Evidence for coherent star-forming processes in the discs of spirals

To what extent is star formation in spiral galaxies controlled by the dynamical geometry of the placental gas? In this article we consider two cases: that of spiral arm formation and that of circumnuclear starbursts, in the search for evidence of phase-coherent star formation, which would accompany the presence of well-ordered shock-fronts passing through the pre-stellar gas clouds. We find cases where, along an arm, the youngest blue stars are coherently displaced from a recent but older generation of redder evolved supergiants and others where the star formation efficiency in arms appears to be responding to a density wave resonance pattern. We also find cases of axisymmetric outflow in the disc components of circumnuclear starburst zones, which could account for an evolutionary starburst→LINER sequence. (author)