ON THE CENTRAL SURFACE BRIGHTNESS PROBLEM IN DISK GALAXIES

We have measured the diameters of a large number of disk galaxies on various Sky Surveys in order to examine the problem of the apparent uniformity of the central surface brightness in disk galaxies. From the ratio of diameters on the various surveys we find that at least 55% of the disk galaxies follow this rule. Another 26% of the galaxies have only a very modest increase in diameters on the SRC-J survey when compared to the Palomar survey prints, indicating the presence of a cutoff in the outer disk. 19% of the galaxies measured have a large increase in diameter. From their integrated H I profiles, these turn out to be mostly dynamically giant galaxies. Several examples are shown. The phenomenon of low surface brightness extensions is greatly reduced for galaxies in the Virgo cluster. From follow-up observations we conclude that the low surface brightness giants contain only about half the amount of luminous material as do ''normal'' spiral galaxies following the 21.65 law.