Hierarchical hub-filament structures of molecular gas: star formation and cluster evolution

Abstract

The first part of this thesis presents a multi-scale investigation of hub–filament structures, analyzing their morphology, kinematics, and evolution from dense core (~1000 AU) to clump (~1 pc), molecular cloud (~10–100 pc), and galaxy-cloud (~1000 pc) scales. Observations reveal a hierarchical, self-similar network of hub–filament structures spanning sub-parsec to kiloparsec scales, underscoring their key role in star formation. Within this hierarchy, a dense core serves as the hub within a clump, a clump functions as the hub within a molecular cloud, and a molecular cloud acts as the hub within a galaxy. Hub–filament structures arise from gravitational contraction, with velocity gradients indicating gravity-driven gas inflows along filaments. Molecular gas is organized into gravitationally coupled networks, where local hubs serve as the main sites of star formation. The second part examines how tidal forces from surrounding material regulate gravitational collapse and shape gas kinematics across scales. These interactions significantly impact velocity dispersion and density contrast, highlighting the environment's key role in structural evolution. The third part studies gas kinematics and dynamics under stellar feedback across scales. Significant correlations between velocity dispersion and column density suggest that velocity dispersion may partly originate from gravitational collapse, a conclusion further supported by the observed velocity gradients. Although feedback disrupts the original cloud complex during star cluster formation, substructures reorganize around new gravitational centers, with ongoing collapse despite structural destruction and regeneration. The fourth part investigates star cluster formation and evolution from clumps to open clusters using observations, simulations, and modeling. Embedded clusters in ATLASGAL clumps show burst-like star formation, with accretion luminosity important in low-mass clumps. Star formation efficiency averages ~30% and strongly anti-correlates with clump mass. The coalescence of embedded clusters within the same parental cloud may drive open cluster formation. Additionally, cluster formation mechanisms and initial gas expulsion are key to black hole formation in open clusters. The fifth part examines mass functions in star-forming regions, focusing on the link between core mass functions (CMFs) in high-mass star-forming regions and the initial mass function (IMF). For clump complexes, the parameters of the clump mass function correlate strongly with cloud mass, while widespread substructures and mass segregation indicate gravitationally driven evolution. On galaxy–cloud scales, the integrated cloud-wide IMF (ICIMF) model is developed to connect the properties of molecular clouds with their internal stellar populations in nearby galaxies. In a hub-filament system, hubs are primary star formation sites. Clumps evolve into embedded clusters defining an IMF, clouds contain multiple clumps forming an ICIMF, and galaxies host multiple clouds producing an integrated galaxy-wide IMF (IGIMF). These hierarchical hub-filament structures (star-forming regions) correspond to hierarchically integrated IMFs. Extending this self-similar physical framework allows us to establish a connection between star formation in the Milky Way and nearby galaxies, linking star-forming regions with their stellar populations across various scales.