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 (Mstar~3e11 Msun) early-type galaxies and its dependence on redshift,
specifically in the range 0<z<1. We start from a large sample of SDSS
early-type galaxies with stellar masses and effective radii measured assuming
two different profiles, de Vaucouleurs and S\'{e}rsic. We assign dark matter
haloes to galaxies via abundance matching relations with standard LCDM 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 S\'{e}rsic and
the input S\'{e}rsic index n to vary with redshift approximately as
n(z)~(1+z)^(-1). This conclusion holds irrespective of the input Mstar-Mhalo
relation, the assumed stellar initial mass function, 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 initial mass
functions. The strong lensing selection effects we have simulated seem not to
contribute to this effect. A steadily increasing S\'{e}rsic 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.
