Embedded Star Formation in S4G Galaxy Dust Lanes
  Star-forming regions that are visible at 3.6 microns and Halpha but not in
the u,g,r,i,z bands of the Sloan Digital Sky survey (SDSS), are measured in
five nearby spiral galaxies to find extinctions averaging ~3.8 mag and stellar
masses averaging ~5x10^4 Msun. These regions are apparently young star
complexes embedded in dark filamentary shock fronts connected with spiral arms.
The associated cloud masses are ~10^7 Msun. The conditions required to make
such complexes are explored, including gravitational instabilities in spiral
shocked gas and compression of incident clouds. We find that instabilities are
too slow for a complete collapse of the observed spiral filaments, but they
could lead to star formation in the denser parts. Compression of incident
clouds can produce a faster collapse but has difficulty explaining the
semi-regular spacing of some regions along the arms. If gravitational
instabilities are involved, then the condensations have the local Jeans mass.
Also in this case, the near-simultaneous appearance of equally spaced complexes
suggests that the dust lanes, and perhaps the arms too, are relatively young.
