M³D: Mosaicking M33 with MUSE datacubes

I. Unveiling the diversity of H II regions in M33 with MUSE

¹ INAF – Osservatorio Astrofisico di Arcetri Largo E. Fermi 5 I-50125 Florence, Italy
² European Southern Observatory Karl-Schwarzschild-Str. 2 85748 Garching bei München, Germany
³ Department of Physics, Faculty of Science, University of Zagreb Bijenička Cesta 32 10000 Zagreb, Croatia
⁴ Dipartimento di Fisica e Astronomia, Università di Firenze Via G. Sansone 1 I-50019 Sesto F.no (Firenze), Italy
⁵ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street 02138 Cambridge MA, USA
⁶ National Radio Astronomy Observatory, 800 Bradbury SE Suite 235 Albuquerque NM 87106, USA
⁷ Institute of Physics, GalSpec Laboratory, Ecole Polytechnique Federale de Lausanne, Observatoire de Sauverny Chemin Pegasi 51 1290 Versoix, Switzerland
⁸ University of Trento Via Sommarive 14 Trento I-38123, Italy
⁹ Max-Planck-Institut für extraterrestrische Physik (MPE) Gießenbachstraße 1 85748 Garching, Germany
¹⁰ Scuola Normale Superiore Piazza dei Cavalieri 7 I-56126 Pisa, Italy
¹¹ Department of Physics, University of Arkansas, 226 Physics Building 825 West Dickson Street Fayetteville AR 72701, USA

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Received: 5 September 2025
Accepted: 12 November 2025

Abstract

We present new VLT/MUSE observations of a 3 × 8 arcmin² mosaic along the southern major axis of the Local Group galaxy M33. These data provide an unprecedented view of the galaxy’s interstellar medium (ISM) and allow us to resolve ionised nebulae at a spatial scale of ≈5 pc. We identified and catalogued 131 H II regions, down to Hα luminosities of ≈5 × 10³⁵ erg s⁻¹, one order of magnitude fainter than previous surveys on nearby galaxies beyond the Local Group, and we compared these regions with the spatial distribution of ionising stars and embedded star clusters. For each region, we extracted the corresponding integrated optical spectra and measured the intensity of key optical emission lines (Hβλ4861, [O III]4959,5007, [N II]λ6548, 6584, Hα, [S II]λ6716, 6731, [S III]λ9069), other weaker optical lines when detectable, and Paschen lines to characterise the physical properties of the ioinised gas, such as density, dust attenuation, and metallicity. Our spatially resolved line ratio and flux maps reveal a remarkable diversity in ionisation properties, from dust-obscured regions hosting young stellar objects to highly ionised bubbles exhibiting high O III/Hβ ratios. Our data reveal a diversity of ionisation fronts, ranging from well-defined to partial to absent. The radial profiles we obtained indicate the presence of both optically thin (density-bounded) H II regions permitting the escape of ionising photons and fully ionised, optically thick (ionisation-bounded) H II regions. The richness of this MUSE mosaic offers an unprecedented view of the ionised ISM at ≈5 pc resolution, providing direct insight into how stellar feedback shapes its environment.