Bimodality of Freeman's Law

Abstract

A cluster sample of 62 galaxies complete to M(B)=-16.5 mag has been observed at B,R,I,K' bands with imaging detectors. The distribution of exponential disk central surface brightnesses is found to be bimodal. The bimodality is particularly significant at K' because obscuration is not a problem and because the high surface brightness galaxies are redder than the low surface brightness galaxies so the bifurcation is greater. The bimodality signal is especially clear when galaxies with close companions are excluded from consideration. High and low surface brightness pairs with essentially identical luminosities and maximum rotation characteristics are compared. It is suggested that the high surface brightness galaxies have self-gravitating disks while the low surface brightness galaxies are halo dominated at all radii. Evidently the intermediate surface brightness regime is unstable. If a disk has sufficiently low angular momentum and it shrinks enough that the disk potential begins to dominate the halo potential locally, then the disk must secularly evolve to the high surface brightness state characterized by a flat rotation curve.