We present a gravitational lensing and photometric study of the exceptional strong lensing system SDSS J1538+5817 , identified by the Sloan Lens ACS survey .
The lens is a luminous elliptical galaxy at redshift z _ { l } = 0.143 .
Using Hubble Space Telescope public images obtained with two different filters , the presence of two background sources lensed , respectively , into an Einstein ring and a double system is ascertained .
Our new spectroscopic observations , performed at the Nordic Optical Telescope , reveal unequivocally that the two sources are located at the same redshift z _ { s } = 0.531 .
We investigate the total ( luminous and dark ) 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 .
Bootstrapping and Bayesian analyses are performed to determine the uncertainties on the quantities relevant to the lens mass characterization .
Several disparate lensing models provide results that are consistent , given the errors , with those obtained from the best-fit model of the lens mass distribution in terms of a singular power law ellipsoid model .
In particular , the lensing models agree on : ( 1 ) reproducing accurately the observed positions of the images ; ( 2 ) predicting a nearly axisymmetric total mass distribution , centered and oriented as the light distribution ; ( 3 ) measuring a value of 8.11 ^ { +0.27 } _ { -0.59 } \times 10 ^ { 10 } M _ { \odot } 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 [ \rho ( r ) \propto r ^ { -2.33 ^ { +0.43 } _ { -0.20 } } ] .
A fit of the Sloan Digital Sky Survey multicolor photometry with composite stellar population models provides a value of 20 ^ { +1 } _ { -4 } \times 10 ^ { 10 } M _ { \odot } for the total mass of the galaxy in the form of stars and of 0.9 ^ { +0.1 } _ { -0.2 } for the fraction of projected luminous over total mass enclosed inside the Einstein radius .
By combining lensing ( total ) and photometric ( luminous ) 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 lens galaxies would improve considerably our understanding of galaxy formation and evolution processes in the \Lambda CDM scenario .