SELF-SIMILAR SPREADING OF A VISCOUS, STAR-FORMING GALACTIC DISC

In a viscous star-forming thin disc in which both the kinematical visc osity and the star formation time-scale are proportional to radius, a self-similar solution exists. It describes an essentially exponential gaseous disc, whose radial scale-length grows with time. Matter flows preferentially outwards. A single parameter describes the shape of the evolving gas distribution - the efficiency of the star forming proces s. The pronounced galactic central gas depression is reproduced, provi ded this efficiency exceeds a certain limit. The location of the gas s urface density maximum is moving outwards, too. Numerical simulations suggest that any viscous axisymmetric gas distribution approaches this self-similar solution quite quickly. The self-similar solution matche s the galactic gas distribution indeed well. To represent the stellar profile would require a radially more extended gas disc than indicated by observations.