We present a multi-wavelength study of the gravitational lens COSMOS J095930+023427 ( z _ { l } = 0.892 ) , together with the associated galaxy group located at z \sim 0.7 along the line of sight and the lensed background galaxy .
The source redshift is currently unknown , but estimated to be at z _ { s } \sim 2 .
The analysis is based on the available public HST , Subaru , Chandra imaging data , and VLT spectroscopy .
The lensing system is an early-type galaxy showing a strong [ OII ] emission line , and produces 4 bright images of the distant background source .
It has an Einstein radius of 0.79 ^ { \prime \prime } , about 4 times large than the effective radius .
We perform a lensing analysis using both a Singular Isothermal Ellipsoid ( SIE ) and a Peudo-Isothermal Elliptical Mass Distribution ( PIEMD ) for the lensing galaxy , and find that the final results on the total mass , the dark matter ( DM ) fraction within the Einstein radius and the external shear due to a foreground galaxy group are robust with respect of the choice of the parametric model and the source redshift ( yet unknown ) .
We measure the luminous mass from the photometric data , and find the DM fraction within the Einstein radius f _ { DM } to be between 0.71 \pm 0.13 and 0.79 \pm 0.15 , depending on the unknown source redshift .
Meanwhile , the non-null external shear found in our lensing models supports the presence and structure of a galaxy group at z \sim 0.7 , and an independent measurement of the 0.5-2 keV X-ray luminosity within 20 ” around the X-ray centroid provides a group mass of M = ( 3 - 10 ) \times 10 ^ { 13 } M _ { \odot } , in good agreement with the previous estimate derived through weak lensing analysis .
Finally , by inverting the HST/ACS I _ { 814 } image with the lensing equation , we obtain the reconstructed image of the magnified source galaxy , which has a scale of about 3.3 kpc at z _ { s } = 2 ( 2.7 kpc at z _ { s } = 4 ) and the typical disturbed disk-like appearance observed in low-mass star-forming galaxies at z \sim 3 .
However , deep , spatially resolved spectroscopic data for similar lensed sources are still required to detected the first stage of galaxy evolution , since the available spectrum shows no clear feature due to the background source .