ALMAGAL

VI. The spatial distribution of dense cores during the evolution of cluster-forming massive clumps

¹ INAF – IAPS, via Fosso del Cavaliere, 100, 00133 Roma, Italy; Istituto Nazionale di Astrofisica (INAF)-Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
² Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 2, 00185, Roma, Italy
³ Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁴ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193, Bellaterra (Barcelona), Spain
⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08860, Castelldefels (Barcelona), Spain
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Straße 2, D-69120 Heidelberg, Germany
⁸ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
⁹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
¹¹ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹² SRON Netherlands Institute for Space Research, Landleven 12, 9747, AD Groninger, The Netherlands
¹³ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁴ Université Paris-Saclay, Université Paris-Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹⁵ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, Firenze, Italy
¹⁶ INAF-Istituto di Radioastronomia & Italian ALMA Regional center, Via P. Gobetti 101, 40129 Bologna, Italy
¹⁷ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
¹⁸ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of ASMAB, AS/NTU No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
¹⁹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²⁰ Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
²¹ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
²² SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK
²³ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
²⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
²⁵ Dipartimento di Fisica e Astronomia, Alma Mater Studiorium Università di Bologna Dipartimento di Fisica e Astronomia “Augusto Righi”, Via Gobetti 93/2, 40129, Bologna, Italy
²⁶ Research Center for Astronomical computing, Zhejiang Laboratory, Hangzhou, China
²⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
²⁹ University of Connecticut, Department of Physics, 2152 Hillside Road, Unit 3046 Storrs, CT 06269, USA
³⁰ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³¹ Max-Planck-Institute for Extraterrestrial Physics (MPE), Garching bei München, Germany
³² LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 75014 Paris, France
³³ East Asian Observatory, 660 N. A’ohoku, Hilo, Hawaii, HI 96720, USA
³⁴ School of Engineering and Physical Sciences, The University of Lincoln, Brayford Way, Lincoln LN6 7TS, UK
³⁵ UK Astronomy Technology center, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
³⁶ INAF – Astronomical Observatory of Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
³⁷ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
³⁸ Center for Data and Simulation Science, University of Cologne, Germany
³⁹ Universidad Autónoma de Chile, Av. Pedro de Valdivia 425, Providencia, Santiago de Chile, Chile

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Received: 21 May 2025
Accepted: 25 November 2025

Abstract

Context. High-mass stars and star clusters form from the fragmentation of massive dense clumps driven by gravity, turbulence, and magnetic fields. The extent to which each of these agents impacts the fragmentation depending on the clump mass, density, and evolutionary stage is still largely unknown.

Aims. The ALMA evolutionary study of high-mass protocluster formation in the GALaxy (ALMAGAL) project, with ∼1000 clumps observed at ∼1000 au resolution, allows a statistically significant characterization of the fragmentation process over a large range of clump physical parameters and evolutionary stages. Our goal is to characterize where and how the dense cores revealed by ALMA are distributed in massive potentially cluster-forming clumps to trace how fragmentation is initially set and how it proceeds before gas dispersal due to stellar feedback.

Methods. We characterized the spatial distribution of dense cores in the 514 ALMAGAL clumps that host at least four cores, using a set of quantitative descriptors that we evaluated against the clump bolometric luminosity-to-mass ratio, which we adopted as an indicator of the evolution of the system. We measured the separations between cores with the minimum spanning tree (MST) method, which we compared with the predictions of gravitational fragmentation from Jeans theory. We investigated whether cores have specific arrangements using the Q parameter or variations due to their masses with the mass segregation ratio, Λ_MSR.

Results. ALMAGAL cores are distributed throughout the entire area of the clump, usually arranged in elliptical groups with an axis ratio e ∼2.2, although high values with e ≥ 5 are also observed. We found a single characteristic core separation per clump in ∼76% of cases, suggesting that multiple fragmentation lengths may be frequently present. Typical core separations are compatible with the clump-averaged thermal Jeans length, λ_J^th. However, we found an additional population of cores, typical of low-fragmented and young clumps, which are on average more widely separated with l ≈ 3 × λ_J^th. By stacking the distributions of the core separations in clumps of similar evolutionary stage, we also found that the separation decreases on average from l ∼22 000 au in younger systems to l ∼7000 au in more evolved ones. The ALMAGAL cores are typically distributed in fractal-type subclusters, while centrally concentrated patterns appear only at later stages, but we do not observe a progressive transition between these configurations with evolution. Finally, we also found 110 ALMAGAL systems with a signature of mass segregation, with an occurrence that increases with evolution.