Golden gravitational lensing systems from the Sloan Lens ACS Survey. I.
  SDSS J1538+5817: one lens for two sources
  We present a lensing and photometric study of the exceptional system SDSS
J1538+5817, identified by the SLACS survey. The lens is a luminous elliptical
at redshift z=0.143. Using HST public images in two different filters, the
presence of two background sources lensed into an Einstein ring and a double
system is ascertained. Our new spectroscopic observations, performed at the
NOT, reveal that the two sources are located at the same redshift z=0.531. We
investigate the total mass distribution of the lens between 1 and 4 kpc from
the galaxy center by means of parametric and non-parametric lensing codes that
describe the multiple images as point-like objects. Several disparate lensing
models agree on: (1) reproducing accurately the observed image positions; (2)
predicting a nearly axisymmetric total mass distribution, centered and oriented
as the light distribution; (3) measuring a value of 8.11 x 10^{10} M_{Sun} for
the total mass projected within the Einstein radius of 2.5 kpc; (4) estimating
a total mass density profile slightly steeper than an isothermal one. A fit of
the SDSS multicolor photometry with CSP models provides a value of 20 x 10^{10}
M_{Sun} for the total stellar mass of the galaxy and of 0.9 for the fraction of
projected luminous over total mass enclosed inside the Einstein radius. By
combining lensing and photometric mass measurements, we differentiate the lens
mass content in terms of luminous and dark matter components. This
two-component modeling, which is viable only in extraordinary systems like SDSS
J1538+5817, leads to a description of the global properties of the galaxy dark
matter halo. Extending these results to a larger number of lenses would improve
considerably our understanding of galaxy formation and evolution processes in
the LCDM scenario.
