The Ursa Major Cluster of Galaxies. The Ursa Major Cluster appears to be unevolved and made up of HI-rich spiral galaxies like one finds in the field. 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 . There is roughly an order of magnitude difference in the mean luminosity densities of high and low surface brightness disks.
Disks avoid the domain between the high and low surface brightness zones.
The few intermediate surface brightness examples in the sample all have significant neighbors within a projected distance of 80 kpc. The existence of separate high and low surface brightness families suggests that there are discrete radial configurations that are stable. Galaxies are driven into one of these regimes. The high surface brightness state has a lower luminosity cutoff. It is likely that the high surface brightness galaxies are dominated by dissipational matter at their centers while the low surface brightness galaxies are dark matter dominated.