The PHANGS-MUSE/HST-Halpha Nebulae Catalogue

TL;DR

This work delivers a parsec-scale view of ionised nebulae across 19 nearby galaxies by combining high-resolution HST Halpha imaging with PHANGS-MUSE spectroscopy. A top-down, homogenised analysis identifies 5177 nebulae (with 4882 classified as HII regions) and provides consistent measurements of sizes $r_{\mathrm{circ}}$, $r_{\mathrm{mom}}$, fluxes, extinction, densities, and ionising photon rates, plus linking nebulae to young stellar associations (median age $\sim3$ Myr, $\sim10^{4}$–$10^{5}\ M_\odot$). The study reveals a median filling factor $\epsilon\approx0.22$ and a luminosity–size relation enabling size recovery down to $\sim1$ pc for the full MUSE sample, highlighting the importance of resolved morphology for interpreting Strömgren-like ionisation in external galaxies. The catalogue establishes a robust, publicly available benchmark for feedback, porosity, DIG, and ISM regulation across diverse galactic environments and provides a foundation for multi-wavelength follow-up with JWST, ALMA, and radio tracers.

Abstract

We present the PHANGS-MUSE/HST-Halpha nebulae catalogue, comprising 5177 spatially resolved nebulae across 19 nearby star-forming galaxies (< 20 Mpc), based on high-resolution Halpha imaging from HST, homogenised to a fixed 10 pc resolution and sensitivity. Combined with MUSE spectroscopy, this enables robust classification of 4882 H II regions and separation of planetary nebulae and supernova remnants. Electron densities for 2544 H II regions are derived using [S II] diagnostics, and nebular sizes measured via circularised radii and second moments yield a median of 20 pc, extending to sub-parsec scales. A structural complexity score traces substructure, showing that about a third of regions are H II complexes, with a higher fraction in galaxy centres. A luminosity-size relation calibrated from the HST sample is applied to 30,790 MUSE nebulae, recovering sizes down to 1 pc. Observed sizes exceed classical Stromgren radii, implying typical volume filling factors of 0.22. We associate 3349 H II regions with stellar populations from PHANGS-HST, finding median ages of 3 Myr and masses of 4-5 log(Msun). The dataset provides a detailed, spatially resolved link between nebular structure and ionising sources, serving as a benchmark for future studies of feedback, diffuse ionised gas, and star formation regulation in the interstellar medium. The full catalogue is made publicly available in machine-readable format.

Figures (23)

• Figure 1: Overview of the nebula catalogues towards NGC 1566. (Upper left) The background colour scale shows the H$\alpha$ emission from the PHANGS-MUSE observations Emsellem2022, overlaid with contours showing the boundaries of the sources identified in the PHANGS-MUSE Nebula Catalogue Groves2023. White boxes indicate the positions of the regions shown in Figs. \ref{['fig_maps_zoom_single']} (solid lines) and \ref{['fig_maps_zoom_complex']} (dashed lines). (Upper centre) The background colour scale shows the H$\alpha$ emission from the PHANGS-HST H$\alpha$ observations Chandar2025, overlaid with contours showing the boundaries of the sources identified in this work in the PHANGS-HST Nebula Catalogue. (Bottom left) The PHANGS-MUSE Nebula Catalogue masks and (bottom centre) PHANGS-HST Nebula Catalogue masks are shown with the same colour scale, where the colours denote the region ID. (Right) Composite of separate exposures acquired with JWST using the NIRCam and MIRI instruments Lee2023Williams2024, and the HST using the ACS/WFC instrument Lee2022. For the JWST part of the image, the assigned colors are Red = F2100W + F1130W + F1000W + F770W, Green = F770W + F360M, Blue = F335M + F300M. For the HST part of the image, the assigned colors are Red = F814W + F656N, Green = F555W, Blue = F435W. Overlaid as contours is the PHANGS-HST Nebula Catalogue (as in the other panels).
• Figure 2: Example of a HII complex region in NGC 1566 (see § \ref{['subsec_complexity']}). (upper left panel) MUSE observations and (upper right) HST H$\alpha$ observations that are smoothed to a physical scale of 10 pc and with a fixed noise level (see § \ref{['subsec_DataHomogenization']}). Each of these are overlaid with contours showing the boundary of the sources identified in the MUSE (white) and HST (black) observations. (lower left panel) A map of the complexity score for the region (see § \ref{['subsec_structure']}). (lower right panel) We show the nebula (orange contours) and 32 pc NUV-identified association (Larson2023; cyan contour) overlaid on a HST filter red (F814W) green (F555W) blue (F438W+F336W) image (see Lee2022). See Fig. \ref{['fig_maps_zoom_complex']} for additional examples of simple, intermediate and complex nebulae within NGC 1566.
• Figure 3: Catalogue completeness. Shown is the number (also see Tab. \ref{['tab_map_compprops']}) and luminosity completeness for the HST catalogue with respect to the MUSE catalogue as a function of luminosity, log($L_\mathrm{H\alpha}$). Also shown as a grey histogram (second y-axis) is the total number of regions within the MUSE catalogue within each luminosity bin, where the grey point with error bars shows the median and standard deviation of the distribution.
• Figure 4: Distribution of source sizes for all galaxies in the PHANGS-MUSE and PHANGS-MUSE/HST-$\mathrm{H}\alpha$ Nebula Catalogue. The histogram distributions of source radii for galaxies identified in both the HST and MUSE observations are shown in blue and orange, respectively. The points with error bars denote the median and standard deviation of the distributions. It is important to note that only sources detected by both HST and MUSE are displayed in the main panel, excluding the full MUSE sample identified by Groves2023. The distributions extend down to radii of approximately $5$ pc, corresponding to the half-width at half-maximum (HWHM) of the assumed Gaussian point spread function (PSF) used to homogenise the observations (see Sect. \ref{['subsec_DataHomogenization']}). The inset panel presents the distribution of source radii for the full MUSE catalogue (restricted to the HST field of view), alongside those detected in the HST catalogue.
• Figure 5: Distribution of source sizes ratios for all galaxies in the Nebula Catalogues relative to MUSE. The ratio of the size of each region identified in both the HST or MUSE observations for all nebulae in the sample is shown in grey. The ratio of the sizes split between complexity scores of 0-1, 2-5, and >5, denoting simple and intermediate and complex regions, respectively are shown as coloured histograms (see § \ref{['subsec_structure']}).