Revisiting the bulge-halo conspiracy - II. Towards explaining its puzzling dependence on redshift

We carry out a systematic investigation of the total mass density profile of massive (log M-star/M-circle dot similar to 11.5) early-type galaxies and its dependence on redshift, specifically in the range 0 less than or similar to z less than or similar to 1. We start from a large sample of Sloan Digital Sky Survey early-type galaxies with stellar masses and effective radii measured assuming two different profiles, de Vaucouleurs and Sersic. We assign dark matter haloes to galaxies via abundance matching relations with standard Lambda CDM profiles and concentrations. We then compute the total, mass-weighted density slope at the effective radius gamma', and study its redshift dependence at fixed stellar mass. We find that a necessary condition to induce an increasingly flatter gamma' at higher redshifts, as suggested by current strong lensing data, is to allow the intrinsic stellar profile of massive galaxies to be Sersic and the input Sersic index n to vary with redshift as n(z) alpha (1 + z)(delta), with delta less than or similar to -1. This conclusion holds irrespective of the input M-star-M-halo relation, the assumed stellar initial mass function (IMF), or even the chosen level of adiabatic contraction in the model. Secondary contributors to the observed redshift evolution of gamma' may come from an increased contribution at higher redshifts of adiabatic contraction and/or bottom-light stellar IMFs. The strong lensing selection effects we have simulated seem not to contribute to this effect. A steadily increasing Sersic index with cosmic time is supported by independent observations, though it is not yet clear whether cosmological hierarchical models (e.g. mergers) are capable of reproducing such a fast and sharp evolution.