Formation of the Galaxy

Abstract

From observations it is deduced that the disk and halo were formed about the same time and that most of the previously gaseous Galaxy became stars then. Dimensional analysis indicates that the flattening of a galaxy is related to its energy,E, angular momentum,H, and mass,M, by the dimensionless number - 2EH2/G5M5. Emphasis is placed on the binding energy, -E, and the energy loss mechanisms of the proto-galaxy, with the aim of explaining Fish's relation -E∝M3/2.Free-bound and free-free emission of hydrogen leads to rapid cooling of proto-galaxies, and dynamical collapse ensues, in which gravitational potential energy is converted into kinetic energy of collapse. As the system becomes flat, this energy is dissipated in violent shocks, behind which zones some 10 to 20 pc thick reach temperatures of 106·5°K and emit strongly in the X-ray region and the ultraviolet. If surrounded by more than a fraction of a gram per cm2, the X-rays will be trapped within the system and eventually converted into Balmer lines, which escape, and Lyman α, which is trapped. About half the total energy of collapse may be left in Lyman α, and it is possible that the system may bounce on this light energy.The readily observed surface brightnesses of galaxies are related to surface densities by the relationship: magnitude 20 per (sec of arc)2= 1·5 gram cm-2, for an assumed mass-to-light ratio of 10.