Discovery of two Einstein crosses from massive post--blue nugget
  galaxies at z>1 in KiDS
  We report the discovery of two Einstein Crosses (ECs) in the footprint of the
Kilo-Degree Survey (KiDS): KIDS J232940-340922 and KIDS J122456+005048. Using
integral field spectroscopy from MUSE@VLT, we confirm their gravitational-lens
nature. In both cases, the four spectra of the source clearly show a prominence
of absorption features, hence revealing an evolved stellar population with
little star formation. The lensing model of the two systems, assuming a
singular isothermal ellipsoid (SIE) with external shear, shows that: 1) the two
crosses, located at redshift $z=0.38$ and 0.24, have Einstein radius $R_{\rm
E}=5.2$ kpc and 5.4 kpc, respectively; 2) their projected dark matter fractions
inside the half effective radius are 0.60 and 0.56 (Chabrier IMF); 3) the
sources are ultra-compact galaxies, $R_{\rm e}\sim0.9$ kpc (at redshift $z_{\rm
s}=1.59$) and $R_{\rm e}\sim0.5$ kpc ($z_{\rm s}=1.10$), respectively. These
results are unaffected by the underlying mass density assumption. Due to size,
blue color and absorption-dominated spectra, corroborated by low specific
star-formation rates derived from optical-NIR spectral energy distribution
fitting, we argue that the two lensed sources in these ECs are blue nuggets
migrating toward their quenching phase.
