The Lack of Structural and Dynamical Evolution of Elliptical Galaxies sincez ~ 1.5: Clues from Self-Consistent Hydrodynamic Simulations

We present the results of a study of the evolution of the parameters that characterize the structure and dynamics of the relaxed elliptical-like objects (ELOs) identified at redshifts z = 0, z = 1, and z = 1.5 in a set of hydrodynamic, self-consistent simulations operating in the context of a concordance cosmological model. The values of the stellar mass M-bo(star), the stellar half-mass radius r(e,bo)(star), and the mean square velocity for stars sigma(3,bo)(star) have been star measured in each ELO and found to populate, at any z, a flattened ellipsoid close to a plane (the dynamical plane, DP). Our simulations indicate that at the intermediate z's considered, individual ELOs evolve, increasing their M-bo(star), r(e,bo)(star) and sigma(3,bo)(star) parameters as a consequence of ongoing mass assembly, but nevertheless, their DP is roughly preserved within its scatter, in agreement with observations of the fundamental plane at different z's. We briefly discuss how this lack of significant dynamical and structural evolution in ELO samples arises, in terms of the two different phases operating in the mass aggregation history of their dark matter halos. According to our simulations, most dissipation involved in ELO formation takes place at the early violent phase, causing the M-bo(star), r(e,bo)(star), and sigma(3,bo)(star) parameters to settle down to the DP and, moreover, the transformation of most of the available gas into stars. In the subsequent slow phase, ELO stellar mass growth preferentially occurs through nondissipative processes, so that the DP is preserved and the ELO star formation rate considerably decreases. These results hint, for the first time, at a possible way of explaining, in the context of cosmological simulations, different and apparently paradoxical observational results for elliptical galaxies.