The Ursa Major Cluster of galaxies .2. Bimodality of the distribution of central surface brightnesses

The Ursa Major Cluster appears to be unevolved and made up of H I-rich spiral galaxies such as those one finds in the held. B, R, I, K' photometry has been obtained for 79 galaxies, including 62 in a complete sample with M-B(b,i) < -16.(m)5 (with a distance to the cluster of 15.5 Mpc). 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 acoin 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 high surface brightness galaxies exhibit a range -21(m) < M-B(b,i) < -17(m), while the low surface brightness galaxies are found with -19(m) < M-B(b,i) down to the completion limit. High and low surface brightness galaxies in the overlap regime -19(m) < <M-B(b,i) < -17(m) that are indistinguishable in luminosity-line width plots have very distinct locations in surface brightness-scale length plots. 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. The high surface brightness family subdivides into those with and without substantial bulges. In either case, these galaxies have essentially the same exponential disk central surface brightnesses. 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 low surface brightness to high surface brightness regimes, and then the transition from exponential disk to disk plus bulge regimes.