Research News

NARIT astronomer discovers a rich population
of compact hub-filament systems in a star-forming complex

A research team including NARIT astronomer Dr. Ram K. Yadav has reported the discovery of an exceptionally dense population of compact hub–filament systems in the W33 star-forming complex-one of the Galaxy’s active nurseries for building stellar clusters and massive stars. The study has been accepted for publication in The Astronomical Journal (AJ).

W33 is a star-forming complex located near the junction of the Scutum and Norma spiral arms, where large-scale Galactic structure, gas flows, and star formation may intersect in especially dynamic ways.

The region contains prominent infrared-dark clouds (IRDCs)-cold, dense clouds that appear in silhouette against the bright Galactic background, and which often represent early stages of star formation.

Using high-quality infrared imaging from Spitzer (8 and 24 μm) and unWISE (12 μm), the researchers identified 45 compact hub–filament systems (HFSs) in W33. A hub–filament system can be thought of as a hub (a dense central concentration) connected by multiple filaments (thread-like structures of gas and dust). In this study, HFSs are defined as locations where three or more filaments converge onto a central hub, often seen in absorption inside IRDCs.

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Figure 1: Infrared view (red: 24 μm, green: 12 μm, blue: 8 μm) of the W33 complex highlighting compact hub–filament systems.
Yellow circles mark the identified HFS candidates across the North/East/West sub-regions;
arrows indicate prominent dark filamentary structures. Two sub-structures in absorption are indicated by arrows. 

The discovered systems are compact, with a median size of approximately 2.4 parsecs.

A major result is how tightly young stars are associated with these hubs. Using Minimum Spanning Tree (MST) analysis, the study shows that protostars cluster near the HFSs with typical separations of ≤0.7 pc, indicating strong clustering within fragmented structures.

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Figure 2: Left: Young-star clustering around hub–filament systems in W33. Markers show the distribution of young stellar objects;
connecting segments illustrate clustering structure, while red circles indicate the locations/footprints of identified HFSs.
Right: Minimum Spanning Tree (MST) linking the hub–filament systems. The MST reveals two dominant groupings
of HFSs across W33, consistent with large-scale organization on ~10–15 pc scales.

Notably, the HFSs form two main groupings spanning 10–15 pc, with member separations of 1–3.3 pc—and roughly 65% of the HFSs are tightly clustered within <2 pc, underscoring the unusually rich small-scale structure of W33.

The study further connects the infrared-discovered hubs to millimeter-wave data from ALMAGAL (ALMA) 1.38 mm continuum observations. In ten hub–filament systems, the ALMA cores—predominantly low-mass and deeply embedded—show a median nearest-neighbor separation of ~0.03 pc, providing a close-up view of how dense fragments are arranged within hubs.

Taken together, the results support a coherent picture: clouds and filaments fragment into clumps, clumps host compact hub–filament systems, and these hubs can facilitate efficient and clustered star formation, including the conditions that often lead to massive stars.

By documenting an unusually high concentration of compact HFSs in a single complex, this work provides a valuable empirical benchmark for theories of clustered star formation and the role of filaments in feeding central hubs.

References: Lokesh K. Dewangan, Ram K. Yadav et al., “Discovery of a rich population of compact hub-filament systems in a single star-forming complex”,  Accepted in Astronomical Journal, https://arxiv.org/pdf/2512.01455
Contact: [email protected] (Ram K. Yadav)