Subluminal Mass

Abstract

This chapter studies the presence of “subliminal matter.” The presence of significant mass in subluminal matter was first suggested in the 1930s by the surprisingly large velocities of galaxies in clusters of galaxies. The chapter traces the history of discovery of astronomical evidence of subluminal matter in large clusters of galaxies, in groups of a few or just two galaxies that are close enough that they seem likely to be gravitationally bound, and in individual spiral galaxies. There must be enough mass in spirals to account for the circular velocities of disk stars, and the mass rotationally supported in the disk must be large enough that gravity can form spiral arms, but this mass component cannot be so large that the spiral arms grow to destroy the observed nearly circular motions in the disk. These conditions require that most of the mass in a spiral galaxy is in a stable subluminal massive halo draped around the outskirts of the luminous parts of the galaxy.