On Boundary Layer Flows in Disk-Halo Galaxies

Extract

The existence of a boundary layer circulation in a system composed of a galactic disk embedded within a surrounding gaseous halo, may be inferred from two physical principles governing the fluid dynamical interactions. The first concerns the presence of a vertical shear in the azimuthal flow of gas, and the localization of this shear to the vicinity of the disk. The second principle concerns the importance of viscous stresses with regard to their influence on the vertical structure of the flow throughout the disk region, and the resultant coupling of the disk to the halo. It is this viscous coupling that gives rise to a meridional circulation extending over several disk scale heights from the galactic plane. In this discussion we shall concentrate on a rather specific model system in order to establish the essential dynamical features of the theory. The details of a somewhat more general analysis are to appear in the Astrophysical Journal in the near future.