Discovery of a rich population of compact hub-filament systems in a single star-forming complex

TL;DR

This study reveals a dense population of 45 compact hub-filament systems (HFSs) in the infrared-dark clouds of the W33 complex, with a median HFS size of $2.4$ pc. HFSs typically arise at intersections of elongated subfilaments and show strong protostellar clustering, while most hubs lack radio continuum emission. Minimum Spanning Tree analyses show protostars clustering at scales of $\leq$ $0.7$ pc and reveal two main HFS groups spanning $10$--$15$ pc, with inter-HFS separations of $1$--$3.3$ pc; ALMAGAL 1.38 mm cores within ten HFSs exhibit a median core separation of $0.03$ pc. The protostellar spacing is larger than the thermal Jeans length ($\sim$0.08 pc at $T\sim18$ K, $n\sim10^5$ cm$^{-3}$), while core spacing remains below the Jeans length, indicating hierarchical fragmentation where cloud/filament fragments form clumps that host HFSs and drive efficient, clustered star formation, often producing massive stars. The W33 environment, at a spiral-arm junction with evidence for supersonic turbulent motions and possible cloud–cloud collisions, appears to sculpt these structures and their evolution toward more developed star-forming activity.

Abstract

We report the discovery of 45 compact hub-filament systems (HFSs; median size $\sim$2.4 pc) in infrared-dark clouds (IRDCs) in the W33 complex, located at the junction of the Scutum and Norma spiral arms. Using {\it Spitzer} 8 and 24 micron, and unWISE 12 $μ$m images, HFSs are identified as regions where three or more filaments converge onto a central hub, appearing as absorption features toward IRDCs. In each IRDC, HFSs mainly lie at the intersections of elongated substructures, associated with groups of protostars and lacking radio continuum emission. Minimum Spanning Tree (MST) analysis shows that protostars are closely associated with the HFSs, with protostellar core separations of $\leq$ 0.7 pc, indicating strong clustering within fragmented structures. The HFSs form two main groupings spanning 10--15 pc, with member separations of 1--3.3 pc. Around 65\% are tightly clustered ($<$ 2 pc), exhibiting rich small-scale structures and emphasizing the uniqueness of the complex. MST analysis of ALMAGAL 1.38 mm continuum cores -- predominantly low-mass and embedded in ten HFSs -- reveals a median core separation of $\sim$0.03 pc. The protostellar spacing ($\sim$0.7 pc) significantly exceeds the thermal Jeans length ($\sim$0.08 pc for temperature $\sim$18 K and density $\sim$10$^{5}$ cm$^{-3}$), whereas the core spacing is smaller than the Jeans length, suggesting that thermal fragmentation may influence core formation but alone cannot explain the larger-scale protostellar distribution. All these findings together support a picture in which fragments of clouds/filaments form clumps hosting compact HFSs that facilitate efficient and clustered star formation, often yielding massive stars.

Figures (10)

• Figure 1: a) The panel shows the Herschel temperature map of a larger area ($\sim$1$^\circ$.1 $\times$ 0$^\circ$.815 ($\sim$50 pc $\times$ 37 pc at d$\sim$2.6 kpc), centered at Galactic coordinates l = 12$^\circ$.946, b = $-$0$^\circ$.1914. The area includes the W33 complex (indicated by the ellipse). The map is overlaid with the positions of the IRDCs (peretto16; see white filled triangles), massive clumps (magenta filled stars representing W33 Main, W33A, W33B, W33 Main1, W33A1, and W33B1; immer14), and 870 $\mu$m dust continuum clumps (green diamonds; urquhart18). The area hosting elongated filaments, fs1 and fs2, is outlined with the dot-dashed box. b) The panel displays a three-color composite map (red: 24 $\mu$m, green: 12 $\mu$m, blue: 8 $\mu$m) of the area highlighted by the dot-dashed box in Figure \ref{['fig1']}a. Dotted circles mark the positions of HFSs identified in the MIR images (see Table \ref{['tab1']}). Two sub-structures in absorption are indicated by arrows. c) The panel presents a three-color composite map (red: 12 $\mu$m, green: DoG processed 12 $\mu$m, blue: DoG processed 8 $\mu$m). The scale bar corresponding to 10 pc at d$\sim$2.6 kpc is shown in each panel.
• Figure 2: a) 12 $\mu$m image of h10-HFS (including h14-HFS) overlaid with the NH$_{3}$ emission integrated over [36, 40] km s$^{-1}$. Contours are 3.706 K km s$^{-1}$$\times$ [0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.98]. b) 12 $\mu$m image of h2-HFS/SDC 13 overlaid with the NH$_{3}$ emission integrated over [35.3, 39.5] km s$^{-1}$. Contours are 6.542 K km s$^{-1}$$\times$ [0.11, 0.16, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 0.98].
• Figure 3: a) Overlay of the positions of HFSs (dotted circles), ATLASGAL clump locations urquhart18), and the NVSS 1.4 GHz continuum emission contour at 4.7 mJy beam$^{-1}$ on the SMGPS 1.3 GHz continuum map. b) The panel displays the 24 $\mu$m image overlaid with the positions of HFSs (dotted circles) and Hi-GAL clumps elia17. Open and filled hexagons indicate the positions of candidate HFSs kumar20.
• Figure 4: Spatial distribution of protostars using MST approach. a) Overlay of protostar positions (open circles) on the 12 $\mu$m image. The ellipse and filled stars are the same as shown in Figure \ref{['fig1']}a. b) MST of protostar positions in the area shown in Figure \ref{['fig1']}a. Blue circles connected by solid lines represent MST branches smaller than the critical branch length, indicative of core-scale groupings. c) MST of protostar positions distributed in the area outlined by the dot-dashed box in Figure \ref{['fig3']}a (also see Figure \ref{['fig3']}b). Large red circles indicate the positions of HFSs (also see Figure \ref{['fig1']}b), and diamonds mark the locations of 870 $\mu$m dust continuum clumps.
• Figure 5: a) MST of the identified HFSs in the area presented in Figure \ref{['fig1']}b. Solid lines connecting red and yellow dots represent MST branches shorter than the critical lengths of 2 pc for cores and 3.3 pc for active regions (ARs), respectively (see text for more details). Purple dots indicate isolated HFSs that are not connected by branches shorter than the respective critical lengths. b) Histogram showing the distribution of MST branch lengths for the HFSs. The red and yellow vertical lines mark the critical branch lengths for cores (2 pc) and ARs (3.3 pc), respectively. c) Cumulative distribution of MST branch lengths for the HFSs. The red and yellow vertical lines indicate the critical branch lengths for cores (2 pc) and ARs (3.3 pc), respectively.