Empirical Scaling Relations

Early-type galaxies do not come in any shape, form, and color. Many of their observable properties obey tight correlations, also known as empirical scaling relations. The correlations are non-trivial, in the sense that they cannot be explained by simple physical or dimensional arguments. A subset of the empirical scaling relations connects baryonic observables with quantities that depend on the total gravitational potential of the galaxies, and thus on their dark matter content. These correlations are a fundamental testbed for our understanding of the formation and evolution of early-type galaxies, and, more in general, of the physical processes that determine the interplay between baryons and dark matter at galactic scales.