  A deep HST image of the MS 2137-2353 core has revealed detailed morphological
structures in two arc systems, which are modelled and well reproduced after a
complete analysis of the lensing properties of the dark matter component.
Latter could have a simple elliptical mass distribution with ellipticity and
angular orientation similar to those of the visible and X-ray light, which
suggests that the MS 2137-2353 is a relaxed cluster at z=0.313. The predicted
density profile ($\rho$ $\sim$ $r^{-1.56\pm0.1}$ with $r_{c}$ $\le$
22.5$h_{50}^{-1}$ kpc) within 150$h_{50}^{-1}$ kpc implies increasing M/L ratio
with the radius, and could be in agreement with predictions from standard CDM
simulations. At least two faint sources (unlensed magnitude, R=23.9 and 26,
respectively) are aligned with the cluster core and are responsible of the arc
systems. They have been reconstructed with details as small as 0".02 (or
160$h_{50}^{-1}$ pc in the source assumed at z= 1), one could be a nearly
edge-on barred spiral galaxy, and the other has a more complex morphology,
which could be related to a close interacting pair and/or to dust. They show
strong signs of star formation indicated by compact HII regions well off their
center. The observation of giant luminous arcs by HST could even resolve the
size of giant HII regions at z$\sim$ 1.
