The bulge-halo conspiracy in massive elliptical galaxies: implications
  for the stellar initial mass function and halo response to baryonic processes
  Recent studies have shown that massive elliptical galaxies have total mass
density profiles within an effective radius that can be approximated as
\rho_{tot}\propto r^{-\gamma'}, with mean slope <\gamma'>=2.08 \pm 0.03 and
scatter \sigma_\gamma'=0.16 \pm 0.02. The small scatter of the slope (known as
the bulge-halo conspiracy) is not generic in LCDM based models and therefore
contains information about the galaxy formation process. We compute the
distribution of \gamma' for LCDM-based models that reproduce the observed
correlations between stellar mass, velocity dispersion, and effective radius of
early-type galaxies in the SDSS. The models have a range of stellar initial
mass functions (IMFs) and dark halo responses to galaxy formation. The observed
distribution of \gamma' is well reproduced by a model with cosmologically
motivated but uncontracted dark matter haloes, and a Salpeter-type IMF. Other
models are on average ruled out by the data, even though they may happen in
individual cases. Models with adiabatic halo contraction (and lighter IMFs)
predict too small values of \gamma'. Models with halo expansion, or
mass-follows-light predict too high values of \gamma'. Our study shows that the
non-homologous structure of massive early-type galaxies can be precisely
reproduced by LCDM models if the IMF is not universal and if mechanisms such as
feedback from active galactic nuclei, or dynamical friction, effectively on
average counterbalance the contraction of the halo expected as a result of
baryonic cooling.
