A multi-wavelength study of the gravitational lens COSMOS J095930+023427
  We present a multi-wavelength study of the gravitational lens COSMOS
J095930+023427 (z=0.89), together with the associated galaxy group located at
$z\sim0.7$ 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", 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 $f_{\rm DM}$ 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 $z\sim0.7$, 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.
