Fundamental relationships in galactic disks

Although there have been a number of correlations demonstrated between observable parameters in galaxies, such as surface brightness, luminosity, metallicity, etc., debate continues as to which of these parameters are truly fundamental. By combining surface photometry, HII region abundances, and H I mapping we have been able to show that the surface density of recent massive star formation, the surface density of stars already formed, the mean oxygen abundance, and the H I column density at a given galactic radius are all fundamentally related within and between the disks of spiral galaxies. Such relationships can serve as powerful constraints on models of galactic evolution, requiring for instance a star formation law dependent not only on gas surface density, but also on the total mass surface density.