Two physical laws for Elliptical galaxies: photometrical consequences
Abstract
In the present paper, we show that elliptical galaxies (Es) obey two laws. The first one accounts for the fact that Es are relaxed systems in a post violent-relaxation stage; they are quasi-equilibrium gravitational systems and therefore have a quasi-constant specific entropy. The second physical law is a scaling relation between potential energy and mass. Assuming that light traces mass, these laws imply that in the space defined by the three Sersic law parameters (intensity Sigma_0, scale a and shape nu), elliptical galaxies are distributed on two 2-manifolds: the Entropic Surface and the Energy-Mass Surface. Using a sample of 132 galaxies belonging to three nearby clusters, we have verified that ellipticals do indeed follow these laws, and their dispersion about the two surfaces is very small. This also implies that they tightly distribute along the intersection line (the Energy-Entropy line), thus they constitute a one-parameter family. These two physical laws (separately or combined), allow to find the theoretical origin of several photometrical observed relations, such as the correlation between absolute magnitude and effective surface brightness, and the fact that ellipticals are located on a surface in the [log R_eff, -2.5 log Sigma_0, log nu] space.Keywords
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