Evolution as a confounding parameter in scaling relations for galaxies

Early-type galaxies are characterized by many scaling relations. Evolutionary classifications find that some of these correlations are indeed generated by diversification. With a simple mathematical formalism, we show (Fraix-Burnet 2011) that even the so-called fundamental plane, a relatively tight correlation between three variables, can be easily explained as the artifact of the effect of another parameter influencing all, without any physical hypothesis. In other words, the fundamental plane is probably a confounding correlation, i.e. not physically causal. The complexity of the physics of galaxies and of their evolution suggests that the confounding parameter must be related to the level of diversification reached by the galaxies. Galaxy mass, central black hole mass or the gas fraction during the last big merger are shown to be possible confounding factors. Consequently, many scaling relations for galaxies are probably evolutionary correlations that are explained by the statistical general evolution of most properties of galaxies. This effect makes the observables not independent, so that it must be removed before statistical and physical inferences could be made.