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Star-forming regions that are visible at 3.6 microns and Halpha but not inthe u,g,r,i,z bands of the Sloan Digital Sky survey (SDSS), are measured infive nearby spiral galaxies to find extinctions averaging ~3.8 mag and stellarmasses averaging ~5x10^4 Msun. These regions are apparently young starcomplexes embedded in dark filamentary shock fronts connected with spiral arms.The associated cloud masses are ~10^7 Msun. The conditions required to makesuch complexes are explored, including gravitational instabilities in spiralshocked gas and compression of incident clouds. We find that instabilities aretoo slow for a complete collapse of the observed spiral filaments, but theycould lead to star formation in the denser parts. Compression of incidentclouds can produce a faster collapse but has difficulty explaining thesemi-regular spacing of some regions along the arms. If gravitationalinstabilities are involved, then the condensations have the local Jeans mass.Also in this case, the near-simultaneous appearance of equally spaced complexessuggests that the dust lanes, and perhaps the arms too, are relatively young.

Embedded Star Formation in S4G Galaxy Dust Lanes