Molecular Cloud Formation in Colliding Flows
by Fabian Heitsch
Stars are the building blocks of galaxies. The (visible) properties of galaxies are determined by that of their dominant stellar population. Stars form in molecular clouds. Although molecular clouds contribute only a fraction to the total mass budget of a galactic disk, their role as reservoirs and stellar birth places is substantial for the evolution of such a disk. Stars are observed to form in very diverse environments. The process of star formation is believed to be controlled to a large extent by the dynamics of the parent molecular cloud. Understanding how molecular clouds are forming will help us to gain deeper insight in the process of star formation. Observations show that molecular clouds are highly filamentary and "turbulent", i.e. the molecular gas is moving in (often) supersonic random motions. What is the reason for such motions? And what generates the long filamentary structures observed? The question points back to the problem of how molecular clouds form. One intriguing mechanism is that of colliding gas streams, e.g. from supernova shells, or - in the extreme - in galaxy mergers. A combination of thermal and dynamical instabilities leads to a very efficient conversion of highly compressible flows into turbulent, chaotic motions. Depending on the physical parameter regime, different instabilities govern the evolution of the colliding flow. The figures show the logarithm of temperature in Kelvin approximately 10 Myrs after the initial flow collision. In Figure 1, the system is dominated by thermal instabilities: the gas streams collide, and the gas can cool efficiently to form a dense, more or less stable slab. Figure 2 shows a system which is dominated by shear flow modes, or Kelvin-Helmholtz-Instabilities. Note the filigrane nature of the "cloudlets". Finally, figure 3 is dominated by the so-called slab-instability which arises at higher inflow speeds, so that the dynamical time scales are on the order of the cooling timescales, thus preventing the slab formation of figure 1. The cold (blue) regions at approx 20 Kelvin are the future sites of star formation.