Evidence for dark matter contraction and a Salpeter IMF in a massive
  early-type galaxy
  Stars and dark matter account for most of the mass of early-type galaxies,
but uncertainties in the stellar population and the dark matter profile make it
challenging to distinguish between the two components. Nevertheless, precise
observations of stellar and dark matter are extremely valuable for testing the
many models of structure formation and evolution. We present a measurement of
the stellar mass and inner slope of the dark matter halo of a massive
early-type galaxy at $z=0.222$. The galaxy is the foreground deflector of the
double Einstein ring gravitational lens system SDSSJ0946+1006, also known as
the "Jackpot". By combining the tools of lensing and dynamics, we first
constrain the mean slope of the total mass density profile
($\rho_{\rm{tot}}\propto r^{-\gamma'}$) within the radius of the outer ring to
be $\gamma' = 1.98\pm0.02\pm0.01$. Then we obtain a bulge-halo decomposition,
assuming a power-law form for the dark matter halo. Our analysis yields
$\gamma_{\rm{DM}} = 1.7\pm0.2$ for the inner slope of the dark matter profile,
in agreement with theoretical findings on the distribution of dark matter in
ellipticals, and a stellar mass from lensing and dynamics $M_*^{\rm{LD}} =
5.5_{-1.3}^{+0.4}\times10^{11}M_\Sun$. By comparing this measurement with
stellar masses inferred from stellar population synthesis fitting we find that
a Salpeter IMF provides a good description of the stellar population of the
lens while the probability of the IMF being heavier than Chabrier is 95%. Our
data suggest that growth by accretion of small systems from a compact red
nugget is a plausible formation scenario for this object.
