On the Formation of Elliptical Galaxies

Abstract

It is shown that the violent relaxation of dissipationless stellar systems leads to universal de Vaucouleurs profiles only outside 1.5 effective radii $R_e$. Inside $1.5 R_e$ the surface density profiles depend strongly on the initial conditions and are in general not in agreement with the de Vaucouleurs law. This result is in contradiction to the observations which show that all elliptical galaxies have de Vaucouleurs profiles inside $1.5 R_e$ and show strong deviations outside $1.5 R_e$. The $r^{1/4}$-profiles of E-galaxies are therefore not a direct result of collisionless violent relaxation and angular momentum redistribution. It is rather a universal initial density profile which is required in order to explain the universal structure of E-galaxies. A new model for the formation of E-galaxies is presented which assumes that ellipticals and bulges formed from isothermal, gaseous spheres which accumulated in the core regions of dark matter halos and condensed into stars through a cooling instability. This scenario predicts that early type galaxies trace directly the core radii and core masses of their dark matter halos, providing important information on the nature of the dark matter component in galaxies.