2026ApJ...998..166M

We use the full-disk Very Large Telescope/MUSE mosaic of NGC 253 to identify 2492 H II regions and study their resolved structure. With an average physical resolution of 17 pc, this is one of the largest samples of highly resolved spectrally mapped extragalactic H II regions. Regions of all luminosities exhibit a characteristic emission profile described by a double Gaussian with a marginally resolved or unresolved core with radius < 10 pc surrounded by a more extended halo of emission with radius = 20-30 pc. Approximately 80% of the emission of a region originates from the halo component. As a result of this compact structure, the luminosity-radius relations for core and effective radii of H II regions depend sensitively on the adopted methodology. Only the isophotal radius yields a robust relationship in NGC 253, but this measurement has an ambiguous physical meaning. We invert the measured emission profiles to infer density profiles and find central densities of n_e ≈ 10-100 cm^–3. In the brightest regions, these agree well with densities inferred from the [S II]λλ6716, 6730 doublet. The central density of H II regions correlates well with the surface brightness within the effective radius. We show that this same scaling relation applies to the recent MUSE + Hubble Space Telescope catalog for 19 nearby galaxies. We also discuss potential limitations, including completeness, impacts of background subtraction and spatial resolution, and the generality of our results when applied to other galaxies.