A MODEL OF THE GALACTIC DARK HALO

Several features of galaxies are consistent with the proposition that the galactic dark halo is composed of pieces of solid matter several to tens of centimeters in size. Direct collisions among these large "grains" vaporize the grains. The vapor, which appears as hot halo gas, flows inward and forms a luminous core, the visible galaxy. Simple approximate expressions for the extent of halo gas and the mass infall rate emerge in a straightforward way. The expression for the mass infall rate, which governs the star formation rate, is consistent with the Tully-Fisher relation. Gray extinction by the grains may account for the paucity of detectable quasars at large redshifts. Because the grains are large, they are an effective cosmic thermalizing agent. Their thermalizing action smooths inhomogeneities in the primordial microwave background radiation while providing a source of nonprimordial background. The expected thermal microwave emission from galactic grain halos is near the current limits of sensitivity of relevant measurements and provides a possible test of the model.