Is there a fundamental relationship between metallicity and density in galaxies?

In addition to the usual metallicity-luminosity relation for elliptical galaxies, we propose that a primary observational correlation exists between B-H colour and surface brightness, and thus that an important physical relationship may exist between the metallicity of an elliptical galaxy and its density. In particular we find a roughly linear correlation between B-H colour and the effective blue surface brightness, which spans at least 5 mag in the latter quantity, from the highest surface brightness giant ellipticals (which are not the most luminous ones) down to low surface brightness dwarfs. (author)