G321.93-0.01: A Rare Site of Multiple Hub-Filament Systems with Evidence of Collision and Merging of Filaments

Hub-filament systems (HFSs) are potential sites of massive star formation (MSF). To understand the role of filaments in MSF and the origin of HFSs, we conducted a multi-scale and multi-wavelength observational investigation of the molecular cloud G321.93-0.01. The $^{13}$CO($J$ = 2-1) data reveal multiple HFSs, namely, HFS-1, HFS-2, and a candidate HFS (C-HFS). HFS-1 and HFS-2 exhibit significant mass accretion rates ($\dot{M}_{||}$ $> 10^{-3}$ $M_{\odot}$ yr$^{-1}$) to their hubs (i.e., Hub-1 and Hub-2, respectively). Hub-1 is comparatively massive, having higher $\dot{M}_{||}$ than Hub-2, allowing to derive a relationship $\dot{M}_{||} \propto M^{\beta}_{\rm{hub}}$, with $\beta \sim1.28$. Detection of three compact HII regions within Hub-1 using MeerKAT 1.28 GHz radio continuum data and the presence of a clump (ATL-3), which meets Kauffmann & Pillai's criteria for MSF, confirm the massive star-forming activity in HFS-1. We find several low-mass ALMA cores (1-9 $M_{\odot}$) inside ATL-3. The presence of a compact HII region at the hub of C-HFS confirms that it is active in MSF. Therefore, HFS-1 and C-HFS are in relatively evolved stages of MSF, where massive stars have begun ionizing their surroundings. Conversely, despite a high $\dot{M}_{||}$, the non-detection of radio continuum emission toward Hub-2 suggests it is in the relatively early stages of MSF. Analysis of $^{13}$CO($J$ = 2-1) data reveals that the formation of HFS-1 was likely triggered by the collision of a filamentary cloud about 1 Myr ago. In contrast, the relative velocities ($\gtrsim 1$ km s$^{-1}$) among the filaments of HFS-2 and C-HFS indicate their formation through the merging of filaments.