their shapes can vary. The researchers used a data‑driven method called Principal Component Analysis (PCA) tomography. PCA rearranges the full spectral data cube so that very small shifts in wavelength — caused by motion toward or away from us — can be measured even when the emission features themselves are wide and not perfectly known. This made it possible to compare dust motions with the motions of different gas phases in the same region.

In M82 the PAH maps mainly trace a rotating disk. The team sees a steep rise in speed close to the center and then a leveling off farther out — a pattern typical of galaxy rotation curves. The combination of the instrument’s field of view and the galaxy’s tilt on the sky made it possible to follow that rotation in detail.

In NGC 253 the PAH signals tell a different story. The 6.2 and 11.3 micrometer PAH features map the launching region of an outflow and sit squarely inside the central starburst. There the ionized gas also shows outflow motion, while the warm molecular gas appears to be mostly rotating. The study finds that molecular gas outflow speeds are lower than those of the ionized gas. The 11.3 micrometer PAH motion matches the ionized gas more than the molecular gas, and the 6.2 micrometer PAH shows even higher outflow speeds in both galaxies. The authors suggest this means PAHs at the base of the outflow are more closely tied to the ionized, hotter phase than to the cool molecular phase.