| Summary: | We present a study of molecular structures (clumps and clouds) in the dwarf galaxy NGC 404
using high-resolution (≈ 0.86 × 0.51 pc2
) Atacama Large Millimeter/sub-millimeter Array
12CO(2-1) observations. We find two distinct regions in NGC 404: a gravitationally-stable
central region (Toomre parameter 𝑄 = 3 − 30) and a gravitationally-unstable molecular ring
(𝑄 ≲ 1). The molecular structures in the central region have a steeper size – linewidth relation
and larger virial parameters than those in the molecular ring, suggesting gas is more turbulent
in the former. In the molecular ring, clumps exhibit a shallower mass – size relation and
larger virial parameters than clouds, implying density structures and dynamics are regulated
by different physical mechanisms at different spatial scales. We construct an analytical model
of clump-clump collisions to explain the results in the molecular ring. We propose that clumpclump collisions are driven by gravitational instabilities coupled with galactic shear, that
lead to a population of clumps whose accumulation lengths (i.e. average separations) are
approximately equal to their tidal radii. Our model-predicted clump masses and sizes (and
mass – size relation) and turbulence energy injection rates (and size – linewidth relation)
match the observations in the molecular ring very well, suggesting clump-clump collisions
is the main mechanism regulating clump properties and gas turbulence in that region. As
expected, our collision model does not apply to the central region, where turbulence is likely
driven by clump migration.
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