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 ~
r^gamma with gamma approx -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 dark
matter density profiles can always be described by a power law with an index of
gamma approx -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 ~ -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 ~ -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.
