The Co-Evolution of Total Density Profiles and Central Dark Matter Fractions in Simulated Early-Type Galaxies
Abstract
We present evidence from cosmological hydrodynamical simulations for a co-evolution of the slope of the total (dark and stellar) mass density profile, gammatot, and the dark matter fraction within the half-mass radius, fDM, in early-type galaxies. The relation can be described as gammatot = A fDM + B for all systems at all redshifts. The trend is set by the decreasing importance of gas dissipation towards lower redshifts and for more massive systems. Early-type galaxies are smaller, more concentrated, have lower fDM and steeper gammatot at high redshifts and at lower masses for a given redshift; fDM and gammatot are good indicators for growth by "dry" merging. The values for A and B change distinctively for different feedback models, and this relation can be used as a test for such models. A similar correlation exists between gammatot and the stellar mass surface density Sigma*. A model with weak stellar feedback and feedback from black holes is in best agreement with observations. All simulations, independent of the assumed feedback model, predict steeper gammatot and lower fDM at higher redshifts. While the latter is in agreement with the observed trends, the former is in conflict with lensing observations, which indicate constant or decreasing gammatot. This discrepancy is shown to be artificial: the observed trends can be reproduced from the simulations using observational methodology to calculate the total density slopes.
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