Indexed on: 05 Mar '97Published on: 05 Mar '97Published in: Astrophysics
The Ursa Major Cluster appears to be unevolved and made up of HI-rich spiral galaxies. B,R,I,K' photometry has been obtained for 79 galaxies, including 62 in a complete sample with M(B)<-16.5 mag. The K' information is particularly important for the present discussion because it is not seriously affected by obscuration. There is reasonably convincing evidence that the distribution of exponential disk central surface brightnesses is bimodal. Disks avoid the domain between the high and low surface brightness zones. The few intermediate surface brightness examples in the sample all have significant nearby neighbors. The high surface brightness (HSB) galaxies exhibit a range -21<M(B)<-17 while the low surface brightness (LSB) galaxies are found with -19<M(B) down to the completion limit. HSB and LSB galaxies in the overlap regime -19<M(B)<-17 that are indistinguishable in luminosity-line width plots have very distinct locations in surface brightness-scale length plots. The existence of separate HSB and LSB families suggests that there are discrete radial configurations that are stable. Galaxies are driven into one of these regimes. The HSB state has a lower luminosity cutoff. It is likely that the HSB galaxies are dominated by dissipational matter at their centers while the LSB galaxies are dark matter dominated. The HSB family subdivides into those with, and without, substantial bulges. Evidently, there are two thresholds probably controlled by angular momentum content or transfer. Passing from high to low specific angular momentum, there is first the transition from LSB to HSB regimes, then the transition from exponential disk to disk plus bulge regimes.