  The process that leads to the formation of the bright star forming sites
observed along prominent spiral arms remains elusive. We present results of a
multi-wavelength study of a spiral arm segment in the nearby grand-design
spiral galaxy M51 that belongs to a spiral density wave and exhibits nine gas
spurs. The combined observations of the(ionized, atomic, molecular, dusty)
interstellar medium (ISM) with star formation tracers (HII regions, young
<10Myr stellar clusters) suggest (1) no variation in giant molecular cloud
(GMC) properties between arm and gas spurs, (2) gas spurs and extinction
feathers arising from the same structure with a close spatial relation between
gas spurs and ongoing/recent star formation (despite higher gas surface
densities in the spiral arm), (3) no trend in star formation age either along
the arm or along a spur, (4) evidence for strong star formation feedback in gas
spurs: (5) tentative evidence for star formation triggered by stellar feedback
for one spur, and (6) GMC associations (GMAs) being no special entities but the
result of blending of gas arm/spur cross-sections in lower resolution
observations. We conclude that there is no evidence for a coherent star
formation onset mechanism that can be solely associated to the presence of the
spiral density wave. This suggests that other (more localized) mechanisms are
important to delay star formation such that it occurs in spurs. The evidence of
star formation proceeding over several million years within individual spurs
implies that the mechanism that leads to star formation acts or is sustained
over a longer time-scale.
