Mirages in galaxy scaling relations

Abstract

We analyzed several basic correlations between structural parameters of galaxies. The data were taken from various samples in different passbands which are available in the literature. We discuss disc scaling relations as well as some debatable issues concerning the so-called Photometric Plane for bulges and elliptical galaxies in different forms and various versions of the famous Kormendy relation. We show that some of the correlations under discussion are artificial (self-correlations), while others truly reveal some new essential details of the structural properties of galaxies. Our main results are as follows: (1) At present, we can not conclude that faint stellar discs are, on average, more thin than discs in high surface brightness galaxies. The ``central surface brightness -- thickness'' correlation appears only as a consequence of the transparent exponential disc model to describe real galaxy discs. (2) The Photometric Plane appears to have no independent physical sense. Various forms of this plane are merely sophisticated versions of the Kormendy relation or of the self-relation involving the central surface brightness of a bulge/elliptical galaxy and the Sersic index n. (3) The Kormendy relation is a physical correlation presumably reflecting the difference in the origin of bright and faint ellipticals and bulges. We present arguments that involve creating artificial samples to prove our main idea.