Clues on regularity in the structure and kinematics of elliptical galaxies from self-consistent hydrodynamical simulations: The dynamical fundamental plane

Abstract

We have analysed the parameters characterizing the mass and velocity distributions of two samples of relaxed elliptical-like objects (ELOs) identified, at z = 0, in a set of self-consistent hydrodynamical simulations operating in the context of a concordance cosmological model. ELOs have a prominent, non-rotating, dynamically relaxed stellar spheroidal component, with very low cold gas content, and sizes of no more than ∼10-40 kpc (ELO or baryonic object scale), embedded in a massive halo of dark matter typically 10 times larger in size (halo scale). They have also an extended halo of hot diffuse gas. The parameters characterizing the mass, size and velocity dispersion both at the baryonic object and at the halo scales have been measured in the ELOs of each sample. At the halo scale, they have been found to satisfy virial relations; at the scale of the baryonic object, the (logarithms of the) ELO stellar masses, projected stellar half-mass radii, and stellar central line-of-sight (LOS) velocity dispersions define a flattened ellipsoid close to a plane (the intrinsic dynamical plane, IDP), tilted relative to the virial one, whose observational manifestation is the observed Fundamental Plane (FP). Otherwise, IDPs are not homogeneously populated, but ELOs, as well as elliptical (E) galaxies in the FP, occupy only a particular region defined by the range of their masses. The ELO samples have been found to show systematic trends with the mass scale in both, the relative content and the relative distributions of the baryonic and the dark mass ELO components, so that homology is broken in the spatial mass distribution (resulting in the IDP tilt), but ELOs are still a two-parameter family where the two parameters are correlated (causing its non-homogeneous population). The physical origin of these trends presumably lies in the systematic decrease, with increasing ELO mass, of the relative amount of dissipation experienced by the baryonic mass component along ELO stellar mass assembly. ELOs also show kinematical segregation, but it does not appreciably change with the mass scale. The non-homogeneous population of IDPs explains the role played by Mvir to determine the correlations among intrinsic parameters. In this paper, we also show that the central stellar LOS velocity dispersion of ELOs, σlos,0star, is a fair empirical estimator of Mvir, and this explains the central role played by σlos,0 at determining the observational correlations. © 2006 RAS.