The PHANGS-MUSE/HST-Hα nebulae catalogue

Parsec-scale resolved structure, physical conditions, and stellar associations across nearby galaxies

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
² Ritter Astrophysical Research Center, University of Toledo, Toledo, OH 43606, USA
³ Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
⁴ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12–14, 69120 Heidelberg, Germany
⁵ INAF — Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁶ International Centre for Radio Astronomy Research, University of Western Australia, 7 Fairway, Crawley, 6009 WA, Australia
⁷ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁸ Department of Astronomy, Ohio State University, 180 W. 18th Ave, Columbus, OH 43210, USA
⁹ Center for Cosmology and Astroparticle Physics, 191 West Woodruff Avenue, Columbus, OH 43210, USA
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹² Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹³ Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁴ European Southern Observatory (ESO), Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
¹⁵ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ap. 70-264, 04510 CDMX, Mexico
¹⁶ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII 3, 28014 Madrid, Spain
¹⁷ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
¹⁸ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Unidad Académica en Ensenada, Km 103 Carr. Tijuana-Ensenada, Ensenada, BC, Mexico
²⁰ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06000 Nice, France
²¹ Astronomy Department, University of Virginia, PO Box 400325, Charlottesville, VA 22904, USA
²² National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
²³ Department of Physics, University of Arkansas, 226 Physics Building, 825 West Dickson Street, Fayetteville, AR 72701, USA
²⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²⁵ Department of Physics, Tamkang University, No.151, Yingzhuan Road, Tamsui District, New Taipei City 251301, Taiwan
²⁶ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
²⁷ Gemini Observatory/NSF’s NOIRLab, 950 N. Cherry Avenue, Tucson, AZ 85719, USA
²⁸ Department of Physics, 4-183 CCIS, University of Alberta, Edmonton, AB T6G 2E1, Canada

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 May 2025
Accepted: 1 October 2025

Abstract

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.