Searcharxiv⌕ Search

arXiv subjects

Ralf S. Klessen

Publications and source records attributed to Ralf S. Klessen.

At least 55 records · Page 3Linked to original sources
arXiv2026

JWST Observations of Starbursts: Dust Processing in the M82 Superwind

We present JWST MIRI and NIRCam imaging of the inner ~5 kpc of the M82 superwind at 0.05-0.375'' (~0.9-6.5 pc) resolution. Targeted filters probe emission from polycyclic aromatic hydrocarbons (PAHs; F335M, F360M, F770W, F1130W) and continuum (F250M, F360M). The images reveal a network of cool wind filaments traced by PAHs. PAH surface brightness declines with the inverse square of distance to the midplane, suggesting that the incident radiation field from the starburst drives the observed PAH intensity out to 2.5 kpc. The 3.3/11.3 and 3.3/7.7 band ratios show uniformity with distance from the starburst, though comparisons with mid-IR dust emission models indicate a modest shift toward larger PAHs. Outside the disk, 11.3/7.7 increases moderately, reflecting that PAHs become more neutral with distance from the starburst as they are exposed to a declining radiation field and ionization parameter. Overall, PAHs in the wind are consistent with standard-to-large sizes and standard-to-high ionization states. Including Spitzer and Herschel data, PAH abundance (qPAH) is set at ~1% in the starburst and remains unchanging out to 5 kpc off the disk. This flat qPAH profile suggests that PAHs are shielded from the hot wind, perhaps residing in the surface layers of cool clouds, with possible replenishment from cloud interiors and enrichment of the halo from previous bursts. In this picture, clouds are not dense enough to promote PAH growth, and they likely undergo radiative cooling and mixing with the hot phase to survive the gauntlet for at least ~20 Myr.

astro-ph.GA↗
arXiv2026

From gas to stars along the spiral wave: CO, HCN, and star formation variations across the spiral arms in NGC 4321 and M51

Molecular clouds form stars from the interstellar medium via gravitational collapse, following a sequence from low-density gas to high-density cores and eventually the formation of stars. In classical density wave theory, gas clouds orbiting the galaxy experience gas compression and triggered star formation, while encountering the gravitational well of spiral arms. We aim to trace these different phases of the molecular cloud life cycle via tracers of molecular gas (CO), dense molecular gas (HCN), and star formation (H$α$, 24 $μ$m) within the spiral arms of two grand-design spiral galaxies: NGC 4321 and M51 (NGC 5194). In the spiral arms of these galaxies, we investigate the relation between molecular gas, dense gas, and star formation (CO-HCN-SFR) at matched physical resolutions of 270 pc and 125 pc in NGC 4321 and M51, respectively. We employed spiral arm masks for these galaxies and investigate trends of HCN/CO and SFR/HCN (SFR/CO), which serve as proxies for the dense gas fraction and dense (molecular) gas star formation efficiency, perpendicular to the spiral arm spines. We find that HCN/CO, SFR/CO, and SFR/HCN increase from the upstream towards the downstream side of both spiral arms of NGC 4321, while their trends are less prominent in M51. Our results indicate that large-scale galactic dynamics (e.g. density waves) can induce a sequence of gas density and star formation-to-gas density variations perpendicular to the spiral arms. This sequence contributes to the increased scatter seen among spectroscopic ratios such as HCN/CO and SFR/HCN at sub-kiloparsec scales.

astro-ph.GA↗
arXiv2026

The Impact of Radiation Environment on the Evolution and Fragmentation of Protostellar Discs

We present high-resolution zoom-in simulations of molecular clouds exposed to an interstellar radiation field and cosmic ray ionisation rate up to 1000 times stronger than that of the solar neighbourhood. We detail the evolution of the accretion discs that form around the first protostar in each simulation, for a total of 7 discs, for up to 100 kyr. The use of a zoom-in procedure allows for the au-scale discs to be well resolved (with resolution < 0.25 au) whilst retaining the structure of the wider parsec-scale molecular cloud. We find that discs exposed to a stronger radiation field tend to be more massive, hotter and denser. Similarly, their host stars grow to become more massive as a result of accreting more rapidly from their surroundings. All the discs show evidence of recurrent instability during the simulations, but only some of them fragment. We investigate whether stability metrics, such as the Toomre $Q$, $α$ viscosity, and $β$ cooling parameter, can predict fragmentation by calculating them just before the discs fragment. We find that the metrics are generally unable to do so, as the discs appear stable even up to a few hundred years before fragmenting. In solar-like environments fragments are typically of planetary mass and often migrate to the centre of the disc, whereas fragments in a high-radiation environment are massive ($\rm > 0.1 \, M_\odot$) and fully disrupt/accrete from the progenitor disc. We conclude that the evolution and properties of circumstellar discs depend on both their radiation and physical environment.

astro-ph.SR↗
arXiv2026

The diffuse gamma-ray sky of a Milky Way analogue: Local diversity and global constraints

Diffuse gamma-ray emission is a key tracer of cosmic rays (CRs) in galaxies, encoding information about their transport, energetics, and interactions with the interstellar medium. Interpreting the Milky Way gamma-ray sky is challenging because the observed emission depends on the three-dimensional distributions of CRs and gas, as well as the observer location within the Galaxy. Using the Rhea suite of CR-MHD simulations of a Milky Way analogue, we study how pion-decay gamma-ray emission varies with galactic environment, local conditions, and CR transport physics. Emission is computed in post-processing under steady-state assumptions, enabling analysis of luminosities, spectra, full-sky maps, and angular power spectra (APS) for multiple observer positions, including those inside Local-Bubble-like cavities. The simulated galaxy reproduces Milky Way-like gamma-ray luminosities and spectral slopes without parameter tuning. While total luminosity is relatively stable, the morphology of the gamma-ray sky varies strongly with observer location due to the complex local gas distribution, consistent with observations. For all observers, the APS traces gas column density rather than the smoother CR energy density, in agreement with previous studies. Comparisons with Fermi-LAT data show good agreement in both the all-sky spectrum and APS, with a diffusion coefficient scaling proportional to E^0.5 providing the best match. These results show that key features of Galactic gamma-ray emission arise naturally in self-consistent CR-MHD simulations. Gas density fluctuations primarily shape emission morphology, while CR transport governs spectral and structural properties. The Rhea simulations provide a physically grounded framework for interpreting diffuse gamma-ray observations and highlight the importance of local environment in tracing Galactic CR physics.

astro-ph.HE↗
arXiv2026

The PHANGS-Hα survey. Ground-based narrow-band imaging of nearby star-forming galaxies

We present PHANGS-Hα, a narrow-band imaging survey that maps Hα emission over a sample of 65 nearby massive star-forming galaxies. The data were obtained using the MPG-ESO 2.2-meter telescope at La Silla and the du Pont 2.5-meter telescope at Las Campanas Observatory, in the framework of the multi-wavelength cloud-scale (50-100 pc) resolution mapping of molecular gas and star formation conducted by the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) collaboration. PHANGS-Hα complements the already published PHANGS-ALMA, PHANGS-MUSE, PHANGS-HST, and PHANGS-JWST surveys, providing an anchor point for the photometric and astrometric calibration of these datasets, as well as samples of H ii regions, and star formation rate maps for the bulk of the PHANGS sample. We present observations, data processing, and calibration of the PHANGS-Hα dataset, as well as the procedures used to derive emission-line fluxes from narrow-band imaging. A subset of galaxies with available spectroscopic Ha mapping from the PHANGS-MUSE survey allows for a detailed comparison with the narrow-band photometry presented here. This informs a series of best practices for the processing of narrow-band Hα imaging that we apply to the full dataset.

astro-ph.GA↗
arXiv2026

The Impact of Fuzzy Dark Matter Dynamics on the Accumulation and Fragmentation of Primordial Gas

Fuzzy Dark Matter (FDM), particularly in the $10^{-22}$ eV mass regime is frequently used to characterize wave-like interference effects. It exhibits macroscopic wave properties, which drive distinct baryonic dynamics within collapsed haloes. Using the hydrodynamical code AREPO with the AxiREPO module and primordial chemistry, we simulate the assembly of haloes with masses $3 \times 10^{8} \le M_{\mathrm{h}} \le 8 \times 10^{9} \: M_\odot$ across a range of axion masses $1 \times 10^{-22} \le m_{\mathrm{a}} \le 7 \times 10^{-22}$ eV. We investigate how small-scale dynamics of the FDM density field affect the accumulation of cold, dense gas essential for primordial star formation. We demonstrate that gas collapse is suppressed by a two-fold mechanism: a delay driven by the geometry of the FDM solitonic core and a secondary dynamical barrier caused by stochastic wave fluctuations. While the flattened solitonic potential profile itself inhibits central gas accumulation, these wave-driven dynamics provide a further layer of disruption and angular momentum support, which in certain regimes prevents gas from reaching the central, compact, high-density configurations characteristic of CDM. Consequently, sites of star formation are shifted away from a single central peak toward a population of lower-mass clusters. Our work provides a physical framework for calibrating halo mass-dependent star formation efficiencies in FDM cosmologies, where internal processes may delay Cosmic Dawn beyond the effects of the initial power spectrum cut-off. These results are essential for interpreting realistic observational constraints from future 21-cm signal observations and the faint-end luminosity functions observed by the JWST, as well as providing an upper bound on the baryonic effects in the context of Mixed Dark Matter scenarios.

astro-ph.GA↗
arXiv2026

A method to derive self-consistent NLTE astrophysical parameters for 4 million high-resolution 4MOST stellar spectra in half a day with invertible neural networks

Modern spectroscopic surveys obtain spectra for millions of stars. However, classical spectroscopic methods can often be computationally expensive, rendering them impractical for the analysis of large datasets. We introduce a novel simulation-based deep-learning approach for the efficient analysis of high-resolution stellar spectra to be obtained with the upcoming high-resolution 4MOST spectrograph. We used a suite of synthetic non-local thermodynamic equilibrium (NLTE) spectra generated with Turbospectrum to mimic 4MOST observations and trained a conditional invertible neural network (cINN) for the purpose of predicting self-consistently stellar surface parameters and chemical abundances. The cINN is a neural network architecture that estimates full posterior distributions for the target stellar properties, providing an intrinsic uncertainty estimate. We evaluated the predictive performance of the trained cINN model on both synthetic data and observed spectra of stars. We found that our new cINN trained on NLTE synthetic spectra is capable of recovering stellar parameters with average errors ($σ$) of $33$ K for $T_\mathrm{eff}$, $0.16$ dex for $\log(g)$, and $0.12$ dex for [Fe/H], $0.1$ dex for [Ca/Fe], $0.11$ for [Mg/Fe], and $0.51$ dex for [Li/Fe], respectively, at a signal to noise ratio of 250 per Angstrom. From the analysis of the observed spectra of Gaia-ESO / 4MOST / PLATO benchmark stars, we verified that our NLTE estimates for stellar parameters and abundances are consistent with results obtained with the independent code TSFitPy. We conclude that the NLTE cINN is robust and can, theoretically, evaluate 4 million high-resolution 4MOST spectra in less than a day, using GPU acceleration.

astro-ph.SR↗
arXiv2026

Impact of initial mass function on the chemical evolution of high-redshift galaxies

Recent observations by the James Webb Space Telescope (JWST) have found evidence for an invariant relation between stellar mass, metallicity, and star formation rate up to $z\sim 8$ and its breakdown at higher redshifts. Understanding the underlying physics driving such correlations is thus crucial. Here, we explore the impact of the initial mass function (IMF) on the chemical evolution of high-redshift galaxies. Indeed, star formation and metal enrichment in galaxies are regulated by supernova (SN) explosions and metal yields from massive stars, which are sensitive to the high-mass end of the IMF. Using the semi-analytical galaxy evolution code \textsc{a-sloth}, we follow galactic baryon cycles along merger trees built from a high-resolution cosmological simulation. Stellar feedback is modeled with up-to-date stellar evolution tracks covering the full metallicity range ($Z \sim 10^{-11} - 0.03$) and a broad stellar mass range ($m_\star\sim2 - 600\ \rm M_\odot$), including metal yields from stellar winds, core-collapse SNe, (pulsational) pair-instability SNe, and Type Ia SNe. Assuming a Kroupa-like IMF with a varying upper mass limit $m_{\max}$, we find that only models with $m_{\max} \gtrsim 200\ \rm M_\odot$ can simultaneously reproduce the observed mass-metallicity-star formation rate relation and cosmic star formation history at $z\gtrsim 4$ owing to enhanced metal yields from pair-instability SNe. Our results confirm that very massive ($\gtrsim 200\ \rm M_\odot$) stars and pair-instability SNe play an important role in the star formation and chemical enrichment histories of high-$z$ galaxies. They also have profound implications for electromagnetic transients and gravitational-wave events.

astro-ph.GA↗
arXiv2026

Self-consistent modelling of the Milky Way's Nuclear Stellar Disc

The Nuclear Stellar Disc (NSD) is a flattened high-density stellar structure that dominates the gravitational field of the Milky Way at Galactocentric radius $30\lesssim R\lesssim 300$ pc. We construct axisymmetric self-consistent equilibrium dynamical models of the NSD in which the distribution function is an analytic function of the action variables. We fit the models to the normalised kinematic distributions (line-of-sight velocities + VIRAC2 proper motions) of stars in the NSD survey of Fritz et al., taking the foreground contamination due to the Galactic Bar explicitly into account using an $N$-body model. The posterior marginalised probability distributions give a total mass of $M_{\rm NSD} = 10.5^{+1.1}_{-1.0} \times10^8 \,{\rm M_\odot}$, roughly exponential radial and vertical scale-lengths of $R_{\rm disc} = 88.6^{+9.2}_{-6.9}$ pc and $H_{\rm disc}=28.4^{+5.5}_{-5.5}$ pc respectively, and a velocity dispersion $σ\simeq 70$ km/s that decreases with radius. We find that the assumption that the NSD is axisymmetric provides a good representation of the data. We quantify contamination from the Galactic Bar in the sample, which is substantial in most observed fields. Our models provide the full 6D (position+velocity) distribution function of the NSD, which can be used to generate predictions for future surveys. We make the models publicly available as part of the software package AGAMA.

astro-ph.GA↗
arXiv2026

3D CMZ V: A new orbital model of our Galaxy's Center, informed by data across the electromagnetic spectrum

The 3D structure of The Milky Way's Central Molecular Zone (CMZ) informs our understanding of star formation cycles, black hole accretion, and the evolution of galactic nuclei. However, a comprehensive 3D model has remained elusive, as no singular dataset nor theory contains the requisite information to describe the orbital motion of the gas. We implement a Bayesian framework to flexibly combine datasets across the electromagnetic spectrum for molecular clouds in our CMZ catalog. We develop near/far metrics for each dataset, including dust extinction, absorption, stellar densities, X-ray echoes, and proper motions; and report a posterior positional probability density function (PPDF) for each cloud. We then use the posterior PPDF distributions for all CMZ clouds to search for a best fitting x$_2$ orbit. We find that no single orbit is a perfect fit, but the structure can overall be represented by nested x$_2$ orbits, with major axes ranging from about $72 < a < 146$ pc. We also present projected line of sight distance estimates for all 31 clouds in the catalog. Our results highlight asymmetries along the line of sight, with most clouds lying on the near side of the Galactic Center, and agree overall with current near/far assumptions for most CMZ clouds, including those in the Sgr A region, which may be much closer to the center. We conclude that the CMZ can be well-described by x$_2$ orbital families, and that the overall gas distribution is more complex than a single closed or open elliptical orbit.

astro-ph.GA↗
arXiv2026

Shaping the diffuse X-ray sky: Structure, Variability and Visibility

The Local Bubble (LB) is a hot, low-density cavity in the solar neighborhood, inside which the Solar System is currently located. The X-ray emission from such bubbles is strongly governed by the gas density, temperature, and the effects of line-of-sight column density. Yet the physical processes that control the formation and evolution of this emission remain incompletely understood. We analyze a LB analogue identified within a magnetohydrodynamical simulation to investigate the key physical factors that shape its X-ray properties. In post-processing, we examine the spatial distribution, variability, and observational constraints of the X-ray emission. Our study reveals three main results: (1) Shortly after a supernova (SN), the bulk of the X-ray emission arises from a small fraction of the bubble's volume, concentrated in hot regions around recent SN sites. Approximately 95% of the X-ray luminosity originates from less than 1% of the total bubble volume. During quiescent phases without recent SNe, the emission morphology changes substantially, with X-ray-bright regions becoming more volume-filling. (2) Column density effects strongly modulate the observable X-ray signal. Gas with column densities exceeding $N_\mathrm{H} \gtrsim 10^{20} \,\mathrm{cm}^{-2}$ efficiently absorbs soft X-ray photons, limiting the depth to which observations can probe. This absorption causes a significant fraction of the sky to be obscured from external soft X-rays. Differences between active and quiescent phases further influence how much of the total bubble emission is visible from within. (3) The X-ray flux shows pronounced temporal variability on Myr timescales, with SN events producing rapid, transient luminosity enhancements, followed by steep declines due to adiabatic cooling. The total flux varies by several orders of magnitude, with SN-driven peaks fading within $10^5$ years.

astro-ph.HE↗
arXiv2026

The SEEDZ Simulations: Methodology and First Results on Massive Black Hole Seeding and Early Galaxy Growth

Here we introduce the SEEDZ simulations, a suite of cosmological hydrodynamic simulations exploring the formation and growth of the first massive black holes in the Universe. SEEDZ includes models for Population III star formation, supernovae explosions and the resulting formation of light seed black holes, metal enrichment and subsequent Population II star formation, heavy seed black hole formation, Eddington and super-Eddington accretion schemes as well as black hole feedback. In this paper, we cover the overall methodologies employed and present our current results at $z=15$. Our main result so far is that black holes initially grow faster than their host galaxy, and hence over-massive black holes are a feature of the high-redshift Universe. The fundamental black hole-galaxy relationships we observe at $z = 0$ (especially the M$_{\rm BH}$ - M$_*$ relationship) likely only emerge in more mature galaxies. At high-redshift, that relationship has not yet been established. We find that even at these high redshifts, MBHs can grow from their initial heavy seed mass of $\sim$10$^4$ M$_\odot$ up to 10$^6$ M$_\odot$. At the high end of our MBH masses, our simulated galaxy M$_{\rm BH}$ - M$_*$ relations match the observed high redshift trends i.e. over-massive BHs with M$_{\rm BH}$/M$_{\rm star} \sim 10^{-2}$. This initial set of simulations will continue to run down to $z=10$, where we will perform a comprehensive comparison of simulated MBH number densities and M$_{\rm BH}$ - M$_*$ relations with JWST observations. Further simulations with higher resolution will then follow.

astro-ph.GA↗
arXiv2026

To What Extent Are Star Cluster Ages Encoded in Their Environments? Exploring the Spatial Distribution of Age-Related Information with PHANGS-HST Imaging and Convolutional Neural Networks

The environments around star clusters evolve as stellar feedback reshapes the interstellar medium and dynamical processes reorganize the structure of the surrounding stellar field. As approximately single-age populations, star clusters can serve as clocks to trace these environmental changes. In this exploratory study, we test whether convolutional neural networks (CNNs) can identify age-dependent changes in cluster environments. We take cluster ages as given from basic SED fitting of five-band UV-optical aperture photometry from the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) HST survey. We first show that CNNs can be trained on image cutouts centered on clusters to recover ages directly from imaging. This demonstration provides the foundation for this study, which examines whether the information used by CNNs to predict age is coherent and physically meaningful. We perform controlled image occlusion experiments as an explainable AI method. These show that the CNNs extract age-predictive environmental cues in the absence of cluster light and when information on SED shape is removed by combining the five filters into one image. We find that reliance on environmental information increases at the youngest (<10 Myr) and oldest (>1 Gyr) ages, where clusters can exhibit similarly red colors. Our results are consistent with the long-recognized picture that cluster environments evolve systematically with age. We demonstrate that this information is encoded at a level detectable by machine-learning and recoverable from broadband imaging. This establishes a path for using new techniques to connect image-based age inference to the physical evolution of cluster environments.

astro-ph.GA↗
arXiv2026

ALMA Central molecular zone Exploration Survey (ACES) V: CS(2-1), SO(2_3-1_2), CH3CHO(5_1,4-4_1,3), HC3N(11-10), and H40a lines data

We present data from the ALMA Central Molecular Zone Exploration Survey (ACES) Large Program, which provides broad spectral-line and 3 mm continuum coverage of the Central Molecular Zone (CMZ) at a spatial resolution of 0.1 pc. The survey delivers homogeneous, wide-field mosaics that enable direct comparisons of the physical and chemical conditions across diverse environments in the Galactic center. In this data release paper, we present the CS(2-1), SO(2_3-1_2), CH3CHO(5_1,4-4_1,3), HC3N(11-10), and H40a lines observed simultaneously within two broad spectral windows. These lines reveal pronounced spatial and chemical variations across the CMZ, tracing distinct components of molecular gas, shock-affected regions, and ionized structures. The high angular resolution and multi-line capability of the ACES dataset make it a powerful resource for future studies of gas dynamics, star formation activity, and the physical connection between the CMZ and Sgr A*.

astro-ph.GA↗
arXiv2026

The Nuclear Star Cluster of M 74: a fossil record of the very early stages of a star-forming galaxy

Nuclear star clusters (NSC) are dense and compact stellar systems, of sizes of few parsecs, located at galactic centers. Their properties and formation mechanisms seem to be tightly linked to the evolution of the host galaxy, with potentially different formation channels for late- and early-type galaxies (respectively, LTGs and ETGs). While most observations target ETGs, here we focus on the NSC in M~74 (NGC~628), a relatively massive, gas-rich and star-forming spiral galaxy, part of the PHANGS survey. We analyzed the central arcmin of the PHANGS-MUSE mosaic, in which the NSC is not spatially resolved. We analyzed the NSC stellar populations in a point spread function (PSF) aperture, and compared it to the host galaxy. Within the PSF size, the NSC is contaminated by the host-galaxy light. We performed a two-dimensional spectro-photometric decomposition of the MUSE cube, employing a modified version of the C2D code, to disentangle the NSC from its host. This method provided different data cubes for the NSC and the host galaxy, allowing for both their comparison in a PSF aperture, and the spatially resolved analysis of the host. Our results show a very old and metal-poor NSC, in contrast to the surrounding regions. While similar properties were found in NSCs hosted by galaxies of different masses and/or morphological types from M~74, they are somewhat unexpected for a relatively massive star-forming spiral galaxy. The spatially resolved stellar populations of the host galaxy display much younger (light-weighted) ages and higher metallicities, especially in the central region (${\sim}500$~pc) surrounding the NSC. This suggests that this NSC formed a long time ago, and evolved passively until today, without any further growth. No significant amounts of gas would have reached the very central region in the last 8~Gyr.

astro-ph.GA↗
arXiv2026

Cloud-scale gas properties, depletion times, and star formation efficiency per free-fall time in PHANGS--ALMA

We compare measurements of star formation efficiency to cloud-scale gas properties across PHANGS-ALMA. Dividing 67 galaxies into 1.5 kpc scale regions, we calculate the molecular gas depletion time, tau_dep= Sigma_mol/Sigma_SFR, and the star formation efficiency per free-fall time, eff=tau_ff/tau_dep, for each region. Then we test how tau_dep and eff vary as functions of the regional mass-weighted mean molecular gas properties on cloud scales (60-150pc): gas surface density, , velocity dispersion, , virial parameter, , and gravitational free-fall time, . and tau_dep correlate positively, consistent with the expectation that gas density plays a key role in setting the rate of star formation. Our fiducial measurements suggest tau_dep \propto ^0.5 and eff \approx 0.39%, though the exact numbers depend on the adopted fitting methods. We also observe anti-correlations between tau_dep and and between tau_dep^mol and . All three correlations may reflect the same underlying link between density and star formation efficiency combined with systematic variations in the degree to which self-gravity binds molecular gas in galaxies. We highlight the tau_dep- relation because of the lower degree of correlation between the axes. Contrary to theoretical expectations, we observe an anti-correlation between tau_dep^mol and and no significant correlation between eff and . Our results depend sensitively on the adopted CO-to-H2 conversion factor, with corrections for excitation and emissivity effects in inner galaxies playing an important role. We emphasize that our simple methodology and clean selection allow easy comparison to numerical simulations and highlight this as a logical next direction.

astro-ph.GA↗
arXiv2026

ALMAGAL VII. Cataloging Hierarchical Mass Structure from Cores to Clumps across the Galactic Disk

Investigating the multi-scale fragmentation of dense clumps into compact cores is essential for understanding the processes that govern the initial distribution of mass in stellar clusters and how high-mass stars ($>8~M_{\odot}$) form. We present a catalog of the hierarchical continuum structure from 904 clumps observed in the ALMAGAL program, a high resolution ($0.15-0.8$\arcsec) 1.38 mm Atacama Large Millimeter/submillimeter Array (ALMA) large program targeting dense clumps capable of high-mass star formation throughout the Galactic disk. We use \verb|astrodendro|, a dendrogram-based algorithm, on a uniform linear resolution (2000 au) version of the data to extract 5160 continuum structures with effective radii spanning $800-42000$ au and estimated masses between $~0.05-670~M_{\odot}$. With our large sample, we statistically examine differences in clump properties for regions with varying levels of hierarchical complexity. We find that clumps exhibiting the richest hierarchical morphology have distributions with higher dust temperatures, surface densities, luminosity-to-mass (\textit{L/M}) ratios, and most massive core (MMC) masses, indicating that these regions tend to be at later evolutionary stages. We find a positive correlation between the mass of cores from the ALMAGAL core catalog and the surface density of their surrounding structures identified in this work. However, this correlation is weaker for cores in more evolved clumps, where lower mass cores can be found at higher local surface densities. This could indicate that some cores accrete mass less efficiently from the intra-clump reservoir than others, despite the total available mass increasing over time, a scenario that is congruent with a clump-fed core accretion model.

astro-ph.GA↗
arXiv2026

Stellar associations powering HII regions $\unicode{x2013}$ II. Escape fraction of ionizing photons

Newly formed stars have a profound impact on their environment by depositing energy and momentum into the surrounding gas. However, only a fraction of the stellar feedback is retained in the cloud and observational constraints are needed to further our understanding of this process. In a sample of 19 nearby galaxies, we match HII regions from PHANGS$\unicode{x2013}$MUSE to their ionizing stellar source from PHANGS$\unicode{x2013}$HST and measure the percentage of ionizing radiation that is leaking into the surrounding diffuse ionized gas (DIG). Based on a catalogue, where each HII region is powered by a single young and massive stellar association, we measure a photon escape fraction of $f_\mathrm{esc}=82^{+12}_{-24}$ per cent. Comparable results are obtained when different procedures are used to match the ionized gas to its source. All samples we study contain a substantial fraction of objects (up to 20 per cent), where the stellar source is not sufficient to produce the H$α$ flux observed from the nebula. Many of them are probably related to uncertain age estimates, but we also find numerous regions, where a significant fraction of the ionizing photon budget is contributed by stars that reside outside the boundaries of the HII region. This motivates the use of an alternative galaxy-wide approach, in which we include all HII regions and stellar sources, not just the ones that show a clear overlap. When summing up the ionization budget over entire galaxies, we measure slightly lower, but consistent values.

astro-ph.GA↗