2026A&A...706A..95B

We present the PHANGS-MUSE/HST-Hα nebulae catalogue, comprising 5177 spatially resolved nebulae across 19 nearby star-forming galaxies (D < 20 Mpc), based on high-resolution Hα imaging from HST, homogenised to a fixed (10 pc) physical resolution and sensitivity. Combined with MUSE integral field spectroscopy, this enables robust classification of 4882 H II regions and the separation of planetary nebulae and supernova remnants. We derive electron densities for 2544 H II regions using [S II] diagnostics and adopt direct or representative electron temperatures for consistent physical characterisation. Nebular sizes are measured using circularised radii and intensity-weighted second moments, yielding a median radius of approximately 20 pc and extending down to (sub-)parsec (deconvolved) radii. A structural complexity score is introduced via hierarchical segmentation to trace substructure, highlighting that around a third of the regions are H II complexes containing several individual clusters and bubbles, with an increased fraction of these regions in galactic centres. A luminosity-size relation, calibrated using the resolved HST sample, is applied to 30 790 MUSE nebulae, allowing the recovery of nebular sizes down to ∼1 pc and providing statistical completeness beyond the HST detection limit. Comparisons with classical Strömgren radii indicate that observed sizes are systematically larger, corresponding to typical volume filling factors with a median of ϵ ∼ 0.22 (10th-90th percentile 0.06-0.78), with larger regions exhibiting progressively lower values. We associate 3349 H II regions with stellar populations from the PHANGS-HST association catalogue, finding median ages of ∼3 Myr and typical stellar masses of around 10⁴-10⁵ M_⊙, supporting the link between ionised nebular and young stellar populations. We also assess the impact of diffuse ionised gas on emission-line diagnostics and after removing confirmed supernova remnants, find no strong variation in line ratios with nebular resolution, indicating minimal systematic bias in the MUSE catalogue. This dataset establishes a detailed, spatially resolved connection between nebular structure and ionising sources, and provides a benchmark for future studies of feedback, DIG contributions, and star formation regulation in the ISM, especially in combination with matched high-resolution observations. The full catalogue is made publicly available in machine-readable format.