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Recent studies have shown that massive elliptical galaxies have total massdensity profiles within an effective radius that can be approximated as\rho_{tot}\propto r^{-\gamma'}, with mean slope <\gamma'>=2.08 \pm 0.03 andscatter \sigma_\gamma'=0.16 \pm 0.02. The small scatter of the slope (known asthe bulge-halo conspiracy) is not generic in LCDM based models and thereforecontains information about the galaxy formation process. We compute thedistribution of \gamma' for LCDM-based models that reproduce the observedcorrelations between stellar mass, velocity dispersion, and effective radius ofearly-type galaxies in the SDSS. The models have a range of stellar initialmass functions (IMFs) and dark halo responses to galaxy formation. The observeddistribution of \gamma' is well reproduced by a model with cosmologicallymotivated but uncontracted dark matter haloes, and a Salpeter-type IMF. Othermodels are on average ruled out by the data, even though they may happen inindividual cases. Models with adiabatic halo contraction (and lighter IMFs)predict too small values of \gamma'. Models with halo expansion, ormass-follows-light predict too high values of \gamma'. Our study shows that thenon-homologous structure of massive early-type galaxies can be preciselyreproduced by LCDM models if the IMF is not universal and if mechanisms such asfeedback from active galactic nuclei, or dynamical friction, effectively onaverage counterbalance the contraction of the halo expected as a result ofbaryonic cooling.

The bulge-halo conspiracy in massive elliptical galaxies: implications for the stellar initial mass function and halo response to baryonic processes