Gravitational collapse in dust lanes and the appearance of spiral structure in galaxies

We consider the possibility that there are two fundamental types of spirals in each spiral galaxy: one results from a spiral density wave, and the other is a stochastic spiral produced by the shear of self-propagating star formation events. It is proposed that there is a physical and causal connection between these two spirals, with the role of intermediary played by the dust lanes, which we assume to be shocked interstellar matter. We show that dust lanes should continuously collapse and fragment into massive cloud complexes by the action of their self-gravitational forces. The origin of the molecular ring at 5--6 kpc from the galactic center seems to be the result of a sharp minimum in the opposition to dust lane collapse by galactic tidal forces at these radii. When the resulting clouds begin to form stars (probably by an independent process), the pressures generated by their own OB associations will push them away from their star clusters and cause them to be shuffled around randomly in the region of the galaxy that lies ahead of the dust lane. Massive star formation may occur in the same clouds on numerous occasions, but they will have a different location each time. This shuffling will not only produce the appearance of a stochastic arm of young, star-forming sites (superposed on the underlying spiral), but it will also give the required stability to the numerical calculations of Gerola and Seiden, when combined with the assumption of a steady state cloud population for the whole galaxy. Numerous observational examples of these processes are cited for our own Galaxy and for other galaxies