The ALMA-QUARKS Survey: Fibers' role in star formation unveiled in an intermediate-mass protocluster region of the Vela D cloud

In this paper, we present a detailed analysis of the IRS 17 filament within the intermediate-mass protocluster IRAS 08448-4343 (of $\sim {10}^3{\mathrm{L}}_{\odot }$), using ALMA data from the ATOMS 3-mm and QUARKS 1.3-mm surveys. The IRS 17 filament, which spans $\sim$54000 au ($0.26\mathrm{p}\mathrm{c}$) in length and $\sim$4000 au ($0.02\mathrm{p}\mathrm{c}$) in width, exhibits a complex, multi-component velocity field, and harbours hierarchical substructures. These substructures include three bundles of seven velocity-coherent fibers, and 29 dense ($n\sim {10}^8\mathrm{c}{\mathrm{m}}^{-3}$) condensations. The fibers have a median length of $\sim 4500\mathrm{a}\mathrm{u}$ and a median width of $\sim 1400\mathrm{a}\mathrm{u}$. Among these fibers, four are identified as fertile", each hosting at least three dense condensations, which are regarded as the seeds" of star formation. While the detected cores are randomly spaced within the IRS,17 filament based on the 3-mm dust continuum image, periodic spacing ($\sim 1600\mathrm{a}\mathrm{u}$) of condensations is observed in the fertile fibers according to the 1.3-mm dust map, consistent with the predictions of linear isothermal cylinder fragmentation models. These findings underscore the crucial role of fibers in star formation and suggest a hierarchical fragmentation process that extends from the filament to the fibers, and ultimately, to the smallest-scale condensations.