Gravo-thermal properties and formation of elliptical galaxies

We have analyzed a sample of galaxies belonging to three clusters: Coma, Ab ell 85, and Abell 496 (real galaxies) and a sample of simulated elliptical galaxies formed in a hierarchical merging scheme (virtual galaxies). We use the Sersic law to describe their light profile. The specific entropy (Bolt zmann-Gibbs definition) is then calculated supposing that the galaxies beha ve as spherical, isotropic, one-component systems. We find that, to a good approximation (similar to 10%), both real and virtual galaxies have an almo st unique specific entropy. Within this approximation the galaxies are dist ributed in a thin plane in the space defined by the three Sersic law parame ters, which we call the Entropic Plane. A further analysis shows that both real and virtual galaxies are in fact located on a thin line, therefore ind icating the existence of another - and yet unknown - physical property, bes ides the uniqueness of the specific entropy. A more careful examination of the virtual galaxies sample indicates a very small increase of their specific entropy with merging generation. In a hier archical scenario, this implies a correlation between the specific entropy and the total mass, which is indeed seen in our data. The scatter and tilt of the Entropic Line, defined by Lima Neto et al. (1999a), are reduced when this correlation is taken into account. Although one cannot distinguish be tween various generations for real galaxies, the distribution of their spec ific entropy is similar to that in the virtual sample, suggesting that hier archical merging processes could be an important mechanism in the building of elliptical galaxies.