The H II Regions' Molecular Law of Star Formation
September 2026 • 2026ApJ..1008..198C
Abstract • We combine imaging data from the HST, JWST, and ground-based millimeter facilities to investigate the correlation between star formation rate (SFR) and molecular gas at the ~100 pc scale of H II regions in three nearby galaxies: NGC628, NGC5194, and NGC5236. The JWST 21 μm maps of the three galaxies offer a unique insight into the dust-absorbed SFR at high resolution. We find that the relation between the surface densities of SFR and molecular gas has a slope of ~1.85 in log--log scale, significantly steeper than previous results for nearby galaxies but closer to the trends found for molecular clouds in the Milky Way. The steep relation also holds on larger, ~500 pc, scales, and results from the high-resolution imaging that cleanly isolates the star-forming region emission from the underlying galaxy's diffuse contribution. The diffuse emission at 21 μm is, in fact, found to correlate with the galaxy's stellar mass. Comparisons with physical models of star formation are inconclusive; they overlap with the locus of the 100 pc data, but have difficulties in reproducing the data scatter. Possible exceptions are models that add a power-law tail to the gas density probability distribution due to the large range of free parameters allowed. We find that local H II regions, high-redshift star-forming clumps, and low- and high-redshift starburst galaxies form a single sequence of star formation over 3 orders of magnitude in gas surface density.
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