91̽»¨

Abstract:

Understanding how the dynamical state of the interstellar medium (ISM) changes across spatial scales can provide important insights into how the gas is organized and ultimately collapses to form stars. To this end, we present ALMA 12CO(2–1) observations at 7 pc (0 .″ 4) spatial resolution across a 1.4 kpc × 5.6 kpc ( 1.′3×1.′3 ) region located in the disk of the nearby (D = 3.5 Mpc), massive, star-forming galaxy NGC 253. We decompose this emission with a hierarchical, multiscale dendrogram algorithm to identify 2463 structures with deconvolved sizes ranging from ∼3 to 300 pc, complete to a limiting mass of 104 M⊙. By comparing the virial parameter of these structures against physical properties including size, mass, surface density, velocity dispersion, and hierarchical position, we carry out a comprehensive search for a preferred scale at which gravitationally bound structures emerge. Ultimately, we do not identify evidence of an emergent scale for bound objects in our data, nor do we find a significant correlation between the virial parameter and structure sizes. These findings suggest that simple observational estimates of gravitational binding cannot be used to define molecular clouds and emphasize the need for multiscale approaches to characterize the ISM.

Abstract:

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM) of near solar metallicity galaxies, where they play a critical role in ISM heating, cooling, and reprocessing stellar radiation. The PAH fraction, the abundance of PAHs relative to total dust mass, is a key parameter in ISM physics. Using JWST and MUSE observations of 42 galaxies from the PHANGS survey, we analyzed the PAH fraction in over 17 000 H  II regions spanning a gas-phase oxygen abundance of 12 + log(O/H) = 8.0–8.8 ( Z ∼ 0.2–1.3 Z ⊙ ), and ∼400 isolated supernova remnants (SNRs). We find a significantly lower PAH fraction toward H  II regions compared to a reference sample of diffuse ISM areas at matched metallicity. At 12 + log(O/H) > 8.2, the PAH fraction toward H  II regions is strongly anti-correlated with the local ionization parameter, suggesting that PAH destruction is correlated with ionized gas and/or hydrogen-ionizing UV radiation. At lower metallicities, the PAH fraction declines steeply in H  II regions and in the diffuse ISM, likely reflecting less efficient PAH formation in metal-poor environments. Carefully isolating dust emission from the vicinity of optically identified supernova remnants, we see evidence of selective PAH destruction from measurements of lower PAH fractions, which is, however, indistinguishable at ∼50 pc scales. Overall, our results point to ionizing radiation as the dominant agent of PAH destruction within H  II regions; metallicity plays a key role in their global abundance in galaxies.

Abstract:

Context. Gas-phase abundances provide insights into the baryon cycle, with radial gradients and 2D metallicity distributions tracking how metals are built up and redistributed across galaxy disks over cosmic time. Aims. We use a catalog of 22 958 H  II regions across 19 nearby spiral galaxies to examine how precisely the radial abundance gradients can be traced when using only the [N  II ] λ 5755 electron temperature as a proxy for temperature-based, direct method metallicities. Methods. Using 534 direct detections of the temperature sensitive [N  II ] λ 5755 auroral line, we measured gradients in 15 of the galaxies. Leveraging our large catalog of individual H  II regions, we carried out a stacking procedure in bins of the H  II region [N  II ] λ 6583 luminosity and radius to recover stacked radial gradients. Results. We found a good agreement between the metallicity gradients from the stacked spectra and those gradients from individual regions and those from strong-line methods. In addition, particularly in the stacked T e [N  II ] measurements, some galaxies show very low (< 0.05 dex) scatter in metallicities, indicative of a well-mixed ISM. We examined the individual high confidence (S/N > 5) outliers and identified 13 regions across nine galaxies with anomalously low metallicities, although this is not strongly reflected in the strong-line method metallicities. By stacking arm and interarm regions, we found no systematic evidence for offsets in metallicity between these environments, suggesting that enrichment within spiral arms is due to very localized processes. Conclusions. This work demonstrates the potential to systematically exploit the single [N  II ] λ 5755 auroral line for detailed gas-phase abundance studies of galaxies. It provides strong validation of previous results, based on the strong-line calibrations, of a well-mixed ISM across typical star-forming spiral galaxies.

Abstract:

We present the PHANGS-MUSE/HST-H α nebulae catalogue, comprising 5177 spatially resolved nebulae across 19 nearby star-forming galaxies ( D < 20 Mpc), based on high-resolution H α 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 10 4 –10 5 M ⊙ , 91̽»¨ing 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.

Abstract:

We present measurements of the masses associated with ∼18,​​​​​000 H ii regions across 19 nearby star-forming galaxies by combining data from JWST, Hubble Space Telescope, MUSE, Atacama Large Millimeter/submillimeter Array, Very Large Array, and MeerKAT from the multiwavelength PHANGS survey. We report 10 pc-scale measurements of the mass of young stars, ionized gas, and older disk stars coincident with each H ii region, as well as the initial and current mass of molecular gas, atomic gas, and swept-up shell material, estimated from lower-resolution data. We find that the mass of older stars dominates over young stars at ≳10 pc scales, and ionized gas exceeds the stellar mass in most optically bright H ii regions. Combining our mass measurements for a statistically large sample of H ii regions, we derive 10 pc-scale star formation efficiencies of ≈6%–17% for individual H ii regions. Comparing each region’s self-gravity with the ambient interstellar medium (ISM) pressure and total pressure from presupernova stellar feedback, we show that most optically bright H ii regions are overpressured relative to their own self-gravity and the ambient ISM pressure and that they are hence likely expanding into their surroundings. Larger H ii regions in galaxy centers approach dynamical equilibrium. The self-gravity of regions is expected to dominate over presupernova stellar feedback pressure at ≳130 and 60 pc scales in galaxy disks and centers, respectively, but is always subdominant to the ambient ISM pressure on H ii region scales. Our measurements have direct implications for the dynamical evolution of star-forming regions and the efficiency of stellar feedback in ionizing and clearing cold gas.