More Like this

1. ALMAGAL: I. The ALMA evolutionary study of high-mass protocluster formation in the Galaxy: Presentation of the survey and early results

   https://doi.org/10.1051/0004-6361/202452702  

   Molinari, S; Schilke, P; Battersby, C; Ho, P_T P; Sánchez-Monge, Á; Traficante, A; Jones, B; Beltrán, M T; Beuther, H; Fuller, G A; et al (April 2025, Astronomy & Astrophysics)

   Context. A large fraction of stars form in clusters containing high-mass stars, which subsequently influences the local and galaxy-wide environment. Aims. Fundamental questions about the physics responsible for fragmenting molecular parsec-scale clumps into cores of a few thousand astronomical units (au) are still open, that only a statistically significant investigation with ALMA is able to address; for instance: the identification of the dominant agents that determine the core demographics, mass, and spatial distribution as a function of the physical properties of the hosting clumps, their evolutionary stage and the different Galactic environments in which they reside. The extent to which fragmentation is driven by clumps dynamics or mass transport in filaments also remains elusive. Methods. With the ALMAGAL project, we observed the 1.38 mm continuum and lines toward more than 1000 dense clumps in our Galaxy, withM≥ 500 M_⊙, Σ ≥ 0.1 g cm⁻²andd≤ 7.5 kiloparsec (kpc). Two different combinations of ALMA Compact Array (ACA) and 12-m array setups were used to deliver a minimum resolution of ∼1000 au over the entire sample distance range. The sample covers all evolutionary stages from infrared dark clouds (IRDCs) to H IIregions from the tip of the Galactic bar to the outskirts of the Galaxy. With a continuum sensitivity of 0.1 mJy, ALMAGAL enables a complete study of the clump-to-core fragmentation process down toM∼ 0.3 M_⊙across the Galaxy. The spectral setup includes several molecular lines to trace the multiscale physics and dynamics of gas, notably CH₃CN, H₂CO, SiO, CH₃OH, DCN, HC₃N, and SO, among others. Results. We present an initial overview of the observations and the early science product and results produced in the ALMAGAL Consortium, with a first characterization of the morphological properties of the continuum emission detected above 5σin our fields. We used “perimeter-versus-area” and convex hull-versus-area metrics to classify the different morphologies. We find that more extended and morphologically complex (significantly departing from circular or generally convex) shapes are found toward clumps that are relatively more evolved and have higher surface densities. Conclusions. ALMAGAL is poised to serve as a game-changer for a number of specific issues in star formation: clump-to-core fragmentation processes, demographics of cores, core and clump gas chemistry and dynamics, infall and outflow dynamics, and disk detections. Many of these issues will be covered in the first generation of papers that closely follow on the present publication. 

   more » « less
2. ALMAGAL: V. Relations between the core populations and the parent clump physical properties

   https://doi.org/10.1051/0004-6361/202554764  

   Elia, D; Coletta, A; Molinari, S; Schisano, E; Benedettini, M; Sánchez-Monge, Á; Traficante, A; Mininni, C; Nucara, A; Pezzuto, S; et al (January 2026, Astronomy & Astrophysics)

   Context. The fragmentation of massive molecular clumps into smaller, potentially star-forming cores plays a key role in the processes of high-mass star formation. The ALMAGAL project, using the Atacama Large Millimeter/submillimeter Array (ALMA), offers highr-esolution data to investigate these processes across various evolutionary stages in the Galactic plane. Aims. This study aims at correlating the fragmentation properties of massive clumps, obtained from ALMA observations, with their global physical parameters (e.g., mass, surface density, and temperature) and evolutionary indicators (e.g., luminosity-to-mass ratio and bolometric temperature) obtained fromHerschelobservations. It seeks to assess whether the cores evolve in number and mass in tandem with their host clumps and to determine the possible factors influencing the formation of massive cores (M> 24 M_⊙). Methods. We analyzed the masses of 6348 fragments, estimated from 1.4 mm continuum data for 1007 ALMAGAL clumps. Leveraging this unprecedentedly large dataset, we evaluated statistical relationships between clump parameters, estimated over ~0.1 pc scales, and fragment properties, corresponding to scales of a few thousand astronomical units, while accounting for potential biases related to distance and observational resolution. Our results were further compared with predictions from numerical simulations. Results. The fragmentation level correlates preferentially with clump surface density, supporting a scenario of density-driven fragmentation; however, it does not show any clear dependence on total clump mass. Both the mass of the most massive core and the core formation efficiency exhibit a broad range and increase, on average, by an order of magnitude across intervals defined by evolutionary indicators such as clump-dust temperature and the luminosity-to-mass ratio. This suggests that core growth continues throughout clump evolution, favoring clump-fed over core-fed theoretical scenarios. However, significant scatter in these relationships indicates that multiple factors, including magnetic fields, turbulence, and stellar feedback, not quantifiable with continuum data, influence fragmentation, as also suggested by comparison with numerical simulations. 

   more » « less
3. ALMAGAL. VII. Cataloging Hierarchical Continuum Structure from Cores to Clumps across the Galactic Disk

   https://doi.org/10.3847/1538-4357/ae2fec  

   Wallace, Jennifer; Kolz, Taevis; Battersby, Cara; Kuznetsova, Aleksandra; Sánchez-Monge, Álvaro; Schisano, Eugenio; Coletta, Alessandro; Zhang, Qizhou; Molinari, Sergio; Schilke, Peter; et al (February 2026, The Astrophysical Journal)

   Abstract Investigating the multiscale 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 (>8M_⊙) form. We present a catalog of the hierarchical continuum structure from 904 clumps observed in the ALMAGAL program, a high-resolution (0 $\stackrel{\text{″}}{.}$ 15–0 $\stackrel{\text{″}}{.}$ 8) 1.38 mm Atacama Large Millimeter/submillimeter Array large program targeting dense clumps capable of high-mass star formation throughout the Galactic disk. We useastrodendro, 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 0.05–670M_⊙. 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 (L/M) ratios, and most massive core 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 intraclump reservoir than others, despite the total available mass increasing over time, a scenario that is congruent with a clump-fed core accretion model. 

   more » « less
4. ALMA-IMF: X. The core population in the evolved W33-Main (G012.80) protocluster

   https://doi.org/10.1051/0004-6361/202347595  

   Armante, M; Gusdorf, A; Louvet, F; Motte, F; Pouteau, Y; Lesaffre, P; Galván-Madrid, R; Dell’Ova, P; Bonfand, M; Nony, T; et al (June 2024, Astronomy & Astrophysics)

   Context.One of the central questions in astrophysics is the origin of the initial mass function (IMF). It is intrinsically linked to the processes from which it originates, and hence its connection with the core mass function (CMF) must be elucidated. Aims.We aim to measure the CMF in the evolved W33-Main star-forming protocluster to compare it with CMF recently obtained in other Galactic star-forming regions, including the ones that are part of the ALMA-IMF program. Methods.We used observations from the ALMA-IMF large programme: ~2′ × 2′ maps of emission from the continuum and selected lines at 1.3 mm and 3 mm observed by the ALMA 12m only antennas. Our angular resolution was typically 1″, that is, ~2400 au at a distance of 2.4 kpc. The lines we analysed are CO (2–1), SiO (5–4), N₂H+ (1–0), H41α as well as He41α blended with C41α. We built a census of dense cores in the region, and we measured the associated CMF based on a core-dependent temperature value. Results.We confirmed the ‘evolved’ status of W33-Main by identifiying three HIIregions within the field, and to a lesser extent based on the number and extension of N₂H⁺filaments. We produced a filtered core catalogue of 94 candidates that we refined to take into account the contamination of the continuum by free-free and line emission, obtaining 80 cores with masses that range from 0.03 to 13.2M_⊙. We fitted the resulting high-mass end of the CMF with a single power law of the form N(log(M)) ∝ M^α, obtainingα= −1.44_−0.22^+0.16, which is slightly steeper but consistent with the Salpeter index. We categorised our cores as prestellar and protostellar, mostly based on outflow activity and hot core nature. We found the prestellar CMF to be steeper than a Salpeter-like distribution, and the protostellar CMF to be slightly top heavy. We found a higher proportion of cores within the HIIregions and their surroundings than in the rest of the field. We also found that the cores’ masses were rather low (maximum mass of ~13M_⊙). Conclusions.We find that star formation in W33-Main could be compatible with a ‘clump-fed’ scenario of star formation in an evolved cloud characterised by stellar feedback in the form of HIIregions, and under the influence of massive stars outside the field. Our results differ from those found in less evolved young star-forming regions in the ALMA-IMF program. Further investigations are needed to elucidate the evolution of late CMFs towards the IMF over statistically significant samples. 

   more » « less
5. ALMA-IMF: III. Investigating the origin of stellar masses: top-heavy core mass function in the W43-MM2&MM3 mini-starburst

   https://doi.org/10.1051/0004-6361/202142951  

   Pouteau, Y; Motte, F; Nony, T; Galván-Madrid, R; Men’shchikov, A; Bontemps, S; Robitaille, J-F; Louvet, F; Ginsburg, A; Herpin, F; et al (August 2022, Astronomy & Astrophysics)

   Aims.The processes that determine the stellar initial mass function (IMF) and its origin are critical unsolved problems, with profound implications for many areas of astrophysics. The W43-MM2&MM3 mini-starburst ridge hosts a rich young protocluster, from which it is possible to test the current paradigm on the IMF origin. Methods.The ALMA-IMF Large Program observed the W43-MM2&MM3 ridge, whose 1.3 mm and 3 mm ALMA 12 m array continuum images reach a ~2500 au spatial resolution. We used both the best-sensitivity and the line-free ALMA-IMF images, reduced the noise with the multi-resolution segmentation techniqueMnGSeg, and derived the most complete and most robust core catalog possible. Using two different extraction software packages,getsfandGExt2D, we identified ~200 compact sources, whose ~100 common sources have, on average, fluxes consistent to within 30%. We filtered sources with non-negligible free-free contamination and corrected fluxes from line contamination, resulting in a W43-MM2&MM3 catalog of 205getsfcores. With a median deconvolved FWHM size of 3400 au, core masses range from ~0.1M_⊙to ~70M_⊙and thegetsfcatalog is 90% complete down to 0.8M_⊙.Results.The high-mass end of the core mass function (CMF) of W43-MM2&MM3 is top-heavy compared to the canonical IMF. Fitting the cumulative CMF with a single power-law of the formN(> logM) ∝M^α, we measuredα= −0.95 ± 0.04, compared to the canonicalα= −1.35 Salpeter IMF slope. The slope of the CMF is robust with respect to map processing, extraction software packages, and reasonable variations in the assumptions taken to estimate core masses. We explore several assumptions on how cores transfer their mass to stars (assuming a mass conversion efficiency) and subfragment (defining a core fragment mass function) to predict the IMF resulting from the W43-MM2&MM3 CMF. While core mass growth should flatten the high-mass end of the resulting IMF, core fragmentation could steepen it. Conclusions.In stark contrast to the commonly accepted paradigm, our result argues against the universality of the CMF shape. More robust functions of the star formation efficiency and core subfragmentation are required to better predict the resulting IMF, here suggested to remain top-heavy at the end of the star formation phase. If confirmed, the IMFs emerging from starburst events could inherit their top-heavy shape from their parental CMFs, challenging the IMF universality. 

   more » « less