Golden gravitational lensing systems from the Sloan Lens ACS Survey. I. SDSS J1538+5817: one lens for two sources

TL;DR

This study analyzes the mass distribution of the lens galaxy SDSS J1538+5817 using lensing and photometric data, revealing a nearly axisymmetric mass profile with a detailed breakdown of stellar and dark matter components.

Contribution

It provides a detailed two-component mass model of a gravitational lens galaxy, combining lensing and photometry to distinguish luminous and dark matter.

Findings

Total mass within Einstein radius: 8.11 x 10^{10} M_{Sun}

Stellar mass estimate: 20 x 10^{10} M_{Sun}

Dark matter fraction inside Einstein radius: 0.9

Abstract

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.