Massive galactic halos. I - Formation and evolution

It is shown that massive halos with approximately an inverse-square density distribution form naturally around galaxies if there is a supply of material left over from galaxy formation which is of the appropriate character, viz., condensed bodies or noninteracting neutral particles. The evidence is strong that halos are made of such stuff, perhaps the most likely candidate being low-mass stars formed very early in the history of the universe. A fairly detailed model for the Local Group fits the total masses, time scales, and dynamics very well with essentially all the mass in halos. Predictions for galaxies in great clusters like Coma are quite different, and calculations indicate that only about 10% of the total mass should be in halos, the rest distributed throughout the cluster. It is argued that the ratio of halo stuff to 'visible' matter is the same everywhere; the arguments for a low-density universe are strengthened thereby.