The dark and luminous matter coupling in the formation of spheroids: a SPH investigation

Using N-body/hydrodynamical simulations which include prescriptions for star formation, feed-back and chemical evolution, we explore the interaction between baryons and dark matter (DM) at a galactic scale. The N-body simulations we performed using a Tree-SPH code that follows the evolution of individual DM halos inside which stars form from cooling gas, and evolve, delivering in the interstellar medium (LSM) mass, both metals and energy. We examine the formation and evolution of a giant and a dwarf elliptical galaxy of total masses 10(12)M. and 10(9)M., respectively. Starting from an initial density profile like the universal Navarro et al. (1996) profile in the inner region, baryons sink towards the center due to cooling energy losses. At the end of the collapse, the innermost part (similar or equal to 1/20 of the halo size) of the galaxy is baryon-dominated, whereas the outer regions are DM dominated. The star formation proceeds at a much faster speed in the giant galaxy where a spheroid of 8 x 10(10)M. is formed in 2 Gyr, with respect to the dwarf galaxy where the spheroid of 2 x 10(7)M. is formed in 4 Gyr. For the two objects the final distributions of stars are well fitted by a Hernquist profile with effective radii of r(e) = 30 kpc and 2.8 kpc, respectively. The dark-to-luminous transition radius r(IBD) occurs roughly at 1 r(e), as in real ellipticals. The DM halo density evolution is non-adiabatic and does not lead to a core radius.