Dark Matter Contraction and the Stellar Content of Massive Early-type
  Galaxies: Disfavoring "Light" Initial Mass Functions
  We use stellar dynamics, strong lensing, stellar population synthesis models,
and weak lensing shear measurements to constrain the dark matter (DM) profile
and stellar mass in a sample of 53 massive early-type galaxies. We explore
three DM halo models (unperturbed Navarro Frenk & White [NFW] halos and the
adiabatic contraction models of Blumenthal and Gnedin) and impose a model for
the relationship between the stellar and virial mass (i.e., a relationship for
the star-formation efficiency as a function of halo mass). We show that, given
our model assumptions, the data clearly prefer a Salpeter-like initial mass
function (IMF) over a lighter IMF (e.g., Chabrier or Kroupa), irrespective of
the choice of DM halo. In addition, we find that the data prefer at most a
moderate amount of adiabatic contraction (Blumenthal adiabatic contraction is
strongly disfavored) and are only consistent with no adiabatic contraction
(i.e., a NFW halo) if a mass-dependent IMF is assumed, in the sense of a more
massive normalization of the IMF for more massive halos.
