Rhea-Silvia Remus, Andreas Burkert, Klaus Dolag, Peter H. Johansson, Thorsten Naab, Ludwig Oser, Jens Thomas
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 with controlled initial conditions as well as the formation within the cosmological framework. On average, the total stellar plus dark matter density profiles can be described by a power law with an index of gamma approx -2.1 with a tendency towards steeper slopes for more compact, lower-mass ellipticals. In the binary mergers the amount of gas involved in the merger determines the 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. At higher redshifts, the slopes of the ellipticals extracted from the cosmological simulations are generally steeper. Each gas-poor merger event evolves the slope towards gamma ~ -2, once this slope is reached further merger events do not change the slope anymore. Independent of their individual slopes or evolution scenarios, 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. In addition, 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.
View original:
http://arxiv.org/abs/1211.3420
No comments:
Post a Comment