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

TL;DR

ALMAGAL VIII constructs a uniform, high-resolution catalog of hierarchical continuum structures across 904 clumps by applying a dendrogram-based extraction to ALMA 1.38 mm data, yielding 5160 structures spanning 800–42000 au in radius. The study demonstrates that clumps with richer hierarchical fragmentation exhibit higher dust temperatures, surface densities, luminosity-to-mass ratios, and more massive cores, indicating a more evolved evolutionary stage. It reveals a direct, evolving link between the most massive core mass and the surrounding local surface density, consistent with clump-fed accretion scenarios, while the broader core population shows a looser correlation, suggesting differential accretion. Taken together, these results support a dynamical, reservoir-driven picture of core mass growth within progressively fragmented clumps, while recognizing uncertainties in temperature assumptions and missing flux that warrant future spectroscopic follow-up.

Abstract

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 the difference 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.

Figures (8)

• Figure 1: Three ALMAGAL targets are selected as examples to depict typical morphology in the Isolated (top), Simple (middle), and Rich (bottom) clump categories. The method used to classify these targets is described in Section \ref{['subsec:clump_class']}. The ID for each target is given at the top of each panel, and it corresponds to the "ALMAGAL ID" panel in Table \ref{['tab:catalog']}. In the right panels, background grayscale fading into heat colorbar shows the ALMAGAL 1.3mm dust continuum emission. Purple contours indicate the branch structures identified by the dendrogram algorithm in each region, while white contours indicate the leaf structures. Cyan circles indicate the position of cores from the ALMAGAL core catalog Coletta_2025. The left panel in each figure shows the dendrogram structure produced from the images shown in the corresponding right panels using the methods described in Section \ref{['subsec:dendro']}. The size of the synthesized 2000 AU beam is shown in the bottom left corner of each image and a scale bar is included in the bottom right corner.
• Figure 2: The maximum dendrogram level distribution for the near (top panel) and far (bottom panel) clumps in our ALMAGAL sample. The vertical dashed lines indicate the 1$\sigma_{\text{level}}$ and 3$\sigma_{\text{level}}$ of the fitted exponential distribution, shown by the solid black line. Since we cannot use the exact 1$\sigma_{\text{level}}$ and 3$\sigma_{\text{level}}$ values as thresholds on the integer-valued dendrogram levels, we instead round each value to the nearest integer and use that as a threshold. The integer ranges for the maximum level are shown for the Rich, Simple, and Isolated targets. The Empty targets do not have a threshold value, since they represent regions with non-detections and do not have an associated dendrogram level.
• Figure 3: Scatter plot showing the difference in the individual RMS values ($\sigma_{\text{rms}}$) and the corresponding 1$\sigma_{\text{rms}}$ point source mass sensitivity for near and far sample targets within the sub-sample of 900 ALMAGAL targets with corresponding clump property measurements. The $\sigma_{\text{rms}}$ values correspond to the standard deviation of the residual images after masking out regions of bright emission in the original (non-smoothed) intensity images Sanchez-Monge_2025. The red, vertical line indicates $d = 4.7$ kpc, the heliocentric distance dividing the near and far sample targets.
• Figure 4: Violin plots showing the distribution of heliocentric distances (Top) and Galactocentric radii (Middle) and the clump mass (Bottom) for the different morphological categories of clumps in the ALMAGAL sample (Empty, Isolated, Simple, Rich and All). The center vertical line indicates the median for each distribution.
• Figure 5: Violin plots highlighting the population differences in clump surface density (Top Left), dust temperature (Top Right), L/M ratio (Bottom Left), and MMC mass (Bottom Right) across the Empty, Isolated, Simple, and Rich clump sub-samples as well as the full sample. The center vertical line indicates the median for each distribution. In general, the morphologically rich targets have a higher median surface density, dust temperature, MMC mass, and $L/M$, all of which are properties that may be related to the evolutionary stage of the clump.