Understanding the surface brightness distribution of disc galaxies

It is difficult to understand why disc galaxies should all form with about the same central surface brightness (central mass density?). A possible explanation is that the data are in fact heavily biased by observational selection. In a number of papers we have shown how this might arise. In this paper we take a new look at van der Kruit's assertion that the observational selection effects are unimportant. We show that his data are incomplete on the low-surface-brightness side and that they are probably constrained by a hidden magnitude constraint that he did not consider. We go on to test the selection hypothesis by analysing a set of observational data for which redshifts are available. The volume occupied by galaxies of different surface brightnesses in this sample are in good agreement with those predicted by observational selection. We conclude that the selection hypothesis has not been disproved and that there is still good reason to expect it to be true.