HYDRODYNAMIC CONSTRAINTS ON THE RADIAL STRUCTURE OF LATE-TYPE GALAXY DISKS

We present models for the radial structure of the gas in late-type galaxy disks which are determined by an active balance of hydrodynamic forces, together with the effects of stellar activity, operating in a fixed (e.g., stellar plus dark matter) gravitational potential. Specifically, in order for the radial surface density profile of a disk to be stable on a hydrodynamic time scale, all radial forces must balance, including the often neglected pressure gradient force as well as the gravitational and centrifugal forces. Moreover, both viscous transport and self-diffusion must be minimized to maintain disk profile stability over a significant fraction of a Hubble time. Self-diffusion is likely to be especially important in an interstellar medium with material transport in the vertical direction via local fountains or chimneys, or in any interstellar medium where the cloud mean free path is long. In the flat rotation curve (FRC) region of a late-type disk the equilibrium gas surface density profile is of 1/r form. A number of authors have previously suggested this profile on the basis of gravitational stability arguments, and it appears to agree well with observations of the FRC regions in late-type galaxies. On time scales comparable to the gas consumption time modest steady flows are expected to develop, but the profile remains of 1/r form to a good approximation. In the inner, rising rotation curve (RRC) region it is not possible to satisfy all the hydrodynamic equilibrium and the gravitational stability conditions simultaneously, so no equilibrium is possible on time scales much longer than a sound crossing time. The consequences include (1) nonsteady gas flows and star formation and (2) steady inflow balanced by gas consumption or vertical wind-fountain flows (a third possibility is that the region is devoid of gas). Specifically, in disks with weak bars or other asymmetries in their central regions, it seems likely that global radial flows will be induced. In such cases the hydrodynamic constraints suggest an especially simple disk structure in all regions. The relative roles of formation conditions and the hydrodynamic constraints in determining the morphological diversity among disks are discussed.