THE DARK HALO-SPHEROID CONSPIRACY AND THE ORIGIN OF ELLIPTICAL GALAXIES

Dynamical modeling and strong-lensing data indicate that the total density profiles of early-type galaxies are close to isothermal, i.e., rho(tot) proportional to r(gamma) with gamma approximate to -2. To understand the origin of this universal slope we study a set of simulated spheroids formed in isolated binary mergers as well as the formation within the cosmological framework. The total stellar plus darkmatter density profiles can always be described by a power law with an index of gamma approximate to -2.1 with a tendency toward steeper slopes for more compact, lower-mass ellipticals. In the binary mergers the amount of gas involved in the merger determines the precise steepness of the slope. This agrees with results from the cosmological simulations where ellipticals with steeper slopes have a higher fraction of stars formed in situ. Each gas-poor merger event evolves the slope toward gamma similar to -2, once this slope is reached further merger events do not change it anymore. All our ellipticals have flat intrinsic combined stellar and dark matter velocity dispersion profiles. We conclude that flat velocity dispersion profiles and total density distributions with a slope of gamma similar to -2 for the combined system of stars and dark matter act as a natural attractor. The variety of complex formation histories as present in cosmological simulations, including major as well as minor merger events, is essential to generate the full range of observed density slopes seen for present-day elliptical galaxies.