  Spitzer Space Telescope InfraRed Array Camera (IRAC) images of M100 show
numerous long filaments with regularly-spaced clumps, suggesting the associated
cloud complexes formed by large-scale gravitational instabilities in shocked
and accumulated gas. Optical images give no hint of this underlying regularity.
The typical spacing between near infrared (NIR) clumps is ~410 pc, which is ~3
times the clump diameter, consistent with the fastest growing mode in a
filament of critical line density. The IRAC magnitudes and colors of several
hundred clumps are measured in the most obvious 27 filaments and elsewhere. The
clump colors suggest that the dust is associated with diffuse gas, PAH
emission, and local heating from star formation. Neighboring clumps on the same
filament have similar magnitudes. The existence of many clumps all along the
filament lengths suggests that the ages of the filaments are uniform. The
observations support a model where interstellar gas is systematically
accumulated over lengths exceeding several kpc, forming spiral-like filaments
that spontaneously collapse into giant clouds and stellar complexes. Optical
wavelengths show primarily the irregular dust debris, HII regions, and
lingering star formation downstream from these primal formation sites.
