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We present a study of molecular structures (clumps and clouds) in the dwarfgalaxy NGC 404 using high-resolution (0.86x0.51 pc^2) Atacama LargeMillimeter/sub-millimeter Array ^{12}CO(2-1) observations. We find two distinctregions in NGC 404: a gravitationally-stable central region (Toomre parameterQ=3-30) and a gravitationally-unstable molecular ring (Q<=1). The molecularstructures in the central region have a steeper size -- linewidth relation andlarger virial parameters than those in the molecular ring, suggesting gas ismore turbulent in the former. In the molecular ring, clumps exhibit a shallowermass -- size relation and larger virial parameters than clouds, implyingdensity structures and dynamics are regulated by different physical mechanismsat different spatial scales. We construct an analytical model of clump-clumpcollisions to explain the results in the molecular ring. We propose thatclump-clump collisions are driven by gravitational instabilities coupled withgalactic shear, that lead to a population of clumps whose accumulation lengths(i.e. average separations) are approximately equal to their tidal radii. Ourmodel-predicted clump masses and sizes (and mass -- size relation) andturbulence energy injection rates (and size -- linewidth relation) match theobservations in the molecular ring very well, suggesting clump-clump collisionsis the main mechanism regulating clump properties and gas turbulence in thatregion. As expected, our collision model does not apply to the central region,where turbulence is likely driven by clump migration.

WISDOM Project XII. Clump properties and turbulence regulated by clump-clump collisions in the dwarf galaxy NGC404