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

TL;DR

This work analyzes the spatial distribution of 5,728 dense cores across 514 massive clumps from the ALMAGAL survey to trace how fragmentation is initiated and evolves prior to gas dispersal. Using minimum spanning tree separations, Jeans-length comparisons, and spatial metrics ($Q$, $\Lambda_{MSR}$), the study finds cores generally arranged in elongated subclusters with typical radii around 0.2 pc; core separations are often close to the clump's thermal Jeans length, though a younger, low-fragmentation population shows larger separations, and some evolve toward sub-Jeans scales due to local effects. The analysis reveals a clear evolutionary trend: average core separations decrease as $L/M$ increases, and mass segregation becomes more common with evolution, indicating that cores (and likely their stars) tend to concentrate over time. Overall, the results support Jeans-like fragmentation as a dominant channel in massive clumps while highlighting a rich diversity of fragmentation modes and the progressive appearance of mass segregation, consistent with a dynamical, multi-scale picture of cluster formation. The findings provide robust, quantitative constraints for models of high-mass star and cluster formation, emphasizing the role of both initial conditions and dynamical evolution in shaping early cluster structure.

Abstract

High-mass stars and star clusters form from the fragmentation of massive dense clumps driven by gravity, turbulence, and magnetic fields. The ALMAGAL project observed $\sim1000$ clumps at $\sim$1000\,au resolution, enabling a statistically significant characterization of this process across a large range of clump physical parameters and evolutionary stages. In this work, we investigated the spatial distribution of dense cores in the 514 massive, potentially cluster-forming, clumps hosting at least 4 cores, to trace fragmentation's initial conditions and early evolution. We used quantitative descriptors, evaluated against the clump bolometric luminosity-to-mass ratio as an indicator of evolution. Core separations were measured with the minimum spanning tree method (MST) and compared with the Jeans gravitational fragmentation theory. We used the $Q$ parameter and the mass segregation ratio, $Λ_{MSR}$, to evaluate whether cores have specific arrangements or differences depending on their masses. ALMAGAL cores are usually arranged in elliptical groups with an axis ratio $e\sim2.2$, but $e\geq$5 is also observed. A single characteristic core separation per clump is found in $\sim76$% of cases, but signatures of multiple fragmentation lengths not rare. Typical core separations are compatible with the clump-averaged thermal Jeans length, $λ^{th}_{J}$, though a population, typical of low-fragmented/young clumps, has wider separations with $l\approx3\timesλ^{th}_{J}$. The core separation decreases on average from $l\sim22000$ au in younger systems to $l\sim7000$ au in more evolved ones. Cores are typically distributed in fractal-type subclusters, with centrally concentrated patterns appearing only at later stages, but without a progressive evolutionary transition. Finally, mass segregation is found in 110 systems, with its occurrence increasing with evolution.

Figures (22)

• Figure 1: Distribution of the ratio $L/M$ and average surface density as function of the clump heliocentric distance for the ALMAGAL sample, and comparison of these two properties one versus the other. Each panel includes the 2D density distribution computed dividing the intervals of $L/M$, $\Sigma_{cl}$ and in $d_{cl}$ with bins of width 0.3 dex, 0.2 dex and 500 pc, respectively. The solid light gray lines indicate the contour levels of the 2D density distribution derived with Kernel Density Estimation corresponding to 10, 30, 50, 70, 95 percentage of the sample.
• Figure 2: Examples of the distribution of cores found in four ALMAGAL clumps overlapped on the dust continuum map at 1.4 mm from which they were extracted. These fields show examples of clustered systems with circular and elliptical patterns and cases of aligned cores. The contour levels correspond to $4n\times\sigma$ level and $\sigma$ equal to the noise level measured on the image. The positions of the cores from the ALMAGAL catalog extracted by Coletta2025 are indicated with gray star symbols. In each field, we indicate the cluster geometrical center and its radius (see the definition in the text) with a black cross and a dark blue circle, respectively. We also show the convex hull polygon (cyan segments) and the corresponding best-fitting ellipse (black dashed line) adopted for the morphological characterization of the system. The solid thick black segments are the MST edges determined with Prim's algorithm Prim1957, connecting all the cores in the field.
• Figure 3: As in Fig. \ref{['Fig:ExampleMST_1']}. These fields shows examples of cores distributed over filamentary features and in well-separated, local substructures, both patterns that are sometimes found in ALMAGAL continuum images.
• Figure 4: Top panel: Distribution of the cluster radius measured from the average core positions for the sample of 514 ALMAGAL clumps with $N_{core}\geq4$ (in gray) and the subsample composed 347 clumps with $d_{cl}\,\geq\,3.7$ kpc (magenta). Bottom panel: Clump heliocentric distance vs measured cluster radius. The dashed line shows the physical size of the ALMAGAL FoV, while the dotted one indicates the area where the sensitivity is constant within a factor of two, i.e. angular sizes $\sim13.5$. Cluster radii exceeding the FoV sizes are possible in systems with a substantial offset between the cluster and the image center.
• Figure 5: Distribution of the ratio between the radius of the core cluster and the clump one (gray histogram). The dot-dashed line, which refers to the top and right axes, shows the distribution of the clump radius derived from Herschel Hi-GAL data at 250 $\mu$m from Elia2021.